Research Plans for the next five year Cherenkov Telescope Array (CTA) 21 September 2012

Transcription

1 Research Plans for the next five year Cherenkov Telescope Array (CTA) 21 September Identification Members: Dr. Carlos José Todero Peixoto (IFSC-USP, post-doctoral fellowship) Prof. Dr. Edivaldo Santos (UFRJ) Prof. Dr. Elisabete M. de Gouveia Dal Pino (IAG-USP) Dr. Grzegorz Kowal (IAG-USP, post-doctoral fellowship) Dr. Gustavo Rojas (UFSCAR) Prof. Dr. Ivone Freire da Mota e Albuquerque (IF-USP) Prof. Dr. João Torres Mello (UFRJ) Prof. Dr. Luiz Vitor de Souza Filho (IFSC-USP) Prof. Dr. Marcelo Leigui (UFABC) Prof. Dr. Ronald Shellard (CBPF)

2 2 - Project Summary Cherenkov Telescope Array (CTA) Observatory is an international collaboration which intends to construct a new generation of ground gamma ray telescopes to study astroparticle physics. The plan is to build hundreds of telescopes in three different configurations in order to detect gamma rays with energies between 10 GeV and 100 TeV. The CTA experiment, which will have 10 times more sensitivity than any currently operating gamma ray detectors, is presently at the study phase and the sites have not been chosen yet. Among these, Argentina is a strong potential candidate to host the CTA branch in the Southern Hemisphere. The prototype phase has just started and the construction phase should begin in Recently, a Brazilian group of scientists listed above has been accepted as members of the experiment and a Memorandum of Understanding was signed by RENAFAE. The Brazilian researchers will participate in several phases of the experiment, such as, the design of the telescope using Monte Carlo simulations, data analysis and the study of the astrophysics of the sources. Considering the experience of the Brazilian scientists in the construction of experiments for high energy physics, the international CTA collaboration is expecting a significant participation of Brazil in the construction of the experiment. This roll will be even stronger if Argentina is chosen as the CTA south site, in which case the participation of Brazilian industries could help to guarantee the success of the construction phase. CTA is a experiment for fundamental science with many technological outcomes. In the fundamental physics scope, CTA is going to contribute in our understanding of: 1. generation of high energetic participles in many astrophysical environment (SNR, Gamma-ray bursts, AGNs etc); 2. Dark-Matter; 3. tests of the Lorentz Invariance over cosmological scales; 4. physics of compact object; 5. Galactic and Extragalactic Astrophysics. At the same time, CTA requires the development of detector technology such as: 1. Precise mechanical structures; 2. Electronics; 3. Special Mirrors; 4. Scintillators; The contribution of Brazilian companies to build mirrors and the mechanical structure of the telescopes has already started. The aim of this research project is to guarantee the continuation of the projects developed in collaboration between the Brazilian scientists in CTA and the Brazilian companies. In particular we focus on the construction of the detector and our intention is to explore other areas which could link our scientific interest to the development of general detector technology which is used for high energy physics in many different kinds of experiments.

3 3- Justification The scientific challenges to be faced by CTA can be organized in three categories: a) design, construction and operation of the experiment, b) raw data reconstruction aiming primary particle information, c) data analysis and interpretation in order to advance our understanding of the science in question. We present some examples below based on the development we have already been working on. Each telescope will consist basically of three parts: a) mechanical structure, b) mirrors c) and photomultiplier camera. Here are the fundamental characteristics of each part: a) Structure Mechanics: support and position the telescope: 1. Ability to move in azimuth and elevation; 2. Structural rigidity allowing an accuracy of 1 mrad pointing; 3. Elastic behavior; 4. Minimization of the shadow of the structure on the mirrors and the camera; 5. Life approximately 30 years; 6. The first natural frequency of oscillation greater than 2.5 Hertz; 7. Minimize costs and installation time. Structures are being studied in carbon fiber and steel. There is an interesting engineering problem in designing such structures. They are very large and heavy as well (30 K Newtons approximately). The heaviest part of the telescope is the photomultiplier camera and its electronics which is positioned one focal length away from the supporting axis of the telescope (the largest telescope focal length is ~ 30 meters). All this illustrates the difficulty of keeping the system well aligned. b) Mirrors: hexagonal segments with size (face to face) ranging from 0.8 to 1.5 m for the small and the large telescopes, respectively. As mentioned earlier, the CTA will have three types of mirrors: Small (S), Medium (M) and Large (L) size ones. The reflectivity of the surface was to be larger than 80% between 300 and 600 nm and the spot size of one segment has to be smaller than 0.8 inches. Several mirror materials and techniques are being tested, among them glass glued on glass fiber, or carbon fiber, or aluminum, hot and cold glass molding and polished aluminum with diamond tools. The mirrors should also be covered by a coating to increase durability. c) Photomultiplier camera: 1. Number of tubes per camera ranges from 2300 for the large telescopes to 900 for the small ones; 2. Each pixel area cm 2 ; 3. Good quantum efficiency. It is not yet defined a reference value; 4. Detection efficiency between 300 and 400 nm; 5. Pixel size ranging between 0.10 and Hamamatsu photomultiplier are being tested as well as new technologies for solid state detectors based on arrays of silicon.

