Science Requirements for CTA

Transcription

1 D. F. Torres (IEEC-CSIC, CTA Physics Coordinator), U. Barres de Almeida (MPIP), C. Boisson (CNRS-OBSPM), S. Casanova (MPIK), J. Conrad, E. de Oa-Wilhelmi (MPIK), M. Doro (UAB), D. Dravins (U. Stockholm), G. Dubus (U. Grenoble), K. Egberts (U. Innsbruck), S. Gabici (CNRS-APC), J. Granot (U. Hertfordshire), J. Hinton (U. Leicester), S. Inoue (ICRR), S. Markoff (U. Amsterdam), D. Mazin (IFAE), P. O Brien (U. Leicester), J. M. Paredes (U. Barcelona), G. Pedaletti (IEEC-CSIC), M. Raue (U. Hamburg), O. Reimer (U. Innsbruck), M. Renaud (U. Montpellier), B. Rudak (CAMK, Warsaw), H. Sol (CNRS-OBSPM), J. Vink (U. Amsterdam), A. Zech (CNRS-OBSPM).

2 Page: 2/54 Executive Summary The field of very high-energy gamma-ray astrophysics has started only recently. In 1989, the Whipple Imaging Atmospheric Cherenkov Telescope (IACT) discovered a clear signal from the Crab Nebula at energies above 1 TeV. Since then, IACTs have evolved to their third generation of instruments, discovering more than 130 galactic and extragalactic sources, and strongly impacting both astrophysics and physics. The current generation of advanced IACTs (H.E.S.S., MAGIC, and VERITAS) has demonstrated the maturity of the detection technique and the huge physics potential of discoveries at these energies. Several astrophysical source classes expected to be TeV emitters have now been firmly established (e.g., pulsar wind nebulae, supernova remnants, pulsars, or blazars), but some have only a few well-studied members (e.g. radio galaxies or binary systems), and some have not yet been detected (e.g. clusters of galaxies or GRBs). However, the discoveries made so far have only scratched the surface of several fundamental topics of modern physics that can be addressed by this technique: Where and how are particles accelerated in our Galaxy and beyond? What makes black holes of all sizes such efficient particle accelerators? What do high-energy gamma-rays tell us about the star formation history of the Universe or the fundamental laws of physics? What is the nature of dark matter? Are there Galactic short-timescale phenomena at very high energies? A natural way to exploring these questions is to aim for a new generation of instruments with much improved performance. The initial working idea for CTA (as described in, e.g., the ASPERA, ASTRONET, and ESFRI roadmaps since 2008, as well as in the US Decadal survey of 2010) is that CTA will consist of two arrays of IACTs aiming to: Increase the sensitivity level of current instruments by up to a factor 10 at 1 TeV, Significantly boost the detection area and hence the detection rate, which is particularly important for transient phenomena, at the highest energies, Increase the angular resolution and hence the ability to resolve the morphology of extended sources, while maintaining a large field of view, Provide energy coverage for photons from some tens of GeV to beyond 100 TeV, thus opening new electromagnetic windows, and Enhance the sky survey capability, monitoring capability, and flexibility of operation, allowing for sub-array and simultaneous multi-mode runs. 2

3 Page: 3/54 Two CTA sites are foreseen in order to provide full sky coverage. In the southern hemisphere, given the wealth of sources in the central region of our Galaxy and the richness of their morphological features, emphasis will be put on Galactic science e.g., via a dedicated Galactic plane survey. In the north, a second array will be primarily (but not uniquely) devoted to the study of extragalactic science and its connection with star formation and evolution. CTA will be operated as a proposal-driven observatory, for the first time in VHE astronomy, with a Science Data Centre providing access to data, analysis tools, and user training. Detailed analysis of the physics and technical reach of such a concept facility have identified two unique aspects of CTA: The ability to produce the deepest surveys of the sky at the highest energies The ability to open up the shortest timescale phenomenology Regarding CTA surveys (both Galactic and extragalactic), they will provide legacy datasets that will enable population studies, as well as individual spectro-imaging analysis of a large number of sources with unprecedented detail. These surveys will, in common with other astronomical facilities, motivate guest investigators to make new observations aimed at in-depth studies of sources of particular interest. At half an hour or less integration times, CTA will feature the ability of detecting events tens of thousand of times weaker than ever before at energies less than 100 GeV. This will chart unexplored territory, possibly uncovering new phenomenology at short timescales, both from unknown or currently believed to be steady sources, mapping possible connections between accretion and ejection phenomena surrounding stellar compact objects, and analyzing in detail the inner structure of AGN flares. Fast reaction times for repointing CTA to any position in the sky will allow focusing quickly on transient events detected at radio, X-ray or other frequencies, including GRBs. Serendipitous transient discoveries while doing the survey will also be possible given the large field of view (FoV). In the latter case, an automated, quick reaction-time analysis will be foreseen, allowing changes to the schedule of the CTA telescope to maximize the scientific return. These instrumental abilities are invaluable tools to tackle the questions that thread the core science program of CTA, in three broad themes: The origin and propagation of leptonic and hadronic cosmic rays (CRs) and their role in the universe; The nature and variety of black holes (BHs) and their use as a probe of star formation history of the universe; The nature of matter and forces beyond the Standard Model of particle physics. Details on key observations for each of these themes are provided in this document. The minimum CTA features needed to make these essential observations are presented on a case-by-case basis. Amalgamating these, a summary table specifying the top-level scientific requirements of CTA is constructed and presented below. This document aims to provide both an introduction to the science of CTA, its main expectations and unique reach, and define the minimum system parameters that would allow such science to happen. A section at 3

4 Page: 4/54 the end describes how the studies on which this document is based were carried out, and where to find further resources. This document also presents further considerations on CTA science: an introduction to its reach per energy domain, and two appendices deal with the concurrent missions and projects at other wavelengths which will be able to alert CTA about transients and provide multi-wavelength coverage, and further technical elaborations on the system requirements and goals. To end, we emphasize that whenever scientific instruments have achieved order-of-magnitude improvements in sensitivity, serendipitous discoveries have been made. The CTA science community too expects surprises and we comment on this below. 4

5 Page: 5/54 CONTENTS 1 The unique instrumental reach of CTA The deepest surveys of the sky at the highest energies Why surveys? Previous surveys Advantages of possible CTA surveys Confusion in the Galactic CTA survey Surveys and monitoring with sub-arrays Possible survey data-taking in divergent mode Survey-driven minimum requirements The high-energy variability at the shortest timescales Why short timescale phenomenology? The Fermi-LAT and higher-energy instrument responses versus CTA Flares from pulsar nebulae? The accretion ejection interface observed at the highest energies? Short timescale variability from black holes beyond the galaxy Detecting GRBs? Alerts Short timescale-driven minimum requirements The core science themes of CTA The origin and propagation of leptonic and hadronic cosmic-rays and their role in the universe Cosmic rays and supernova remnants Pevatrons Diffusion of cosmic rays Cosmic rays in galaxies Clusters Pulsar wind nebulae

6 Page: 6/ Pulsars Ultra high energy cosmic rays Key elements for this theme The nature and variety of black holes, and their use as a probe of the star-formation history of the universe Active Galactic Nuclei Radiogalaxies and Seyfert galaxies Star formation history Intergalactic magnetic field Key elements for this theme The nature of matter and physics beyond the Standard Model Key elements for this theme A look at the main physics cases per energy domain The low-energy end of CTA (< 100 GeV) The mid-energy range of CTA (100 GeV 10 TeV) The high-energy end of CTA (> 10 TeV) CTA: not only a gamma-ray experiment CTA as a cosmic-ray experiment Key elements related to the fulfilling of this additional science aim CTA as an intensity interferometer Key elements related to the fulfilling of this additional science aim Appendix A: Separate requirements for essential astrophysics cases 36 6 Appendix B: Impact of variations in the minimum requirements On energy threshold On the sensitivity requirement On the consequences of reaching the arcmin scale

7 Page: 7/ Fine spectro-imaging studies in the brightest TeV SNRs Pinpoint the nature of VHE sources in the Galaxy as shell-type SNRs or PWNe Alleviating the confusion in the Galactic Plane Other physics reasons On the repositioning time On the sensitivity across the Field of View Appendix C: The multi-wavelength scenario in the CTA era Radio Optical / Infrared X-rays Lower and higher energy gamma-ray domains Appendix D: References of further studies 47 9 Appendix E: Acronyms used in this document 48 7

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10 Page: 10/54 1 THE UNIQUE INSTRUMENTAL REACH OF CTA 1.1 The deepest surveys of the sky at the highest energies Why surveys? Surveys of different extents and depths are amongst the scientific goals of all major astrophysical facilities. The reasons are many: surveys constitute a systematic, exploratory approach that favours discoveries of unknown source classes, provide testing ground for new theoretical ideas, facilitate population studies at an unbiased sensitivity threshold, allow for scheduling ease and homogeneous data reduction, and provide legacy datasets for lasting future reference. Surveys are particularly critical for energy domains that are just opening up observationally. And, in the context of an open observatory, surveys are an indispensable tool to assist the community in formulating guest investigator proposals for additional in-depth studies of sources. For all these reasons, making both a deep Galactic plane survey, as well as a blind, e.g. a π-sky (i.e. 1/4th of the sky) survey of regions out of the Galactic plane is of great importance for CTA. In both of these aims, CTA is unique: compared to previous IACTs, surveys with CTA will benefit from all its features: increased sensitivity (detection of fainter sources), larger field-of-view (to study multiple or extended sources and reduce the observation time it takes to make the surveys uniform), improved angular resolution (to alleviate source confusion), broader energy range (facilitating detecting soft spectrum sources because of its lower-energy reach - as well as pevatrons because of its higher-energy reach), and better energy resolution (to help determine the sources spectral energy distributions, SEDs) Previous surveys The Galactic Plane survey carried out by H.E.S.S. led to the detection of dozens of sources, many of which were unexpected; some with no obvious counterparts at other wavelengths (Aharonian et al. 2005, 2006, 2008). The H.E.S.S. Galactic Plane Survey (GPS) now covers 90 0 < l < 60 0 ( 2.2% of the sky) with the detection of close to 50 sources and a sensitivity reaching 20 mcrab (Chaves 2009). The current Galactic Plane survey is estimated to be complete down to 85 mcrab (Renaud 2009). It took a total observing time of 1500 hr. At higher energies, the Milagro collaboration has surveyed the region 30 0 < l < 120 0, b < 10 0 ( 7.3% of the sky) using 6.3 years of observations with a water Cherenkov extensive air shower (EAS) array. Eight sources above a median energy of 20 TeV were detected, down to a sensitivity 200 mcrab, some of which have an extension of several degrees (Abdo et al. 2007). Milagro and Tibet-AS have also carried out a survey for sources in the Northern hemisphere down to an average sensitivity of a few hundreds of mcrab above 1 TeV (Atkins et al. 2004, Amenomori et al. 2005). The future HAWC Project, which will be contemporaneously working with CTA, aims for sensitivity to 1 Crab sources in a day (50 mcrab in a year), a median energy around TeV, and a 10 angular resolution at 700 GeV. 1 EAS arrays have lower angular resolution ( 10), higher energy thresholds (> 1 TeV) and are

11 Page: 11/54 less sensitive than what CTA aims. However, because their high duty cycle and FoV, they would alert on transient events that could then be followed up by CTA. Above 100 MeV, the survey is actually the default mode of operation of the Fermi-LAT satellite. The Fermi- LAT catalog (Abdo et al. 2010, Nolan et al. 2012) allowed statistical studies of blazars and pulsars for the first time, and has led to the discovery of emission associated with new objects, e.g., the nova outburst from V407 Cyg (Abdo et al. 2010b), or the new gamma-ray binary 1FGL (Abdo et al. 2012). The second all-sky Fermi-LAT catalog (Nolan et al. 2012) contains 1873 sources (i.e., an average of one HE source per every 20 square degrees which, assuming a FoV of a few degrees for CTA, would be equivalent of having one Fermi-LAT source in every CTA FoV). Of these sources detected in the 100 MeV to 100 GeV range, about 30% are of yet unknown nature. While Fermi-LAT and CTA would share little simultaneous operating time, if any at all (the 10-year expected mission of Fermi would allow operations only until 2018), the connection between Fermi-LAT and CTA surveys would bring significant astrophysical information Advantages of possible CTA surveys A ten-fold improvement in sensitivity at the central energy regime of CTA would mean, for example, that by using standard techniques, CTA can carry out a GPS of the region l 60 0, b 20 in 200 hr (less than 1/4th the available time per year at one location) down to a uniform sensitivity of 3 mcrab. 2 Such survey sensitivity would already be 3 times better than what is typically achieved in the deepest 100 hr exposures carried out with the current generation of IACTs on a few selected individual objects (e.g. the supernova remnant SN 1006, Aharonian et al. 2010). A CTA GPS would thus give access to dozens of supernova remnants (SNRs) and pulsar wind nebulae (PWNe) with no a priori pointing, enabling the first population VHE studies of these objects to proceed (e.g. building luminosity vs SNR age or vs pulsar power diagrams). Indeed, more than 300 sources are expected at such sensitivity based on an extrapolation of the current log N - log S diagram for VHE sources (see comment in more detail below, also Renaud 2009). Such a dataset can be used to search for emission from cosmic-ray interactions with molecular clouds, stellar clusters, dark accelerators, binaries, and of course, fundamental physics and the unexpected. For example, the GPS could provide the possibility of detecting pevatrons: Given the featureless cosmicray spectrum extending to PeV energies, and the fact that secondary gamma-rays produced by these PeV particles would have a factor of 10 less energy, there should be cutoffs at TeV in the spectra of young Galactic sources. The GPS survey may have a high-energy sensitivity of 0.2 Crab units (for an observation of 8 hours) at 50 TeV and would allow, e.g., selection of sky-directions from which emission produces at least a 3? flux measurement above 50 TeV. For example, a source such as RX J (cited here just an illustrative spectrum and not to imply that it is of hadronic origin), even with 50% reduction in normalization of its 1 TeV-flux, would produce such excess if its spectrum has a 100 TeV cutoff. Such a source would additionally be detected at lower energies with high significance, which would allow a firm detection above post-trials. This kind of candidate source would thus be ideal for further dedicated observations, and would come at no cost as a result of the survey. 2 For details on the dependence of the sensitivity to survey pointing steps, size, or spectral indices of the sources see Dubus et al (quoted in the references) and the details commented in the Appendix. For instance, using the minimum required sensitivity flatness of 2.50 the survey times would be 190h, 290h, and 200h, for Galactic Surveys with one and two rows, and the extragalactic survey as described, respectively. 11

12 Page: 12/54 Once the most promising sources have been identified, the community-at-large will request dedicated pointed observations or detailed spectral-imaging or variability monitoring of sources, as is usual for any astronomical observatory. The density of known VHE sources increases towards the Galactic Center, favouring the use of the CTA array in the southern site for such an exploration. However, a full exploration of the Galactic plane requires both the southern and northern arrays. Deeper GPSs will be possible, since required observations would not impose demanding constraints on CTA operation. Extending the survey to higher longitudes, or going to deeper sensitivity can be accommodated in the first years of observations with the CTA observatory. For instance, just scaling the improvement with the square root of time, accumulating 2000 hr of GPS (achievable in 5 years) would provide an uniform sensitivity of 1 mcrab, better than all current individual pointed detections, regardless of their exposures. The survey itself can be repeated within the lifetime of CTA to check for variability and to enhance statistics of the steady sources. For an IACT, a quarter of the sky (10 4 square degrees) is accessible when keeping only zenith angles < 60 0, so as to ensure an energy threshold 100 GeV. If CTA reaches a sensitivity of 20 mcrab at 5σ level in just above 30 min (e.g., Actis et al. 2011, page 62), a fraction of the sky could be surveyed at the 22 mcrab level (for energies larger than 100 GeV), with an evenly-spaced grid (i.e. 200 hr, compared to the 1000 hrs available in a year of operations, for the required sensitivity flatness). This sensitivity is similar to the flux level of the faintest AGNs currently detected at VHE energies. The sensitivity above 1 TeV ( 38 mcrab) would be also better than the one-year sensitivity aimed at by HAWC above the same energy threshold ( 50 mcrab). But a large-scale CTA survey like this would bring improvements over HAWC in survey depth, energy threshold (a major advantage for extragalactic sources, which tend to be soft) and angular resolution, even with a moderate investment in time. A handful of counterparts to Fermi-LAT sources should be seen by a CTA survey as described, and based on blazar population modeling (the dominant extragalactic source population) about 10 to 20 sources are expected. 3 Such a blind survey has not been done by IACTs as yet. It could uncover new, unsuspected classes of extragalactic VHE sources, put constraints on dark matter in the TeV mass range, probe diffuse emission on scales of several degrees (limited by the FoV), constrain the distribution of cosmic rays in our Galaxy and the presence of a Galactic wind. This survey can be correlated with sky maps obtained by detectors of ultra-high energy cosmic rays (UHECR) and high-energy neutrinos (although these are less sensitive) and localized anisotropies in the arrival directions of multi-tev charged particles could thus be investigated. The northern CTA site, for which the sky regions observed can be matched with data from e.g. HAWC, LHAASO, and IceCube would be well suited to survey the extragalactic sky. However, both southern and northern sites are required for accessing the whole sky Confusion in the Galactic CTA survey The Galactic distribution of, e.g. PWNe (see Renaud 2009) which produce the largest number of Galactic VHE sources, is well fitted with a 2D Gaussian around the Galactic centre, with a standard deviation of 40 0 in l and in b, and a maximal value of 4 (N P W Ne /500) sources per square degree. This implies that CTA could detect nearly 200 sources in the central regions of the Galaxy, at l < 30 0 and b < 0.5 0, i.e. 3 sources per square degree, on average. Given that a large fraction of VHE-emitting, middle aged PWNe are expected to be extended (on scales of pc which is equivalent to at 6 kpc), 3 This estimate does not take into account flaring episodes, which may modify this number and/or provide information about the poorly known duty cycle of blazars. 12

13 Page: 13/54 source confusion within the Galactic Plane survey performed with CTA may become an issue. It is a sign of a maturing astronomy that confusion may affect VHE observations for the first time. Possible mitigating strategies include source identification using the highest energies, where the angular resolution of CTA should improve at the same time that PWNe and other sources are expected to be more compact due to larger particle losses. An exemplary case for this is the observation of HESS J (Aharonian et al. 2006c), where an energy dependent morphology has been established at TeV energies. The decomposition of the VHE photon statistics in energy bands clearly shows the larger extension of the nebula at lower energies. The distance from the pulsar at which the surface brightness drops to 50% of the flux at the pulsar position can be taken as a measure of the extension at different energies. This decreases from > 10 at 1 TeV energies, to 0.20 at E > 10 TeV (see e.g. Funk et al. 2007). The percentage of the survey occupied by sources will therefore decrease from 10.5% (extension of 0.10) to 2.6% if the objects are identified as point sources (i.e. extension of or less) at the highest energies, thus alleviating the confusion problem Surveys and monitoring with sub-arrays The large number of individual telescopes involved in CTA can provide flexibility in operation and the ability to pursue more than one different scientific objective in parallel by using sub-sets of the full array. Use of sub-arrays may also become useful after a first, full-cta Survey is conducted in some region, e.g. as a means to test for variability in longer timescales of the brighter sources detected. Moreover, apart from the sub-array, the remaining telescopes in the array could continue to survey the sky with an acceptable loss of sensitivity while the subset is used for a critical monitoring Possible survey data-taking in divergent mode Provided that the intricacies of background estimation when telescopes observe close but different portions of the sky are understood, an alternative strategy for a survey with CTA is to shift from a convergent to a divergent pointing of the telescopes. Considering a 25 medium-sized telescope sub-array, the angles between telescope pointing directions can be adjusted such that a patch of sky can be covered with an average of 2-3 telescopes observing a given event. This situation can be approximated by considering the sensitivity of a H.E.S.S.-like sub-array (four medium-size telescopes, with the sensitivity being essentially set by the telescope multiplicity) but with uniform exposure over a FoV. An exposure duration of 3-4 hr is required to reach about 20 mcrab over 100 GeV-100 TeV, for a source with spectral index 2.5. To cover 1/4th of the sky, about 25 pointings of the enlarged FoV are required, making a total of about 100 hr. This is 4 times less than the total time for convergent pointing with the full array. The small-sized telescope array could be used to cover the same FoV with increased telescope multiplicity, or cover a wider FoV due to the increased number and FoV of the individual telescopes. This mode sacrifices depth, with energy and angular resolution being comparable to H.E.S.S., but allows for large sky-coverage. 13

14 Page: 14/ Survey-driven minimum requirements Several CTA requirements are derived from the intention to conduct survey studies. CTA sensitivity is required to improve significantly from current instruments (about 1 order of magnitude in the central energy regime of CTA), but also to extend in energy acceptance both to low (30 GeV is preferred both for the serendipitous detection of short timescale phenomena and the resultant increase of the observable volume due to reduced absorption on the EBL, which is discussed more in depth below) and also to high energies (up to 300 TeV, to look for pevatrons). The FoV needs to span several degrees (values used for prior estimations were 24 m (diameter) telescopes with 50 FoV, 12 m telescopes with 80 FoV, and 7 m telescopes with 100 FoV). The flatness in sensitivity within the FoV should reach 50% of peak sensitivity out to 2.50 (radius) at few hundred GeV energies to allow for such deep surveys to be done in relatively short times. To combine depth (see Tables in 4 for sensitivity values) with alleviating as much as possible the issue of confusion in the crowded Galactic Center region, a minimum angular resolution of at > 1 TeV is required. Divergent modes of operation would be the ideal way of doing a larger and blind sky-coverage, out of the Galactic plane. The array would also benefit from allowing for sub-array monitoring. The Tables at the end of this document summarize these requirements. 1.2 The high-energy variability at the shortest timescales Why short timescale phenomenology? The short timescale phenomenology at HE and VHE (in timescales of less than 1 hour) has been clearly observed from sources such as AGNs, and hinted at from microquasars and other binaries. From theoretical considerations (see, e.g. Atoyan & Aharonian 1999, Gupta et al. 2006), as well as from multiwavelength studies and the limitations of the current IACTs, VHE observations opening up this time-domain astronomy will be crucial for the development of high-energy astrophysics The Fermi-LAT and higher-energy instrument responses versus CTA At the highest energy end for Fermi-LAT (the lowest for CTA), it is photon starved, and therefore the SED (E 2 dn/de) sensitivity worsens with increasing energy. The Fermi-LAT sensitivity is uneven across the sky due to the bright diffuse gamma-ray emission in its energy range from cosmic-ray interactions in our Galaxy. But since the Galactic diffuse emission has a steeper spectrum than E 2, it is therefore increasingly less dominant at the highest energies of the Fermi-LAT domain (say, beyond 10 GeV) and can be ignored thereafter. Contrary to Fermi-LAT, CTA is systematic error-dominated in the overlapping energy range. Therefore longer observations do not help in increasing the CTA sensitivity, and consequently, do not significantly shift the energy at which the Fermi-LAT and CTA sensitivity curves cross each other. Due to inherent fluctuations in the particles above the Cherenkov threshold, Fermi-LAT would outperform groundbased instruments in both angular and energy resolution below 100 GeV. Even if the differential sensitivity of Fermi-LAT and CTA is the same at a given energy, say at 50 GeV, Fermi-LAT will be able to do a better measurement of a source if it is able to detect it. But since Fermi-LAT is signal-limited at these energies, its sensitivity decreases rapidly with event duration. Thus, for short-duration HE phenomenology, CTA has 14