4 4 - Research Plan The CTA in general aims at improving our understanding of astrophysical mechanisms in a huge range of galactic and extragalactic objects which produce gamma-ray photons and accelerate particles above ev. Figure 3 taken from reference i shows succinctly and clearly CTA potentials in this field of investigation. It shows the expected average flux from the main galactic and extragalactic gamma rays sources. The solid line shows the sensitivity of the best telescopes presently in operation (H.E.S.S.). The dashed line shows the CTA sensitivity. Clearly, it is not possible to measure most of the Universe emission of gamma rays with the current instruments. This figure, does not include fluxes from less studied sources or unknown mechanisms. From Figure 3, it is possible to foresee CTA's relevance based on the success of the H.E.S.S. experiment. Table 1 shows the number of publications by the H.E.S.S. Collaboration over the past three years and illustrates the importance of the area and productivity. Figure 1: Expected flux of typical sources compared to the energy threshold of current experiments and to the future CTA threshold. Journal No. Nature 3 Science 4 MNRAS 1 Physical Review Letters 3 Astronomy & Astrophysics 57 Astroparticle Physics 10 Astron. Astroph. Letters 3 Astrophysical Journal 4 Physical Review D 2 Total 89 Table 1: Number of publications by the H.E.S.S. observatory in the last three years.

5 It is important to notice that Cherenkov telescopes do not compete with current gamma ray experiments based on satellites (Swift ii and Fermi iii ) or wide aperture telescopes on the ground (Milagro iv and HAWC v ). Cherenkov Telescopes are unique in being directional, ie, one can track a single source for a long time thus making an integrated measurement. Experiments in orbit have a small collection area which dramatically reduces the detection ranges at high energies (~ TeV). Furthermore, the limited field of view of the existing ground large aperture telescopes decreases their ability to detect potential new sources emitting at low gamma-energies (~10 GeV). Nonetheless, Cherenkov Telescopes, satellites and ground large aperture observatories are complementary techniques. As stated above, CTA's scientific scope covers several crucial astrophysics and high energy particle physics unsolved questions, such as the origin of cosmic rays, the nature of the dark matter, the physics of compact objects, from pulsars to galactic and extragalactic black holes, binary stars, supernovae, and giant molecular clouds Besides, dwarf galaxies (which are rich in dark mater), starburst galaxies, galaxy clusters and cosmic magnetic fields are also among the top list topics to be investigated with the CTA. Considering this broad scope it is difficult to write a project on all aspects in detail. Therefore we have chosen a few items to describe in our project. Of course, these were chosen accordingly to the interests of our scientific team. Reference vi describes in detail all of CTA's scientific goals. 4.1 Origin of charge cosmic rays The acceleration of charged particles in astrophysical objects almost invariably ends up producing an associated gamma-ray flux. It can be shown that charged cosmic rays with energies between and ev are accelerated in supernova remnants by acceleration mechanisms based on the first order Fermi mechanism. The most accepted particle acceleration model provides an energy threshold in supernova remnants that depends directly on the charge (Z) of the accelerated particle. If this model is correct, the flux of cosmic rays with low Z must be extinguished sooner than the flux of cosmic rays with high Z. The KASCADE Experiment vii measured the extinction of the proton flux. Now it would be necessary to measure the high Z (iron) extinction in order to confirm this model. The measured flux of charged particles together with theoretical assumptions allows one to estimate the associated gamma ray flux. Figure 4, from reference viii, shows a flux calculation using two gamma-ray models of acceleration of charged particles. The expected flux is compared to the detection threshold of the best observatory in operation (H.E.S.S.). CTA intends to lower the detection threshold of flux by a factor of 10 compared with H.E.S.S. in order to distinguish between the two models assumed in the calculation. It is important to note that Cherenkov telescopes have the advantage of being able to investigate a single source at a time since gamma rays are not deflected on its way to Earth. On the other hand, cosmic ray experiments (e.g., AUGER ix ) detect the integrated flux from all sky sources together since the particles are deflected in magnetic fields, erasing the information about the exact location of the sources.