15 Page: 15/54 a significant advantage over Fermi-LAT which may easily translate into many more orders of magnitude in sensitivity for variable events with duration of the order of minutes. To give an example, the differential sensitivity improvement at 50 GeV between the possible CTA configurations presented in Actis et al. (2011) and Fermi-LAT reaches a factor of for a 30 min observation. This kind of improvement will open a new domain of HE and VHE phenomenology. Alternative detection techniques, either non-imaging Cherenkov detectors or particle detector arrays, like HAWC, may ultimately become more cost-effective for the very bright short transients, despite having a higher energy threshold and far inferior gamma-hadron discrimination. However, these alternative highenergy instruments can never compete with the CTA angular or energy resolution. Even if they were to detect excesses, they will have problems in identifying the sources which, given their variability, will most likely be compact. Short-timescale HE and VHE astronomy, currently terra incognita, is thus a unique reach of CTA. Which phenomena could one expect to uncover? Flares from pulsar nebulae? One of the most exciting surprises in HE gamma-ray astronomy has been the detection of flares (up to 30 times larger flux at GeV energies, with a duration from days to a couple of weeks) coming from supposedly steady sources, such as the Crab nebula (Tavani et al. 2011, Abdo et al. 2011b, Buehler et al. 2012). The rapid flux variations seen, e.g. doubling times t d within 8 hours for the April 2011 flare, allow for a strong causality-based argument: the emission region must be compact, with a length L < ct d pc. The emitted (unpulsed) isotropic power at the peak of the flare of erg s 1 corresponds to 1% of the total spin-down power of the pulsar. If these flares are related to synchrotron emission of electrons from the wind termination shock (as the extrapolation of the Fermi-LAT spectrum to low energies as well as the cooling timescale would suggest), only the highest energy electrons above 1 PeV can be responsible for this emission. The observation of peak synchrotron energy of 380 MeV is among the highest seen from an astrophysical source. The observation is surprising as particle acceleration in the presence of synchrotron cooling is expected to limit synchrotron emission to photon energies below 150 MeV (Guilbert et al. 1983; de Jager et al. 1996). Where within the nebula does the emission come from? What produces the flux variations? How were the emitting particles accelerated? How are the flares related to the milder variability observed on timescales of years and months? How does this variability show up at other frequencies? Possible solutions include a relativistic Doppler boost (e.g. Komissarov & Lyutikov 2011, Khori et al. 2012, Bednarek & Idec 2011, Yuan et al. 2011), acceleration by electric fields in the reconnection layer (e.g. Uzdensky et al. 2011), and the sudden concentration of magnetic field (e.g. Bykov et al. 2012). Observations with CTA at higher energies may be crucial disentangling the alternatives (see, e.g., Khori et al. 2012) The accretion ejection interface observed at the highest energies? Microquasars (MQs) are a subclass of X-ray binaries displaying non-thermal continuum emission in all wavebands. These sources are analogous to AGNs, which exhibit jets as a result of accretion processes. However, due to the much smaller mass of the Galactic compact objects, their associated timescales are 15

16 Page: 16/54 orders of magnitude shorter than those in extragalactic jets (e.g. Mirabel & Rodriguez 1999). Because of the relative proximity and shorter timescales found in MQs, it is possible to establish the relativistic motion of the sources of radiation, and to better study the physics of accretion flows and jet formation near the horizon of a BH. Despite this, the accretion-ejection connection, the formation of jets, their launch off the disc, the way the jets couple to the BH spin, their matter content, the details of their energy source, their interaction with the ambient environment, and the processes at the termination region remain unclear. Is there any gamma-ray emission associated with X-ray and radio state changes? What produces it? Can it be used to learn about accretion disc physics? Which are the drivers of jet launching and quenching? All these issues relate, in one way or another, to understanding the physics and characterizing the phenomenology of the short timescales in high-energy processes. Cyg X-1 and Cyg X-3 are the only MQ claimed to have flared at TeV or GeV energies (Albert et al. 2007, Abdo et al. 2010c, Tavani et al. 2009, Sabattini et al. 2011). Cyg X-3 regularly becomes the brightest radio source amongst the known binary systems, with large flares attributed to its relativistic jets. Emission had been detected up to 300 kev whilst the system shows a complex X-ray spectrum that transits between soft and hard states; the source is known to flare in radio when entering the soft state, with associated relativistic plasma ejection events (e.g. Fender et al. 2004). Fermi-LAT presented unambiguous evidence for the detection of Cyg X-3 in two flaring episodes through finding the 4.8-hour orbital period, thus linking the gamma-ray activity to periods of relativistic ejection events. Cyg X-1 is also believed to be powered by accretion and is seen to transit between soft and hard X-ray spectral states. Searching for time-varying signals resulted in the Fermi-LAT claim - at 4.1σ after correcting for trials - in 79 min of effective on-time. This TeV excess was coincident with a X-ray flare detected by INTEGRAL, Swift-BAT and RXTE-ASM. CTA should have the capability to explore even shorter timescales. In addition, the quiescent state should be measured even if it is up to 1 order of magnitude less luminous than the flaring state. 4 Opening up shorter timescales in high-energy astrophysics may also provide clues on the formation of relativistic outflows from binaries that may be formed not by BHs, but by highly magnetized pulsars such as PSR B (Aharonian et al. 2005b). Perhaps other such examples of this class are LS 5039 (Aharonian et al. 2006b) or LS I (Albert et al. 2006), presenting recurrent, anti-correlated, orbital GeV and TeV emission. Additionally, short timescale variability is hinted at already in the currently measured light curves of these objects. Short variability can additionally be expected on top of averaged periodic behavior, for instance, as a result of granularity or clumping in stellar winds, or of the appearance of random shocks in the inner wind of the pulsar, which may modify local conditions away from the average. Whereas the anti-correlation of GeV and TeV fluxes is a generic feature of detailed inverse Compton (IC) models, the specific details of these models (place of origin of the radiation, absorption and cascading processes, geometry, possible influence of discs, etc) can be disentangled only if the evolution of their luminosity and spectra is measured on short-timescales. Gamma-ray astronomy can be also used to acquire information regarding the particles energy distribution within the pulsar wind zone itself and thus probably impact on current models for dissipation in pulsar winds (see, e.g. Jaroschek et al. 2008). In the GPS runs, depending on implementation, each location could be visited at least 8 times in runs of 4 As an example, consider the claimed flare in Cyg X-1: should such a flare happen again, the minimum timescale for a 5σ detection of a flare (within 10% the reported spectrum from Cyg X-1) is in the range of 2-3 min. In order to have a spectrum determination, one can consider a 10σ detection threshold. With this constraint, the minimum timescale required should be in the range min. 16

17 Page: 17/ hr, giving a sensitivity to variability with amplitudes larger than 20 mcrab, relevant for instance for the discovery of new gamma-ray binaries. Population synthesis models for binaries show that there should be 30 binaries in our Galaxy where the pulsar is young, and thus are candidates to emit gamma-rays (see e.g. Meurs and van den Heuvel 1989 or Portegies Zwart & Verbunt 1996). Some of these could be detected as variable, point-like sources in the survey. To give an example, consider a case where the orbit-averaged flux is at the limit of detection (5σ) for 1 pointing of 30 minutes of exposure (the limiting flux in this case is similar to the average flux of LS5039) and the amplitude of variation is of order 50% of the integral flux. If the initial survey pointings are divided into 8 integrations of 30 minutes spanning a period of 4 months (so as to maintain a good zenith angle), variability of the detected object could be resolved if its orbital period is 6 months or less. This would enable pinpointing new gamma-ray binary candidates and would also come at no cost as a result of the sky surveys Short timescale variability from black holes beyond the galaxy The high states of AGN are dominated by strong burst-like flaring episodes. These are short, aperiodic variability events possibly dictated by the fast cooling times of the 10 TeV electrons thought to be responsible for the VHE emission. The detection and detailed studies of such fast transient events is important in order to put constraints on the physics of the emission process(es) and the astrophysics of the jets. CTA will be an ideal instrument for the study of timing properties of blazars. Given that most extragalactic sources have steep spectra, the low energy threshold and the increased effective area of CTA will contribute to opening up cosmological volumes to blazar studies, since the universe is essentially transparent at 30 GeV. The current generation of IACT instruments is able to detect variability on the scale of several minutes in the case of the brightest blazar flares, but CTA would be in principle capable of measuring timescales much shorter than this, testing blazar variability to its limits. For instance, the fastest and highest fluence flare ever observed in the VHE range was seen from PKS (Aharonian et al. 2007b). For this particular event, the shortest flare rise time detected by H.E.S.S. was of order 180 s, whereas CTA will be able to extend the current detection limits and probe much shorter scales down to 30 s, if such variations are indeed present in the lightcurve. Tighter upper limits on the smallest variability timescale put constraints on the size of the emission region, taking us further in the search for an absolute shortest timescale indicative of the physics driving these systems. A finer sampling of the light-curves of blazars as allowed by CTA will allow performing, for the first time, systematic frequency-domain studies of these objects. The determination of the source s power spectral density (PSD), for example, allows characterizing the stochastic mechanisms behind the observed flux variability. An important question in this study is to probe if such mechanisms have a fundamental timescale, associated to certain physical parameters of the source that are ultimately responsible for the variability. Whereas such studies (common in the X-ray domain; e.g., Marcowitz et al. 2003) are possible today for a few extreme outbursts, they might become routine with CTA. In fact, simulations of such extreme events as they would be seen by CTA (see studies quoted in 8) have shown that a significant improvement (by a factor of few), is expected in terms of resolving high-frequency variability events. 17

18 Page: 18/ Detecting GRBs? In the framework of the standard fireball model, VHE gamma-rays are expected from GRBs during the prompt to afterglow phases. While prompt VHE emission (i.e. from internal shocks) may suffer from attenuation due to pair-production before they escape the source, VHE emission from IC scattering off normal afterglow or X-ray flare photons may offer the best opportunities to be detected by IACTs. A light curve is desired to map out the GRB properties: Lorentz factors, micro-physical shock parameters and density of the surrounding medium. A possible contribution from hadronic components should not be neglected since GRBs are one of a few potential sites able to accelerate protons and heavy nuclei to UHE. Detection of distant GRBs (z > 2) would provide beacons to probe star formation history: Measuring spectral cutoffs in sources at very high redshifts would directly probe the evolving UV background at these redshifts, providing invaluable insight into the cosmic reionization epoch. The recent detections by Fermi/LAT of dozens of GRBs including GRB C at z = 4.35 (Abdo et al. 2009c) demonstrate that at least some GRBs have luminous emission extending to few tens of GeV. Simulations show that these GRBs can be observed by CTA if it features a 30 GeV threshold and 30% energy resolution below 100 GeV, with the sensitivity noted above (see 6). To maximize the chance of detection for CTA, the instrument has to react fast enough: The afterglow emission can potentially be detected for hundreds or even thousands of seconds, but unique science can be done with the prompt emission (the variability and short timescale are much better for LIV studies, the larger fluence is better for EBL studies, and of course in order to study the GRB physics related to the prompt emission it must be observed directly, rather than just the later afterglow emission). The only natural timescale in this problem is the typical duration of long GRBs, which is tens of seconds. Thus to ensure observations of the GRB prompt emission, CTA must be able to repoint within a few tens of seconds. More on this issue is commented in the Supplement. The attenuation of the gamma-rays due to the EBL depends strongly on gamma-ray energy, and whereas at a redshift 3, only 50% of the gamma-ray flux is absorbed for 30 GeV gamma-rays, there is 90% (99%) absorption for 70 GeV (100 GeV) gamma-rays (see, e.g. Dominguez et al. 2010). A low energy threshold is, therefore, essential for the chance to detect GRBs, given the high-z location of most of the bursts. Finally, a large FoV (especially at low energies, 50 or larger) would increase the chance of making a GRB discovery especially since instantaneous localization of triggers (e.g. as currently circulated in GBM circulars) may have typical uncertainties of a few to several degrees Alerts Short timescale phenomenology needs either a monitoring program (e.g., over a known particular source or during a survey) or an alert. X-ray, radio, or higher-energy instruments can provide the latter. In the supplement we discuss which instruments will be able to alert CTA. As such, short time phenomenology can be both, discovered by, and followed up by CTA. Surveying will be an intrinsic part of the CTA program of observations. Thus CTA should, in real-time (i.e. during the data acquisition) and in an automated way, be able to check on the on-going observation to detect unexpected events (e.g. flares from Galactic or extra-galactic sources). The CTA observatory should be able to change the short-term schedule (when alerts are generated by the monitoring system) in order to follow-up on a flaring source. This will enable CTA to realize an effective follow-up of transient phenomena. In addition, the monitoring program should be able to generate alerts for other instruments. 18

19 Page: 19/ Short timescale-driven minimum requirements Several requirements are derived from allowing for the shortest timescales. Among them, and apart from those commented already in the survey section above, the obvious one is on the energy range (particularly, the threshold energy) and the corresponding necessary sensitivity at short timescales. The threshold requirement is set at 30 GeV, to maximize the volume accessibility (and as well to provide good overlap for steady sources with the expected sensitivity of Fermi-LAT survey at 10 years). Additionally, to detect transient phenomenology such as GRBs in the prompt phase, and since most GRBs last < 50 s, this is the requirement on the largest repointing time, so as to allow observing during the prompt emission. Given that Fermi-LAT has detected GeV emission within the first tens of seconds, a reasonable goal of 20 s repointing results, particularly for the telescopes mostly impinging on the lowest energies. The Tables at the end of this document summarize these requirements. 2 THE CORE SCIENCE THEMES OF CTA Embedded in these unique instrumental possibilities of CTA, at least three prominent research themes can be recognized. These are connected with key observations. Some are concurrent with the survey, but some are deep observations of key individual objects of the populations detected. We discuss these and provide individual scientific requirements below. 2.1 The origin and propagation of leptonic and hadronic cosmicrays and their role in the universe Cosmic rays and supernova remnants According to the most popular scenario, galactic CRs are accelerated at SNRs via diffusive shock acceleration (for a review, see, e.g. Hillas 2005). The main argument supporting this scenario is the fact that SNRs alone would be able to maintain the CR population at the observed level, if some fraction ( 10%) of their kinetic energy were somehow converted into CRs. Moreover, diffusive shock acceleration predicts a spectral shape for the accelerated particles that, after taking into account propagation effects in the Galaxy, fits fairly well with the observed CR spectrum, and, under certain assumptions for particle injection, is roughly compatible with the observed chemical composition of CRs (see, e.g., Ptuskin et al and references therein). Despite all these facts being encouraging, conclusive proof for (or against) the validity of this scenario is still missing. Very tight connections exist between CR studies and gamma-ray astronomy, due to the fact that CR protons can undergo hadronic interactions with the interstellar medium producing neutral pions that in turn decay into gamma-rays (e.g. Stecker 1971, Dermer 1986). This is of particular relevance for the identification of CR sources because the production of gamma-rays is inevitable at some level during CR acceleration. Since the explosion energy of a supernova is a remarkably constant quantity close to erg, a rough estimate of the expected gamma-ray flux from a given SNR can be obtained if one knows the density of the ambient medium, and the SNR distance. Such estimates fall within the capabilities of currently operating Cherenkov 19

20 Page: 20/54 telescopes (Drury et al. 1994, Naito & Takahara 1994). In agreement with these estimates, several SNRs have indeed been detected at TeV energies (see, e.g. Hinton & Hofmann 2009 for a review). However, although their detection fits well with the general scenario described above, the origin of such radiation might still be leptonic and thus unrelated to the acceleration of hadronic CRs. For the best studied SNR in TeV gamma-rays, RX J (Aharonian et al. 2004), evidence has been put forward both from X-ray (Ellison et al. 2010) and GeV gamma-ray (Abdo et al. 2011) observations supporting a leptonic origin of the TeV emission. This does not mean that the SNR RX J is not accelerating hadronic CRs at the level required to explain the flux of galactic CRs, but that the leptonic contribution to the gamma-ray production is dominant over the contribution from hadrons. This may happen naturally if the ambient gas density is low. On the other hand, the gamma-ray emission detected from the historical SNR Tycho, seems to favour a hadronic origin (see Giordano et al. 2012, and Atoyan & Dermer 2012 for an alternative interpretation). Multi-wavelength studies of SNRs are needed in order to find conclusive evidence of the fact that SNRs, as a class, accelerate hadronic CRs and are quantitatively able to provide the measured flux of CRs. Central to this problem is the fact that the as yet small sample of sources detected limits population analysis, which CTA data should provide. For the sake of estimation, assume that the SNRs detected so far in TeV gamma-rays are good representatives of the whole class of SNRs. The maximum distance at which a generic SNR would be detected is the horizon of detectability. If a SNR is bright in TeV gamma-rays for?3000 yr (this is approximately the age of Vela Jr), and 2.8 supernovae are expected to explode each century in the Galaxy, the number of SNRs currently emitting TeV gamma-rays is 80. Should CTA have the features described above, e.g. for producing the GPS, essentially all of them should be detected in less than 20 hours integration time for each. Normally, TeV-bright SNRs are detected by pointing the telescope in the direction of SNRs known at other wavelengths. However, it can also happen in reverse order, as in the case of the SNR HESS J where the discovery of the radio shell followed the detection of the SNR in gamma-rays. Similar situations are expected to happen frequently for CTA, once the number of detections has increased. Of course, if the gamma-ray source clearly exhibits a shell-like structure, it can be confidently identified as a SNR, while for unresolved sources, follow-up at other wavelengths is mandatory. For this reason, it seems appropriate to define a horizon of resolvability: the maximum distance up to which the shell of a SNR can be spatially resolved and distinguished from a simple Gaussian shape. Again, should CTA have the features described above, e.g., for producing the GPS, simulations have shown that a SNR HESS J like source would be clearly resolvable up to distances of several kpc. For the first time, then, CTA can provide the community with a population of TeV-emitting SNRs from which to learn about cosmic-ray acceleration and propagation. Non-thermal X-ray filaments are observed in most of the young Galactic SNRs with measured widths between a few arcseconds in the youngest SNRs (such as Cas A, Tycho and Kepler, e.g. Parizot et al. 2006) to the arcmin scale in older SNRs (such as RX J , Vela Junior and RCW 86). The nature of these filaments is still debated (e.g. Pohl, Yan & Lazarian 2005). If a goal PSF of about 1 arcmin at 10 TeV is achieved, CTA could probe these filaments in the VHE gamma-ray domain. In the simplified (one-zone) leptonic scenario, given that the same electron population should radiate synchrotron and inverse-compton (on CMB only) photons at 1 kev and 10 TeV for a magnetic field of 15 µg, CTA should then be able to study the morphology of any putative TeV counterpart to these X-ray filaments in the TeV-bright SNRs RX J and Vela Jr (and, to a lesser extent, RCW 86). Conversely, in the hadronic scenario, these filaments are expected to behave differently with energy (see Marcowith & Casse 2010). Therefore, by constraining the width of the TeV-counterpart filaments in several energy bands compared to those measured 20

21 Page: 21/54 in X-rays, such spectro-imaging studies could shed light on the nature of the VHE emission. The supplement gives further details on these aspects Pevatrons At the highest energies, PeV particles are accelerated at the beginning of the Sedov phase ( 200 yrs), when the shock speed is high. They quickly escape as the shock slows down. The highest energy particles are released first, and particles with lower and lower energy are progressively released later, see, e.g. Ptuskin & Zirakashvili The details of CR diffusion in the interstellar medium are, however, also not fully understood (see e.g. Yan & Lazarian 2004 and references therein), but from considerations on the relevant timescales, one can conclude that a SNR is a pevatron for a very short time. We expect roughly 10 such objects to be active in the Galaxy, and as discussed above in the Survey section, CTA would be searching for them Diffusion of cosmic rays By assuming that CR diffusion proceeds isotropically, and that at a given time CRs have diffused away from the source and fill a region of radius R, it is possible to estimate the average energy density of CRs, see, e.g., Aharonian & Atoyan (2006): w CR 0.55(W CR /10 50 erg)(r/100pc) 3 ev/cm 3, where W CR is the total energy carried by CRs when released by the source. This means that in regions up to 100 pc away from SNRs (or from any other source injecting 1050 erg in form of CRs), and at some given time depending on the details of CR escape and diffusion, the CR intensity might well be above the background intensity of galactic CRs: 1 ev/cm 3. As a consequence, the gamma-ray emission from CR interactions also would be correspondingly enhanced. Thus, searching for excesses of gamma-ray emission from the regions surrounding candidate CR sources might provide an indirect way to identify and locate CR sources and learn about the properties of the medium in which they have propagated. The detection of this radiation is more likely if massive molecular clouds (MC) are located within the region filled by CRs, since they would provide a dense target for CR hadronic interactions (see, e.g. Aharonian & Atoyan 2006, Gabici et al. 2009). An investigation of the capabilities needed for CTA to detect MCs considers four different situations: i) a passive MC (i.e. with no CR accelerator in its proximity); ii) a MC with a CR accelerator inside; iii) a MC illuminated by CRs from a nearby accelerator; iv) a CR accelerator located in a region filled with dense, diffuse gas. In all four cases, should CTA have features compatible with the discussed above for the survey (see references for further studies below), significant detections are expected. These could lead to the determination of the diffusion characteristics in regions far from our Solar system Cosmic rays in galaxies CRs escaping from SNRs and from any other CR source in the Galaxy eventually mix with the background CR and sustain it against losses due to escape. Except for localized (both in time and space) excesses around CR sources, the CR intensity is expected to be, both spatially and temporally, quite homogeneous throughout the Galaxy. The interaction between CRs and the interstellar gas makes the Galactic disk a prominent source of diffuse gamma-rays at energies above 100 MeV (Abdo et al. 2009). No diffuse emission has been firmly 21

22 Page: 22/54 detected at TeV energies from the galactic disk, though some evidence for the presence of diffuse emission at 15 TeV has been presented by the MILAGRO collaboration (Abdo et al. 2008). Studying the diffuse emission is of crucial importance in order to probe large-scale spatial variations in the distribution of CRs and thus constrain the properties of their propagation in the turbulent galactic magnetic field. The detection by the Fermi satellite of gamma-rays from nearby galaxies (Abdo et al. 2010d,e,f), as well as the detection in both the GeV and TeV energy range of the starburst galaxies NGC 253 and M82 by H.E.S.S. and VERITAS (Abdo et al. 2010g, Acero et al. 2009, Acciari et al. 2009) can also be interpreted as the result of CR interactions with the interstellar gas. Similarly to what is done for our Galaxy, such gamma-ray emission can be used to infer the CR intensities in these objects. Thus, the study of this emission and the detection of more galaxies in gamma-rays will serve to investigate the possible differences in the acceleration of CRs in galaxies different from our own, and in their transport and confinement properties. To date, the scientific return beyond the physics scenarios that led to the prediction of their high-energy emission from starbursts is still limited, as the spectrum is only crudely measured. Accordingly, no observations exist that could address the question of the maximum energy, or the potential existence and shape of spectral cutoffs. CTA should provide detailed spectrum capabilities for the closer starbursts and LIRGs/ULIRGs. The lessons learned from these systems will also indirectly contribute to solving the problem of the origin of galactic CRs, and its association with star-formation processes. The low energy acceptance of CTA will also facilitate extrapolating the cosmic-ray spectrum down to low energies, relevant for ionization of molecular clouds and the ISM. The connection between astro-chemistry and cosmic-ray studies (see e.g., Shuppan et al. 2012, or Padovani et al. 2009) will benefit from CTA observations of molecular clouds in our Galaxy as well as in starbursts Clusters After leaving a galaxy, if it is a member of a cluster, CRs will remain confined for cosmological times in the magnetized ( µg) intracluster medium: the very large (Mpc) scale size of these objects makes the confinement time of CRs larger than the Hubble time (e.g., Volk et al. 1996). As a consequence, all the CRs injected by all the sources within a cluster of galaxies accumulate in the intracluster medium, and for this reason clusters of galaxies often have been considered as potential gamma-ray sources. Despite the very low ambient density ( 10 4 cm 3 ), one may still expect a copious production of gamma-rays from protonproton interactions due to the huge amount of CRs that possibly can be stored within clusters. With the anticipated sensitivity of CTA, the minimal hadronic model for the gamma-ray flux in a galaxy cluster (see Alecksic et al. 2010) will be testable. So far, the deep exposure of the Perseus cluster obtained by the MAGIC telescope (Aleksic et al. 2010) places the most stringent constraints from gamma-ray observations regarding CR energy (E CR /E th < 3-5%), CR- to-thermal pressure (< 8% for the entire cluster and < 4% for the core), maximum shock acceleration efficiency, and value of the magnetic field in a galaxy cluster. Since the upper limits are only a factor of 2 larger than the model prediction for the CR-induced gamma-ray emission, the discovery potential for finally detecting a galaxy cluster in gamma-rays is clear. A non-detection at CTAsensitivity level would imply CR fluxes that are far too small to produce a sufficient number of electrons through hadronic interactions with the ambient matter to explain the observed synchrotron emission - thus raising yet another intriguing science aspect for studies of galaxy clusters. 22