6 Figure 2: Predicted gamma ray flux for charged particles accelerated in SNRs. Using this same steering potential, CTA may contribute to the understanding of the mechanisms of acceleration of ultra-high energies cosmic rays (> ev). Take as an example the generation of cosmic ray energies above ev in Centaurus A (Cen A) which is the closest active galaxy to the Earth. Classified as an FRI radio-active galaxy, Cen A is a complex object and considered a prototype of this class of active galaxies. Two bipolar jets are observed at length emerging from the radio nucleus of this galaxy with an angle of 8 x 4, extending for millions of light years in the intergalactic space. The structure of the jets is also quite rich, with sub-jets and bright knots along its length that are formed by magnetized shocks where it is believed that cosmic rays are accelerated. Recent estimates consider a central black hole with mass of about 5x10 7 solar masses x and a kinetic power of the jets of the order of erg / s xi. Given the complex and extended structure of Cen A, several models for particle acceleration above ev have been proposed: a) the acceleration on the outskirts of the central black hole; b) direct acceleration in the internal knots along the jets; c) re-acceleration of the particles, originally produced in the central source, along the jet, d) acceleration behind the bow shocks at the edge of the jets where they impact supersonically with the local intergalactic medium, among others, and it is quite probable that all these processes are occurring concomitantly. Current cosmic ray experiments that measure the charged particles, such as the Pierre Auger Observatory, have a maximum angular resolution of about 1.0 xii. With this resolution it is impossible to trace the sub-structures of Cen A and therefore, the exact acceleration site(s). Other mechanisms of acceleration of ultra-energetic cosmic rays point to compact sources as the magnetosphere of pulsars located in the body of normal galaxies (not active) in the vicinity of our galaxy. Gamma ray telescopes have angular resolution far better than the charged cosmic-ray experiments because they are directional, ie, they can track the source and obtain an integrated measure of the signal of a single object or substructure. The CTA will have a resolution of the order of 0.03, thus being able to make a map of possible sources of high energy cosmic rays. Using the correlation

7 between acceleration of charged particles and gamma rays we will have also important information about the processes of particle acceleration to energies above ev. 4.2 Dark Matter Indirect search for dark matter (DM) is among CTA's goals. Experimental evidences show that most of the halo of our galaxy consists of this kind of matter which interacts gravitationally but not electromagnetically. Dark matter should be captured in astrophysical objects and sink to its core due to the gravitational pull. Gamma rays will result from DM annihilation in the core of these objects and can therefore be an indirect DM signal. Astrophysical sites to search for these gammas include clusters of galaxies, the galactic center and spheroidal dwarf galaxies. The two latter ones have a very large ratio between total mass and luminosity, indicating that the dark component dominates the matter in these objects. Several gamma-ray telescopes have been searching for these signs. It is worth noting the results of Pamela published in Nature xiii, where they obtained a signal above the expected range for the diffuse background. Among various explanations was that it could result from dark matter annihilation. The Fermi experiment has also recently reported results xiv which constrain several dark matter models, including some of those which were compatible with the signal found by Pamela. One of the advantages of detecting gamma rays produced by DM annihilation, is the fact that it will allow to determine the energy spectrum and thus constrain the nature of the dark matter. In our galaxy, the galactic center is the most promising target for gamma production from DM annihilation. However, despite the expected large signal from the galactic center, a very large background noise from luminous sources is also expected. Therefore several techniques have been developed to separate both signals. Independently of the separation technique, the signal to noise discrimination is strongly dependent on the angular resolution, since this is crucial to eliminate the background coming from point sources. Other requirements are to have a moderate field of view (greater than 7 degrees approximately), good energy resolution, and to locate the telescope in the southern hemisphere to point directly to the galactic center. As DM detection is among CTA's goals, its design will consider an optimization to increase the DM signal to noise ratio. This type of indirect DM detection is complementary to searches carried out in the Large Hadron Collider (LHC) experiment, in direct detection experiments and indirect detection in neutrino telescopes. The CTA can certainly play a central role in elucidating the composition of matter in the universe. 4.3 Compact luminous astrophysical sources As we have seen, gamma rays can be emitted by different classes of compact luminous astrophysical sources such as pulsars, supernovae, supernova remnants, neutron star binaries, stellarmass black holes immersed in binary systems and supermassive black holes located in the nuclei of active galaxies. Besides these, there are the mysterious GRBs which are isotropically distributed throughout the universe. These objects are traditionally observed in other bands of the electromagnetic spectrum and the combination of data from a wide range, from radio to gamma rays, provides a powerful tool for modeling these sources. 20 years ago, only one source (Crab nebula) was detectable at the gamma-ray part of the spectrum. Currently, the FERMI catalogue registers more than 1500 gamma-ray sources in the sky, most of which associated to the different classes of objects listed above. Researchers of this project have been investigating for several years these classes of sources using theoretical and numerical magnetohydrodynamical tools to model their origin and evolution. In particular, highly collimated relativistic outflows like those mentioned in paragraph 2.1, are commonly