23 Page: 23/ Pulsar wind nebulae On the other hand, pulsar wind nebulae (PWNe) currently constitute the biggest class of identified Galactic VHE gamma-ray sources, with the number detected increasing from a few in the early 90s to more than two dozens of sources nowadays. The discovery of this large population of TeV PWNe has provided a large input for multi-wavelength models of particle evolution and acceleration in these objects. Theoretical MHD models describe well the observed emission by a relativistic wind of particles driven by a central pulsar into the ambient medium creating a termination shock. The particles are believed to be accelerated somewhere between the light-cylinder and the termination shock, although this question is still open. The broadband spectrum of a PWNe thus provides constraints on the integrated energy injected by the pulsar over its lifetime as well as on the effects of adiabatic expansion and the evolution of the magnetic field. Combined observations at VHE, X-rays, and radio wavelengths are then crucial to constrain the evolution of the nebula magnetic field as well as the magnetic-to-kinetic energy conversion. Observations at VHE have revealed PWNe as a very efficient Galactic source of gamma-rays, even allowing the detection of such systems outside our own Galaxy (Komin et al. 2011). Observations with CTA will allow a homogeneous sampling of the PWNe of the Galaxy. In fact, under the same assumptions as in the prior studies for SNRs, a large fraction ( ) of G21.5-, HESS J1356- and Kes75-like nebulae should be detectable by CTA. If the estimated lifetime of TeV-emitting leptons in such nebulae is 40 kyr (for B = 3µG, similar to what has been found in several PWNe such as Vela X), between 300 and 600 PWNe will be detected. Such an unprecedented number of TeV objects of the same class will enable detailed population studies. The majority of PWNe at VHE gamma-rays have a very large size, up to 1.2 deg depending on their evolutionary stage and proximity (see i.e. Vela X or HESS J ). Thus, a minimum of radial, flat response is again required for CTA in order to sample all the possible sizes of the PWNe detected up to now. One of the key factors for the exponential increase of the PWNe number in the last years has been the large field of view of the third generation of Cherenkov Telescopes, which has allowed the detection of the large PWNe gamma-ray halo produced by cooled electrons, and which enabled efficient surveys of the Galactic plane to be conducted to reveal formerly unknown plerions. The large energy range covered by CTA will be crucial to understand cooling effects in the electron parent populations, including the resolution of internal structures and the disentanglement between synchrotron or adiabatic losses. One of the most intriguing aspect of the new observations is the interaction between the host SNR and its PWNe, and raises the question how much of the gamma-ray emission is originated by the former. A good angular resolution (see 8 and below) is required to understand those composite systems, which are often unresolved when observed with the present instruments (e.g., Kes 75 or G ). It is expected that the CTA observations will both constrain and provide input for MHD simulations of PWNe, in particular estimates of magnetic field strength, electron population generating the gamma-ray (and synchrotron) emission, spectral characteristics, etc Pulsars Regarding the pulsars themselves, one of the key issues is the question of how far the non-thermal spectra of pulsed radiation from known gamma-ray pulsars actually extends to high energies and what are the shapes of their cutoffs and tails, if any. Pulsed VHE emission from the Crab pulsar is of particular interest for pulsar 23

24 Page: 24/54 physics. In the context of outer gap models its properties depend strongly on the actual location of the outer boundary of the outer gap with respect to the light cylinder. The discovery of pulsed VHE emission shows that the outer gap models need to undergo major modifications if the gaps are indeed to be closer to the light cylinder. More accurate treatment of the electrodynamics, including currents, and possibly going beyond the light cylinder, will be driven by CTA observations, since high-quality spectral, phase-resolved properties of the VHE radiation from Crab are essential to help develop realistic models of the magnetospheric gaps. Additionally with the adopted CTA sensitivity, the combined effects of several dozen millisecond pulsars in globular clusters is expected to be detectable. These observations would allow studies of the detectability criteria (number of MSPs vs. ambient magnetic field strength) and perhaps even be able to resolve different spectral contributions Ultra high energy cosmic rays The astrophysical objects described above are possible accelerators of cosmic rays up to maximum energies of 1015 ev. However, cosmic rays are detected at the ground level up to ultra high energies ev. The origin of these UHECRs is one of the most important open questions in astrophysics. The Pierre Auger Observatory (PAO), built to study these particles, has recently reported several new results: a suppression feature in the UHECR flux at energies ev (Abraham et al. 2010); tentative evidence for anisotropy in the arriving UHECR flux (Abraham et al. 2007, Abreu et al. 2010); and air shower measurements indicating that the UHECR flux composition becomes heavier at energies above ev (Abraham et al. 2010b). A favoured scenario for the origin of the UHECR is that they are produced by AGN. However, one cannot identify AGN (or other classes of objects) as the actual sources of UHECRs since these trace the distribution of matter in the local Universe, such that a multitude of potential acceleration sites sit in the same angular region of the sky. Collectively, the limited angular resolution of Auger ( 1 0 ), the expected deflections in galactic and extragalactic fields ( Z 2 0 where Z is the CR charge), and the very low flux rates, all imply that making a solid identification using UHECRs alone would be very difficult for current instruments. CTA can in fact contribute in the search for the origin of UHECRs. In particular, if photons from a single nearby (within 30 Mpc) source are responsible for several of the UHECR events detected, the UHE photon signal could be detected by Auger, while a cascade gamma-ray signature at lower energies could be detected by CTA (Taylor et al. 2009). Accompanying the flux of UHE photons arriving at the Galaxy, a secondary flux of UHE electrons is also generated. When these enter the magnetized halo of the Milky Way, they would lose their energy via synchrotron emission, which, depending on the initial luminosity of the UHECR source, its distance and the value of the intergalactic magnetic field (IGMF), is well within CTA detection capabilities. One interesting aspect for such a scenario is that the angular extension of this emission is likely to be dominated by the angular spread of the arriving UHE electrons, expected to be 0.50 for 0.1 ng strength magnetic fields in the intergalactic medium. Another possibility is to use CTA to detect the accelerators of UHECRs by observing the gamma-ray emission associated to the particles produced in the cascades induced by the interaction of UHECRs. Two main scenarios have been proposed: if the accelerator is embedded in a region with a relatively strong magnetic field, the electrons produced in the cascade initiated by the photo-pion interaction of a proton close to the source leads to synchrotron emission in the gamma-ray band (Gabici & Aharonian 2005; Kotera et al. 2011). 24

25 Page: 25/54 On the other hand, if the magnetic field in the intergalactic medium is weak, gamma-rays are produced as a result of the cascade initiated in the photo-pion and Bethe-Heitler processes with cosmic microwave background photons, and the gamma-rays point back to the source (Ferrigno et al. 2005). In the first scenario, electrons will emit high energy synchrotron radiation close to the source and the resulting emission is expected to be point-like, steady, and detectable in the GeV-TeV energy range if the magnetic field is at the ng level. Due to extragalactic absorption, a low energy threshold is key to detect this emission (50 GeV would allow the detection of accelerators able to promote particle s energies up to ev, at 1 Gpc, with an intermediate magnetic field of 1 ng). Alternatively, for the second scenario, a steady cascade gamma-ray flux will be produced below TeV energies. Whether the emission is point-like or more diffuse around the arrival direction of UHECR protons depends critically on the strength of the intergalactic magnetic field. For the present lower bounds in the fg range (Neronov et al 2010), and for sources at 100 Mpc distance which can plausibly be sources of the UHECRs observed at Earth, it is possible for the emission to still look point-like Key elements for this theme Design Concepts features foreseen for CTA (in particular, a minimum sensitivity a factor of almost 10 larger than current instruments at 1 TeV) would allow increasing dramatically the number of known TeV sources, including PWNe, SNRs, MCs and SNR/MC associations. This will make possible to perform for the first time population studies at TeV energies. Both PWNe and SNRs/MCs are extended objects (as an example, Vela Junior and the MC complex in the galactic ridge have an extension of a couple of degrees) and thus a large (camera) field of view (several degrees) is needed to study and image them. A large field of view is also needed in the context of MCs illuminated by CRs coming from nearby SNRs. In this case, the detection of one MC (or possibly more), and of the SNR within the same field of view, will enable estimations of the CR diffusion coefficient. A very large FoV might in principle allow us to observe OB associations or superbubbles, which are very extended objects which have also been proposed as sources of galactic CRs With a low energy threshold (of tens of GeV) we will have, for the first time, a partial overlap between the energy domains of ground and space based gamma-ray observations. Since spectra of known gamma-ray sources are typically steep, moving the energy threshold to low energies will thus increase the number of detectable sources. For SNRs in particular this is true since the cutoff in the gamma-ray spectrum is expected to move downward in energy with time. Thus, a low energy threshold will allow detection of older SNRs. A large energy acceptance for the detected spectrum would constitute the key to achieving an unambiguous proof for the presence of CR protons in the source with PeV energies. At energies > 50 TeV, the signal dominates clearly above background. Therefore an increase in observation time can be matched by the same increase in effective area. An improvement in effective area translates into an improvement in sensitivity. At the very least a flat response of the camera up to should be achieved. On the other hand, a good angular resolution is needed to localize the sites of gamma-ray production as sub-structures in the total emission. Given the smaller size and complexity of, e.g. the nebulae detected in X-rays, an 25

26 Page: 26/54 angular resolution of a few arcmin and a systematic pointing error of 5 per axis both at CTAs central energy range would be needed to allow detailed multiwavelength comparisons. More quantitative details on the requirements of a few essential cases for this theme are given in the appendices of this document. 2.2 The nature and variety of black holes, and their use as a probe of the star-formation history of the universe Active Galactic Nuclei AGN produce powerful outflows that offer excellent conditions for efficient particle acceleration in internal and external shocks, turbulence, and magnetic reconnection events. The jets, as well as particle accelerating regions close to the SMBH at the intersection of plasma inflows and outflows, can produce readily detectable VHE gamma-ray emission. As of now, more than 45 AGN including 41 blazars and 4 radiogalaxies have been detected by the present IACTs, which represent more than one third of the cosmic sources detected. Under the requirements stated for the survey, CTA will boost the sample of AGN detected in the VHE range by about one order of magnitude, shedding new light on AGN population studies, and AGN classification and unification schemes. CTA will be a unique tool to scrutinize the high-energy tail of accelerated particles in SMBH environments, to probe the central engines and their associated relativistic jets, and to study the particle acceleration and emission mechanisms, particularly exploring the missing link between accretion physics, SMBH magnetospheres and jet formation. Monitoring of distant AGN will be a rewarding observing program, which will inform us about the inner workings and evolution of AGN. One of CTAs aims is thus to provide a measurement of a large sample of AGN (both in flaring and quiescent state), covering various AGN types and redshift ranges for studies on classification, unification scheme(s), evolution, and populations. Individual detailed studies will derive constraints, from the VHE spectra, on particle acceleration and emission models. For this, a blind imaging survey is the most unbiased way to search for populations, and has been discussed above. The capability of CTA for the exploration of the shortest timescales will provide insights into the jet and BH physics (Fermi mechanism(s) in turbulent jets and shocks, alternative accelerations mechanisms, such as BH magnetospheres) and the geometry and location of the emitting zone Radiogalaxies and Seyfert galaxies CTA will also have the capability of studying extragalactic objects other than blazars, and for which the HE and VHE emission are also related to their central BHs. The four radiogalaxies detected so far at VHE, namely M 87 (Aharonian et al. 2006d; 2003), Cen A (Aharonian et al. 2009b), IC 310 (Aleksic et al. 2010b, Neronov et al. 2010), and NGC 1275 (Aleksic et al. 2012), have all been tentatively classified in the literature as Fanaroff-Riley type I radio sources, even though each source has some peculiarities. The detected TeV fluxes do not exhibit any clear correlation with the non-thermal radio and X-ray fluxes. The four sources are located in rich environments, and show signs of galaxy interaction or mergers. Both M 87 and NGC