8 observed in compact objects (GRBs, active galactic nuclei, galactic black-hole and neutron-star binaries) spanning a vast range of sizes, of about 7 orders of magnitude in length and 9 orders of magnitude in the mass of the source progenitors. It is widely believed that in all these cases, the jet is driven by rotating, twisted magnetic field lines emerging from an accretion disk around the source (or directly from the spinning source) xv. The rotation twists up the magnetic field into a toroidal component and plasma is ejected by the combination of centrifugal and magnetic tension forces. Particles in the ejected plasma are then accelerated possibly through a first-order Fermi mechanism, either behind internal shocks along the jet, as mentioned earlier, or by magnetic reconnection at the launching region xvi ; xvii ; xviii. These jets are commonly detected mainly through synchrotron radio emission that is produced by relativistic electrons spinning around the magnetic field lines, but their real matter composition is unknown. It could be either leptonic or both, leptonic and hadronic. Close to the launching base of stellar black hole relativistic jets, X-ray and gamma-ray emission have been observed that could be produced either in the accretion disk, by the falling material into the black hole, or by jet material. Some questions that can be answered by CTA are: To what extent do they produce very high energy (VHE) gamma rays? What is the jet composition? Can one catch the accelerators in the act? Can one elucidate the real acceleration site and mechanism in the jets? Besides, CTA's observations combined with observations at higher wavelengths may allow to probe the real structure of these jet/accretion disk systems and, perhaps elucidate the jet launching mechanism. Likewise, the nuclei of active galaxies (AGNs), which host supermassive black holes, also produce relativistic jets, with the difference that they may extend up to millions of light years into the intergalactic medium, rather than only light years, as the galactic counterparts. Besides the investigation of this extended component, CTA will also help to constrain models of the central engines of these AGNs. Furthermore, it will be able to detect a large number of AGNs due to its high sensitivity which is a factor 10 times larger than any current instrument. This will enable population studies that will be a major driver of this theme. Extragalactic background light and galaxy clusters studies are also connected to this field. In particular, as seen previously, our galaxy also hosts a supermassive black hole in its core with 2 million solar masses. At a distance of only 8 kpc from the Earth, CTA is the best opportunity that we have to study the vicinity of a supermassive black hole and its gamma radiation, especially if we combine these measurements with those already performed at other frequencies, allowing a multi-band observation. Also, the careful analysis and the ability of separating the gamma ray contribution associated to the nuclear source, will help to determine the amount of gamma emission due to DM only. Finally, gamma-ray bursts, or GRBs, are the most energetic events in the Universe and have as yet unknown origin. A gamma-ray burst is usually followed by an "afterglow" of smaller energy and longer duration that emits at longer wavelengths (X-ray, ultraviolet, optical, infrared, and radio). The scenario called fireball explains the ability of these sources to produce collimated relativistic ejecta with Lorentz factors > 100 xix. First, the flux is dissipated by internal shocks that promptly produce gamma-rays. Subsequently, this flux interacts with the surrounding matter and produces an external shock wave that in turn develops the hyper-sonic afterglow. There is still much work to be performed before a true understanding of the origin of such huge Lorentz factors can be achieved. The particle acceleration efficiency in these events and their potential astrophysical sources are also open questions. Members of this proposal are involved on various aspects of basic research on these GRBs and CTA will be decisive in this search. In particular, the large area of CTA (~5km 2 ) will allow a high detection rate of these transient sources. GRBs of long period (i.e., longer than 2 s) seem to be associated to collimated emissions caused by the collapse of a high-mass star with a rapidly rotating black hole (e.g., T. Piran, AIP Conf. Sers., 784, 164, 2005). The origin of short GRBs (lasting <2s) is even more uncertain. They could have been