27 Page: 27/54 are prominent cluster galaxies with a very massive central black hole. Cen A is presumably a recent merger, in a group of galaxies. IC 310 is located in the Perseus cluster, with radio jets in strong interaction with the intracluster gas. They have intermediate viewing angles (about 200 for M 87, 400 for Cen A, and between 200 and 500 for NGC 1275), no direct alignment between their radio compact VLBI core and their extended radio structures (grossly misaligned by 70 0 in M 87 and 45 0 in Cen A). This is possibly enhanced by projection effects and inhomogeneous external medium, or related to specific properties of their central engines. Three different main emitting zones have been considered for M 87: (i) the peculiar knot HST-1 located at about 65 pc from the nucleus, (ii) the inner VLBI jet, and (iii) the central core itself. No conclusive identification of the emission region has come out from multiwavelength correlations. Results favour scenarios where most of the VHE emission comes from the inner VLBI jet (multi-zone models inspired from standard blazar scenarios; e.g., Lenain et al. 2008, Tavechhio et al. 2008, Giannos et al. 2010) or from the central core (particle acceleration in the black hole magnetosphere; e.g., Neronov & Aharonian 2007, Rieger & Aharonian 2008, Levinson & Rieger 2011). However the situation still remains unclear and suggests the possibility of different types of VHE flares (Abramowski et al. 2012). An accuracy of 5-6 in source localization will allow ruling out (or detecting) all bright and relatively compact knots along the kpc jets from CTA data alone. There is growing evidence that relativistic jets are seen not only in blazars and radio galaxies but in several types of Seyfert galaxies as well. About 5% of narrow-line Seyfert 1 (NLS1) galaxies are radio-loud (RL) (Komossa et al. 2006), and show flat spectra together with variability in the radio band, suggesting the presence of relativistic jets. This hypothesis has recently been confirmed by the detection of a small number of RL-NLS1s with Fermi-LAT (Abdo et al. 2009b). The measured GeV spectra are typically steep, with Γ = , which makes the detection of RL-NLS1s with CTA challenging. However, at least two RL-NLS1s (PMN J and SBS ) have shown significant variability (Foschini et al. 2011, Donato & Perkins 2011), with gamma-ray luminosities that would make them detectable with CTA during outbursts. Gamma-ray emitting RL-NLS1s have inferred SMBH masses typically 12 orders of magnitude smaller than blazars, while their accretion rate reaches extreme values, up to 80% of the Eddington rate, which have never been found in gamma-ray loud AGNs but are usual for NLS1s (e.g. Foschini et al. 2011b). Detecting RL-NLS1s with CTA would significantly increase the range of parameters of the accretion-ejection process explored at VHE Star formation history The star formation history is one of the fundamental quantities of cosmology and is closely linked to structure and galaxy formation. It is mainly studied by combining the specific star formation rate of individual galaxies as inferred from tracers like, e.g., the ultraviolet or infrared emissivity, with galaxy number counts. Up to a redshift of 1 to 2 the star formation rate is reasonably well measured (spread of 20-50%). At higher redshifts, data are rare and mostly lower limits are provided. The history of the galaxy and star formation is also imprinted in the amount and spectral energy distribution of the light emitted by the stars in galaxies over their lifetime. Part of the starlight is absorbed by dust within the galaxies and reemitted at higher wavelengths. The resulting diffuse radiation field in the ultraviolet to far infrared wavelength regimes, commonly referred to as the Extragalactic Background Light (EBL), is the second largest background, in terms of its energy content, after the CMB at 2.73 K (Hauser & Dwek 2001). The EBL evolves with redshift reflecting the evolution of the stars and galaxies. The integrated EBL density depends not only on the 27

28 Page: 28/54 number and properties of stars and dust in galaxies but also on the cosmological model of the universe. The direct measurement of the EBL is complicated and has large uncertainties due to strong foreground emission in our solar system, especially due to uncertainties in the modeling of the zodiacal light. However, indirect EBL measurements are possible using the VHE emission of distant AGNs. The idea behind this is that VHE gamma-rays are absorbed via interaction with low energy photons of the EBL if the photon energies involved are above the threshold for electron-positron pair creation. The VHE gamma-ray absorption, which is energy dependent and increases strongly with the redshift of the AGN, results in a clear imprint of the EBL on the measured VHE spectra. A fundamental difficulty of the method is that one has to distinguish between intrinsic effects in the AGN and the absorption effect produced by the EBL. This requires some assumptions about the intrinsic spectra, which can, e.g., be derived from nearby sources or unattenuated parts of the spectrum. In addition, EBL attenuation should be a cosmological phenomenon affecting the spectra of all sources in a consistent way. In particular, sources at the same distance should show the same attenuation imprint from the EBL. A large sample of AGN will allow this to be tested. To accurately model the intrinsic parameters of distant sources one requires a simultaneous measurement of the EBL attenuation (at high energies), together with the unattenuated intrinsic spectrum (at the lowest energies). The division in attenuated and unattenuated energy ranges depends on the redshift of the source. While for a source at z = 0.1 the unattenuated part of the spectrum (defined as say less than 30% absorption by the EBL) can be measured for energies E < 400 GeV, this range shifts to energies E < 160 GeV for z = 0.4 and to E < 70 GeV for z = 1.0. Thus, in order to measure an unattenuated part of the spectrum (with a minimum lever arm of half a decade in energy) for sources at a redshift of at least z = 1.0 (which corresponds to about 50% of the universe), an energy threshold of 30 GeV or lower is required. An increase of the CTA energy threshold by factor of 2 (3) to 60 GeV (90 GeV) would reduce the accessible redshift range of the sources for such studies to z < 0.6 (0.4), effectively decreasing the available volume by a factor of about 5 (10). Note that most of the extragalactic sources used for the EBL studies are variable in flux, and, therefore, archival Fermi-LAT data at lower energies cannot be used for the purpose of accurate determining the unattenuated part of the spectrum (note that Fermi will not be available for most of the lifetime of CTA) Intergalactic magnetic field High-energy gamma-rays from AGN offer a unique potential to probe weak intergalactic magnetic fields (IGMF) too. A very powerful probe of a weak IGMF is offered by the secondary GeV-TeV components accompanying the primary TeV emission of blazars, which result from IC emission by e+ e? pairs produced via intergalactic gamma-gamma interactions between the primary TeV and EBL photons. Depending on the IGMF value, such secondary components may be observable either as pair echos, which arrive with a time delay relative to the primary emission (e.g., Plaga 1995), or as a pair halo, an extended emission with a spatial extension around the primary source (e.g., Aharonian et al. 1994). The properties of the extended emission depend on the IGMF strength. Strong enough IGMF (> G) leads to full isotropization of the cascade emission and formation of a physical pair halo, while weaker magnetic field leads to appearance of an extended emission with an IGMF-dependent size. If the IGMF strength is in the range, B 10 16?10 12 G, the spatially-extended emission may be detectable and resolvable by CTA by virtue of its high sensitivity and angular resolution; e.g., for a source at a distance of 120 Mpc, the size of the extended emission would range between 2.50 to 0.50, and thus fit into the FoV / flatness sensitivity of CTA. Detailed estimates of 28

29 Page: 29/54 CTA sensitivity to pair halos have been done and have shown that under the requirements stated for the survey (and even when halo fluxes are below a couple of percent compared to the primary 100 GeV emission), the distinction between a point-like and a point-like + extended source is possible Key elements for this theme A minimum sensitivity of a factor of 10 larger than current instruments at 1 TeV would allow a significant enhancement of the AGN population. Good astrometry and angular resolution are essential to spatially identify emission regions in nearby radiogalaxies. A minimum accuracy on the absolute position of 7 arcsec (rms space angle) is required (5 arcsec would be a goal). Flexibility in operating the CTA arrays is very important to follow up AGN flares. Repositioning time to alerts should be of the order of a few minutes for the full array (see above for repositioning requirements in the case of GRBs). Also, to allow for self-triggers using sub-arrays, online analysis tools on near-real time, as in the case of current IACTs, is required. Access to a sample large enough for reliable statistical studies of the various AGN types requires coverage of both hemispheres. An energy threshold around 30 GeV is required to simultaneously measure the unabsorbed and the absorbed part of the AGN spectra, and to detect high redshift (z > 1) sources. Angular resolution is not essential from the point of view of possible confusion given that resolution of 5 arcmin, i.e. similar to the current generation of IACTs, is adequate for avoiding this between extragalactic objects. However, it is welcome for studies of nearby galaxies. More quantitative details on the requirements of a few essential cases for this theme are given in the appendices of this document. 2.3 The nature of matter and physics beyond the Standard Model A major open question for modern physics is the nature of dark matter (DM). At scales from kpc to Mpc, there are numerous lines of evidence for the presence of an unknown form of mass that cannot be accounted for by the Standard Model (SM) of particle physics. The observation of the acoustic oscillations imprinted into the cosmic microwave background measured by the WMAP satellite quantifies this dark component as contributing to about 25% of the total Universe energy budget. Being over dominant with respect to the baryonic component which accounts for only about 4% of the total energy density, DM shaped the formation of cosmic structures. By comparing the galaxy distribution in large galaxy redshift surveys, and through N-body simulations of structure formation, it emerges that the particles constituting the DM had to be moving non-relativistically at freeze-out to reproduce the observed structure of the Universe, hence the term cold DM (CDM). The observational evidence has led to the establishment of a concordance cosmological 29

30 Page: 30/54 model, dubbed ΛCDM. Despite the fact that a cosmological model has been built around the DM paradigm, we still have no idea of the nature of the DM. One of the most popular scenarios for CDM is that of weakly interacting massive particles (WIMPs), that includes a large class of non-baryonic candidates with a mass typically between few tens of GeV and few TeV and an annihilation cross section of the order of the weak interaction. Natural WIMP candidates are found, e.g., in supersymmetric (SUSY) extensions of the SM. The success of the WIMP scenario relies on the theoretical observation that weak scale masses (GeV TeV) and standard coupling yield an annihilation cross-section that generically fixes the CDM density to be close to the currently observed value fixed by WMAP. This cross-section ( cm 3 s 1 ) is therefore a baseline benchmark that CTA has the potential to reach through searches for the annihilation of dark matter particles, as has been attempted already by all operating IACTs. Obtaining convincing evidence for dark matter from excesses in the measured energy spectrum of gamma-rays needs to deal both with the unknown micro-physical parameters such as the mass and the cross section of the dark matter particle, and with the uncertain astrophysical backgrounds. There is significant effort, especially at the LHC in attempting to create DM directly in the laboratory. Direct detection experiments that measure the recoil energy of the nuclei of a well-shielded detector when hit by a DM particle are beginning to report events in their signal region (e.g. XENON100, Aprile et al. 2011, CDMS-II, Ahmed et al. 2011, and EDELWEISS-II, Armengaud et al. 2011) although these are all consistent with being residual background. Moreover, controversial evidence has been presented of a signal coming from a DM candidate with low mass (originally presented by DAMA/LIBRA (Bernabei et al. 2010) collaboration but, more recently, also by the CoGeNT (Allseth et al. 2011) and CRESST-II experiments (Angloher et al. 2012). Concerning indirect detection, CTA will have a better chance of DM detection compared to the current generation of IACTs: its extended energy range will allow searches for WIMPs with lower mass; the improved sensitivity in the entire energy range will improve the probability of detection or identification of DM through the observation of spectral features, if any; the increased FOV with a homogeneous sensitivity as well as the improved angular resolution will allow for more efficient searches for extended sources and spatial anisotropies, and finally, the improved energy resolution will increase the chances of detecting a possible spectral feature in the a DM-induced photon spectrum. By observing the region around the Galactic Center, and by adopting dedicated observational strategies (see Doro et al. 2012), CTA will reach the canonical velocity-averaged annihilation cross-section of cm 3 s 1 in only 100 h observation DM mass above 300 GeV. This will be the first time for ground-based IACTs to reach this sensitivity level. Together with the constraints from Fermi-LAT for DM lighter than a few hundred GeV, this can seriously constrain the WIMP paradigm for CDM in case of no detection. Models with a large photon yield from DM annihilation will be constrained to even smaller cross-sections. In conclusion, the WIMP scenario, either through detection or non-detection will be significantly affected by the first years of operation of CTA. If signatures of DM appear in direct-detection experiments or at the LHC, gamma-ray observations will provide a complementary approach to identify DM thanks to the universality of cosmic DM spectra, and the typical cutoff of the DM spectra will allow for a precise mass determination. If such experiments do not detect DM, as may be the case for sufficiently heavy DM candidates, CTA may be the only way to look for such particles. CTA will also be an excellent experiment for other fundamental physics searches, and especially for possible Quantum Gravity (QG) induced Lorentz Invariance Violation (LIV) and Axion-like particle (ALP) searches. 30

31 Page: 31/54 It has been suggested that QG effects may induce time delays between photons with different energies travelling over large distances due to a non-trivial refractive index of the vacuum. The observation of very distant, strong flaring blazars will provide the strongest constraints of LIV compared to the current generation of IACTs, and at no additional cost, since AGN observation will be routine astrophysical targets for CTA. On the other hand, axions, which are a proposed solution to the strong-cp problem of QCD (or ALPs in general), are also valid candidates to constitute a part or all of CDM. They are expected to convert into photons (and vice versa) in the presence of magnetic fields. In the case of a very distant AGN, the ALP/photon can cause either attenuation or enhancement of the photon flux (in competition with the EBL absorption), depending on the ALP mass Key elements for this theme Given the underlying uncertainties in both the particle physics and astrophysics of cosmic DM searches, general statements in terms of minimum requirements for detection cannot be made. However, exclusion of the canonical WIMP model with averaged annihilation cross-section of cm 3 s 1 should be possible with CTA with 100 h observation of the GC vicinity for WIMP masses above 300 GeV. For LIV studies, the important parameters are sensitivity and energy resolution (less so the latter, since there exist already algorithms that can bypass much of the problems with energy-resolution in time-lag determinations Barres de Almeida & Daniel, 2012). With improved values for the system parameters of CTA, light curves of AGN flares can be reconstructed more precisely if the energy resolution is at least 20% at medium-low energies ( 200 GeV) and the sensitivity at the highest energies increases the photon statistics sufficiently to enable construction of separate light curves for each energy range. For observations of Mrk 421 (z = 0.03) with a sensitivity of erg cm 2 s 1 for E 10 TeV photons (30 min at 10 TeV), first-order Planck scale effects ( E 2 /M P l ) would be expected to induce a time delay of 1 s/tev. This means that CTA needs to resolve flare features of 30 s duration, which is well within its reach as discussed above. Higher redshift sources will benefit from the lower energy threshold. 3 A LOOK AT THE MAIN PHYSICS CASES PER ENERGY DOMAIN The energy spectra measured by current TeV instruments are often power-laws. With no characteristic energy scale it can be hard to disentangle the effects of different processes (including acceleration, cooling and escape) and identify the emission mechanism and the nature of the parent particles. The measurement of spectra over a wide energy range, and the identification of spectral curvature, cut-offs and/or the emergence of new spectral components, will be key to the success of CTA. In this Section we briefly comment on the science of CTA in different energy ranges, and refer back to the main document for details. 31

32 Page: 32/ The low-energy end of CTA (< 100 GeV) The Universe goes from being optically thin to gamma-rays of few tens of GeV to having a gamma-ray horizon at a redshift < 1 at TeV energies (e.g., Gould & Schreder 1966, Fazio & Stecker 1970). This limits the distance at which objects (such as AGNs, and GRBs) can be detected in the central and high-energy range of CTA, whereas it opens up the observable volume at lower energies. At the same time, absorption on the way to Earth is a probe of the intermediate background light level, ultimately reflecting the star formation history of the universe. In the absence of absorption by the EBL in the low-energy band of CTA, we may expect a dramatic increase of the number of detectable blazars up to redshift 1 and beyond. Having a low threshold would allow separation of intrinsic from propagation effects, which is essential to measure the EBL level. It is also essential to observe GRBs, most of which are beyond the local Universe. This energy regime is also important for galactic sources. For instance, spectral measurements may provide relevant information as to whether the TeV emission from shell-type SNRs and plerions has an inverse- Compton or pion decay origin. Inverse-Compton origin of the radiation from compact binaries may also be unveiled by utilizing a lower threshold, since at GeV energies they are optically thin, and higher energy photons suffer intrinsic local absorption. Detailed studies in this regime could help disentangling, for instance, the level of cascading involved in these sources. Several sources discovered by Fermi-LAT (e.g., Eta Carina) present additional components in this regime, which are yet to be understood; and of course, serendipitous discoveries may be made in this relatively unexplored energy range. The low-energy regime of CTA holds the key to uncovering for the first time completely new phenomenology opening up the time domain astronomy in timescales shorter than half an hour, with sensitivity tens of thousands of times better than achieved by previous experiments. Both extragalactic (AGN flare lightcurves mapped with timescales of seconds) and the Galactic realm will benefit from the ability to explore such short events in this energy domain. In the latter case, the accretion ejection interface will be tracked at high-energies, providing precious information about the constitution and dynamics of relativistic jets from microquasars, and their relation to lower energy state transitions. 3.2 The mid-energy range of CTA (100 GeV 10 TeV) This is the central and most natural regime for IACTs and addresses most of the core science themes. In this energy domain, the good reconstruction of the arrival direction of primary photons, the high statistics, and the efficiency of gamma-hadron separation, allow deep sensitivity and arcmin angular resolution to be reached. The flux sensitivity of erg cm 2 s 1 aimed for translates into source luminosities of (d/10kpc) 2 erg s 1 for Galactic sources and (d/100mpc) 2 erg s 1 for extragalactic objects. This sensitivity should allow for detection of all CR accelerators, including SNRs (and the interaction of cosmic-ray particles accelerated by them with nearby MCs), PWNe, clusters, as well as AGNs, radio galaxies, etc. and is expected to lead to the detection of over 1000 sources. Thus CTA, in its central energy regime, will allow tackling a plethora of topics related to CRs, their acceleration, propagation, diffusion, convection, and relation with star-forming processes. The large FoV will allow doing deep (up to the level of the weakest sources currently detected by IACTs in 100 hours of observation) GPSs relatively fast, and exploring the possible diffuse emission of the Galactic disc, as well as the origin of 32

33 Page: 33/54 the extended features in the GC. Indeed, data from the Fermi-LAT revealed two large gamma-ray bubbles extending 50 above and below the Galactic center with a width of about 40 in longitude (Su et al. 2010). These bubbles are spatially correlated with the hard-spectrum microwave excess known as the WMAP haze; their edges also line up with features in the ROSAT X-ray maps at kev. The origin of these bubbles is far from clear (star-formation? black hole activity?), and their detailed study will likely illuminate our understanding of recent energetic events in the GC. This is an example of the new studies that the ability to cover a large FoV, a relatively large flat region within it (where the sensitivity is close to that at the camera center), and a deep sensitivity, will allow in this energy regime. 3.3 The high-energy end of CTA (> 10 TeV) Above 10 TeV our knowledge of the gamma-ray sky is very limited. At these energies, current instruments typically run out of photon statistics for all but the brightest objects. At ultra-high photon energies (approaching 1 PeV) gamma-gamma absorption on the CMB limits the horizon to within our own galaxy. An unopened electromagnetic window therefore still exists in the range of a few tens to a few hundreds of TeV. The TeV emission of many objects has been attributed to inverse Compton (IC) emission from accelerated electrons. In this scenario, a high energy steepening of the spectrum is expected due to the Klein-Nishina effect, with almost no IC emission expected beyond 100 TeV due to the scarcity of photons with energies lower than those of the CMB. Then, the highest energy domain of CTA will provide the opportunity to measure the emergence of any pion-decay component and thus unambiguously identify hadronic cosmic ray (CR) accelerators. The peak energy output from pion decay occurs at gamma-ray energies a factor of 10 lower than the parent proton energy. Galactic pevatrons should therefore produce hard spectrum powerlaw gamma-ray emission with gradual cut-offs in the TeV range. CTA, with sensitivity up to 300 TeV, will be able to measure such cut-offs and identify these acceleration sites. High energies also provide access to electrons with short cooling times, which are confined to a volume close to their acceleration sites, and the angular resolution necessary to resolve structures on the critical scale of the CR streaming instabilities, which are seen in magnetohydrodynamic/particle-in-cell simulations. The ability to separate particles and magnetic fields on such scales may lead to a breakdown in the correlation of X-ray, synchrotron and gamma-ray IC radiation and provide insights into the processes of CR acceleration and CR driven magnetic field amplification. Measurements at > 10 TeV are also critical for objects where the gamma-ray emission is probably dominated by IC scattering, such as in PWNe, where electrons can be accelerated to close to PeV energies at ultrarelativistic shocks. But X-ray telescopes measuring emission at a few kev probe electrons with energies >100 TeV, whereas current TeV IC measurements probe electrons of significantly lower energies (which produce undetectable optical-uv emission). At 30 TeV, the direct IC counterparts to the known X-ray PWN will be found, and the arcminute resolution achievable at these energies will be sufficient to resolve most of these objects. Outside the Milky Way, nearby active galaxies are the prime targets. M87 in particular, with its short timescale variability and hard spectrum, presents an excellent opportunity to study particle acceleration driven by accreting SMBHs. Gamma-gamma absorption on the EBL is expected to suppress the detected gamma-ray emission from M87 above 40 TeV, but the TeV window provides an opportunity 33

34 Page: 34/54 to constrain the emission mechanism and site and gives access to the particles with the shortest cooling timescales. In addition, the absorption signature provides the opportunity to measure the energy density of the far infrared EBL. Variability studies of (relatively) nearby AGN with huge collection area will also provide a sensitive probe of possible violations of Lorentz Invariance through time of flight studies at the highest possible photon energies. 4 CTA: NOT ONLY A GAMMA-RAY EXPERIMENT 4.1 CTA as a cosmic-ray experiment CTA can provide measurements of the spectra of cosmic-ray electrons and nuclei in the otherwise inaccessible energy range between balloon/satellite experiments and ground-based air shower arrays. Bridging spaceborne and ground-based measurements will provide crucial information regarding spectra, cosmic-ray sources and air-shower physics. The potential of CTA comprises the measurement of the high-energy end of the CR electron spectrum. TeV electrons have very short lifetimes and thus propagation distances due to their rapid energy losses. Local CR electron accelerators must therefore dominate the upper end of the electron spectrum (which is not accessible by current balloon and satellite experiments). Thus, a measurement of the electron spectrum can provide valuable information about characteristics of the contributing sources and of CR electron propagation. CTA will be able to measure CR electrons up to 30 TeV and higher, while at the same time providing a comfortable overlap with balloon and satellite experiments at lower energies. Additionally, CTA has some capability to perform nuclear CR composition measurements. The composition of CR has so far been measured by balloon- and space-borne instruments (e.g. TRACER) up to 100 TeV. Starting at about 1 PeV instruments can detect air showers at ground level (e.g. KASCADE). Such groundbased air shower experiments have, however, difficulties in identifying individual nuclei, and consequently their composition results are of lower resolution than direct measurements. Cherenkov telescopes are the most promising candidates to close the experimental gap between the TeV and PeV domains, and will probably achieve better mass resolution than ground-based particle arrays. Furthermore, with its overlap with direct measurements, CTA can test hadronic interaction models and their performance in the atmosphere and thus provide crucial input to improve ground-based cosmic-ray measurements beyond PeV energies. While such measurements of charged cosmic rays with CTA involve analyses that differ from the conventional gamma-ray studies, proof-of-principle measurements have already been performed with the H.E.S.S. telescopes (Aharonian et al. 2007, 2008b, 2009). Monte Carlo studies reveal the potential of CTA to widen the energy range of the electron measurement significantly and demonstrate the capability to observe spectral features caused by close electron accelerators Key elements related to the fulfilling of this additional science aim For CR electrons: It is crucial to have a large effective collection area due to limited statistics at high energies. Since cosmic rays are isotropic, a large FoV is also helpful. 34

35 Page: 35/54 For CR composition: For good separation between different nuclei, small pixel sizes are required for the identification of the direct Cherenkov light. Also good time resolution can contribute to secure identification. For sufficient statistics, large effective collection areas are again needed and as for the CR electrons a large field of view is advantageous. 4.2 CTA as an intensity interferometer With its unprecedented light-collecting area for night-sky observations, CTA holds great potential for optical stellar astronomy, in particular as a multi-element intensity interferometer for realizing imaging with sub-milliarcsecond angular resolution. Such an order-of-magnitude increase of the spatial resolution achieved in optical astronomy will reveal the surfaces of rotationally flattened stars with structures in their circumstellar discs and winds, or the gas flows between close binaries. Image reconstruction is feasible from the second-order coherence of light, measured as the temporal correlations of arrival times between photons recorded in different telescopes. This technique pioneered by Hanbury Brown and Twiss (see references) connects the telescopes only by electronic means and is practically insensitive to atmospheric turbulence or to imperfections in telescope optics. Detector and telescope requirements are very similar to those for Cherenkov light measurements, the main difference being the signal processing. Observations of brighter stars are not limited by sky brightness, permitting efficient CTA use also during bright-moon periods. While other concepts have been proposed to realize kilometric-scale optical interferometers of conventional amplitude (phase-) type, both in space and on the ground, their complexity places them much further into the future than CTA, which could thus become the first km-scale optical imager in astronomy. The question of how faint are the sources that can usefully be observed has been examined, with the conclusion that a practical limit for two-dimensional imaging with a large CTA-like array is around mv = 6. However, if only some one-dimensional measure would be sought (e.g., a stellar diameter or limb darkening), the data can be averaged over all position angles, and the limiting magnitude will become somewhat fainter. In any case, there are thousands of stars bright enough to be observable. Even if only a handful of them are actually observed with high S/N ratio, it would be an extremely rewarding program, years in advance of dedicated facilities Key elements related to the fulfilling of this additional science aim It is desirable not to place the larger telescopes on an exact east-west grid, in order to increase the number of projected interferometric baselines sampled by a source as it moves across the sky (rising in the east, moving towards west). However, the exact placement of the many smaller telescopes is not critical. It is highly desirable to provide a provision to mount small dedicated cameras on the outside of the ordinary Cherenkov camera shutter lids. It is not yet possible to tell whether the ordinary detectors will be practical to use also for interferometry of [very] bright sources. Such a provision will enable optimized detectors to be installed while minimizing disturbances to the main camera. For possible experiments with the one central pixel in the ordinary camera, it would be desirable to have it equipped with a provision (e.g. mounting holes) for a mechanical holder for small (cm-size) optical elements to be placed in front of it (such as a colour filter or polarizer to reduce the flux from 35

36 Page: 36/54 bright stars). It is highly desirable that the signal-timing precision of the photon-pulse train from the detector to a central computing location is not much worse than 2 ns. 5 APPENDIX A: SEPARATE REQUIREMENTS FOR ESSEN- TIAL ASTROPHYSICS CASES The separate tables attached at the end of this document presents a summary on an individual basis of the main minimum requirements that CTA needs to fulfill in order to access the essential physics up to the level described in the text of the previous sections. The sensitivity requirements are essentially driven by the survey depth and the accessibility to short timescales in individual survey runs. It is assumed below that that the southern observatory will attain sensitivity beyond 100 TeV, as described earlier in the text, and thus conduct the GPS. The northern observatory will focus more on extragalactic science, and sensitivity at the CTA highest energy end is not herein considered as a minimum requirement. It is also assumed that the northern observatory will conduct most of the extragalactic survey. In constructing the following tables, several aspects of the same physics case have been put together. For example, for measuring the EBL, different approaches can be pursued depending on the AGN redshift, including looking at the residuals of the spectral fitting after assuming a given EBL model, or using the low-energy (and unattenuated) part of the AGN spectrum to determine the intrinsic level of emission. Requirements from both of these science aspects are unified under the same physics case. For such details, the reader is referred to the references for further studies. The penultimate column in the table below refers to an estimation of how many hours of observation are needed to achieve the physics discussed. The observation times needed for the science aims can be compared with an expectation of the CTA lifetime ( 30 years), and the typical amount of hours ( 1000 hours) of a possible duty cycle per year at each of the two observatories, to give an idea of possible CTA overall focus. Moonlight observations are not considered in these estimations, however observations under (partial) moonlight are expected to be performed and will provide a wealth of additional time (typically 20-30%) albeit with limited performances. The last column (generically called Impact) emphasizes that the cases included are all part of the core science of CTA and thus considered essential to fulfilling its physics aims. Hence relative judgments should not be made among them. A summary table of science requirements for CTA appears at the beginning of this document, within the Executive Summary. 36

37 Page: 37/54 6 APPENDIX B: IMPACT OF VARIATIONS IN THE MINI- MUM REQUIREMENTS In this section we explore the consequences of variations in the minimum requirements, noting the main impact that under doing those will have on the science aims. 6.1 On energy threshold The energy threshold of the telescope arrays is a key performance parameter, especially for the detection of extragalactic sources, the fluxes of which suffer from redshifting and from absorption on the extra-galactic background light (EBL). This has already been commented regarding the survey volume accessible to observations at the highest energies as a function of energy threshold, and the need to lower the threshold as much as possible in order to measure the unattenuated part of the spectra of distant sources. Here, further details of the impact on the number of detections are provided. To study the impact of the energy threshold on the number of detectable sources in a more systematic way, a set of instrument response curves for a maximum exposure time of 50 hr per field of view has been considered. For this study, the effective area, the energy and angular resolution curves, and the background rates corresponding to a configuration with 5 LSTs, were artificially shifted to lower and higher energies, away from the regular 30 GeV energy threshold. The number of detectable sources, based on this estimate, would decrease rapidly with an increasing energy threshold: e.g., for the class of FSRQs, which have steeper spectra in the VHE domain than BL Lac objects, a shift from 30 to 40 GeV reduces their number to below 30. A similar situation is expected for sub-classes of BL Lac sources (LBL and IBL). Note that for this simple estimate, the background rate is shifted together with the effective area and changes in the background cosmic-ray flux with energy are neglected (see Sol et al. 2012, quoted in main doc 6 for details). A low energy threshold is also very important to constrain the high-energy peak in the spectral energy distribution for BL Lacs. Current instruments detect in general only the spectral slope above the peak, which makes it difficult to strongly constrain emission models, especially during rapid flares where the spectral evolution below the peak cannot be described adequately with Fermi LAT data. For bright, nearby sources, such as Mrk421, Mrk501 or PKS , coverage down to at least 30 GeV is required if one wants to follow the fast evolution of the peak and therefore confirm or dismiss specific emission scenarios for nearby BL Lacs. For FSRQs, on the other hand, recent observations of spectral features with Fermi LAT at a few TeV to a few 10 TeV were interpreted as signatures of the absorption of the blazar emission on the broad line region (BLR). A low energy threshold, at least as low as 30 GeV, would provide sufficient overlap between Fermi LAT and CTA spectra to validate this interpretation and to extract information on characteristics of the BLR. Finally, the energy threshold will also impact significantly on the possible detection of GRBs. Apart from being far, and thus suffering from absorption, those detected by Fermi-LAT have generally steep spectra. According to the simulations, between 30 GeV and 100 GeV we found a factor of 5 decrease in the expected GRB rate of detections, which would make them very rare, if at all detectable events. 37

38 Page: 38/ On the sensitivity requirement The configurations explored for CTA (see, e.g., Actis et al. 2011) feature a similar structure: large size telescopes (LSTs, dish 23m) placed at the center of the array; tens of medium size telescopes (MSTs, dish 12m) placed in a surrounding ring; an outer ring will be composed of small size telescopes (SSTs, dish 7m). The LSTs are those impacting the most at the lowest energies. The minimum requirement described in the main documentation relative to the low energy domain ( 50 GeV, see Table 2), can in principle be accommodated with configurations having only 3 LSTs. However, if one of the LSTs fails, the remaining 2 LSTs are not sufficient to achieve the minimum physics goals described in the main document. From MC simulations (see Bernhloer et al. 2012, quoted in 6 of the main doc.), the sensitivity drop is at least 40% at the lowest energies ( 50 GeV) if only 2 LSTs are present, the energy resolution is not greatly affected, while the angular resolution degrades by only 10% at 30 GeV and less at higher energies. In the case of short flares, the most important performance quantity when looking at the sensitivity would be the effective area, which drops by at least 50% at threshold for arrays that have the same characteristics (number of MSTs, LSTs, spacing and total covered area), but differ on the amount of LSTs: 3 versus 2. This applies especially in the case of strong flares, where the background estimations plays a minor role and it would reduce the number of GRBs detected at high redshifts. Also less extreme flares would be affected. For example, a source at high redshift (>3) flaring for 2 hours with the same spectrum as the Crab Nebula, will not be detected if there are only 2 LSTs. Even for long observation times, the sensitivity reached with only 2 LSTs would be limited. Adopting a nave scaling in distance, in order to have the same flux measurement on a source, 1 LST will be limited to 80% of the distance that would have been possible with 3 LSTs. This lowering of sensitivity would translate also to a much longer observation time needed to detect AGNs at redshift > 2 (from 50 hours to 120 hours per source, for a scaling of the sensitivity with the square root of time). Regarding populations studies, for a fixed observation time and FoV and assuming that the sources are uniformly distributed in the universe, the number count would drop by a factor of 2 for a drop in sensitivity of 40%. This would affect the capability of cataloging an extragalactic population: for an exemplary sky survey of 250 hours, the expected number of sources would drop from 26 to 15. At the highest energies, the sensitivity is usually signal limited, and background does not play an important role. Therefore an enlarged effective area would dictate an improvement in sensitivity. An improvement of 20% is considered next. This gain in sensitivity could be met with a higher number of SSTs. At energies larger than 50 TeV, the improvement in effective area would translate into an enlarged horizon of detectability, but only by 10%. For a pevatron similar in spectral shape to RX J but with a 100 TeV cutoff and a distance of 5 kpc, the observation time needed to reconstruct the cutoff would be reduced from 200 to 160 hours. Such a gain can thus be mimicked by an increase of the observation time. 6.3 On the consequences of reaching the arcmin scale The minimum requirement on angular resolution (68% containment radius) has been defined as at 0.1 TeV and at > 1 TeV energies. However, several high-performance reconstruction techniques, recently developed within the different collaborations and successfully applied to current IACT data, lead to a significant improvements in terms of angular and energy resolutions, and background rejection. Therefore, it is of interest to quantitatively explore what would be the impact of a plausible improvement of the CTA 38

39 Page: 39/54 Figure 1: Images of the two brightest TeV shell-type SNRs: RX J (upper row) and Vela Jr. (NW rim, lower row). XMM-MOS images are shown in the left column. CTA simulated images of these two SNRs (for E > 1 TeV, configuration I as described by Benloahr et al. (2012), and 50 h observations) are shown in the middle and right columns, (assuming the same morphology as that seen in X-rays and the VHE spectra as measured with H.E.S.S. the flux from the NW rim of Vela Jr was assumed to amount to one tenth of that of the whole SNR), with the minimum required, and goal angular resolution, respectively. angular resolution beyond the minimum science requirements. The simulations shown below assume a gain of 2 in angular resolution over the whole energy range, with other instrument characteristics (point-source sensitivity, energy resolution, effective area) unchanged. Arguments are given to justify such a goal angular resolution from a physics perspective Fine spectro-imaging studies in the brightest TeV SNRs The debate about the nature of the gamma-ray emission detected towards several Galactic SNRs is tightly linked to those of the small-scale structures (filaments, dots) observed in X-rays along the SNR shell-type morphologies. Probing their VHE counterparts with CTA would then provide direct insight into particle acceleration mechanisms, in particular the magnetic field amplification/damping processes and its cosmic-ray energy content. CTA images of the two brightest large SNRs detected with H.E.S.S., namely RX J and Vela Jr., have been simulated based on their respective X-ray morphologies and VHE spectra, as shown in the figure below. While small-scale structures such as the double-arc feature in the W region of RX J and the 50 arcsec-wide NW rim of Vela Jr will be barely resolved with the minimum required CTA angular resolution, the simulated images with the goal resolution show clearly that joint X-ray and VHE spectro-imaging studies could be performed in order to constrain the nature of the gamma-ray emission. 39