9 produced during the phase transition of a neutron star (NS) to a quark star xx, or by collision between orbiting NSs in a binary system xxi. The sources of GRBs are in general at distances of billion light years from the Earth which makes their observation very difficult. Again, CTA's high sensitivity and large effective area will boost the detection rate. Another transient phenomenon related to GRBs are the so-called Soft Gamma-Repeaters (SGRs), which are mostly of galactic origin. These are persistent X-ray emitters that sporadically emit short bursts of gamma-rays. In the quiescent state they have luminosities of erg/s, while during the bursts of gamma-rays they release up to erg/s, at about 0.1s. These are usually explained in the context of the "magnetar," model, i.e., assuming that the source is a neutron star with an extremely intense magnetic field (B ~ G) xxii, which is difficult to explain theoretically. An alternative model, tries to explain the SGR's behavior as well as the X-ray anomalous pulsars (AXPs) postulating the existence of a (accretion) disk made of material left over around the neutron star from the earlier supernova explosion. In this case, the magnetic field is sub-critical ~ G xxiii. This model can explain the production of gamma ray flares and also provides a natural connection between shortduration GRBs and SGRs sources in nearby galaxies. Observations of these events with CTA may help to elucidate this impasse. 4.4 Schedule CTA International Collaboration : Prototype Phase : Construction Phase CTA Brazilian Collaboration 2013: 2014: Berlin. 2015: 2016: 2017: a) Participation in meetings of the international collaboration; b) Construction of the prototype quadrupod; c) Tests of the prototype quadrupod; a) Final Construction of one quadrupod; b) Installation of the quadrupod to be installed in the official prototype telescope in a) Coating chamber construction for large mirror b) Continuously coat mirror segments a) Construct Quadrupos for the Construction Phase b) Continuously coat mirror segments c) Development of data analysis a) Calibration b Data analysis

10 2018: a) Begining of science analysis b) First science results

11 5 - Coordination with, and relevance to the larger Collaboration CTA experiment was considered a priority in all evaluation panels to which it was submitted, including: ASPERA (Astroparticle European Strategy for Physics), Astronete (A Strategic Plan for European Astronomy) and ESFRI (European Strategy Forum on Research Infrastructures) in Europe, and PASAGE (Particle Astrophysics Scientific Assessment Group) and ASTRO2010 (Decadal Survey Information of the American Astronomical Society) in the United States. It is worthwhile to mention here the opinion of Astronete on the CTA: "The Cherenkov Telescope Array (CTA), an array of optical telescopes to detect gamma rays from high energy black holes and other extreme phenomena in the Universe. Building on existing successful European experiments, the CTA - the first true observatory at such energies - is expected to bring a breakthrough in our understanding of the origin and production of high energy gamma rays." CTA is an experiment on astroparticle physics in its essence, so that the successful participation of a group of researchers in CTA depends on joint efforts in particle physics and high energies astrophysics. An important step in astroparticle physics experiments is the reconstruction of the extensive air shower initiated by gamma interactions in the top of the atmosphere. Monte Carlo simulations of the cascade development in the atmosphere and the signal in the detector are an essential tool in this step. This reconstruction allows one to infer the properties of the source mechanisms. At this stage, astrophysical and cosmological models can be tested against measured data. As can be verified by the Curriculum Vitae of the scientists involved in this project, our team has worked successfully in various stages of construction, simulation and data analysis of astrophysical experiments, as well as in particle physics and astrophysical investigation related to the science to be performed by CTA. In the experimental side, some of the scientists of this project have been involved in the construction and data analysis in the Pierre Auger Observatory. In the theoretical side, some scientists in this project have been conducting studies of astrophysical models concerning the nature of compact astrophysical sources, energy spectra and acceleration mechanisms of cosmic rays as well as analysis on dark matter candidates. The researchers that compose our team are well fit to contribute in the achievement of CTA's goals. Their participation in this experiment can be viewed as a natural contribution of scientists who have long been involved with high energy astrophysics and astroparticle physics at various stages. This project is proposed to guarantee their participation in the CTA experiment which will certainly produce innovative and high-quality science. Currently the Brazilian Members of CTA are well incorporated and participating in the international collaboration. We have collaborations with the DSM/IRFU/SPP CEA-Saclay (J-F Glicenstein) and INFN Sez. Di Padova (Prof. Mosè Mariotti) for developing mirrors for CTA. We have also collaborated with Desy-Zeuthen (Dr. Stefan Schlenstedt ) in the development of the mechanical structure of the telescopes. Besides that we have participated activily in the the astrophysics, darkmatter and simulation working packages.

12 6 - Technology and knowledge transfer CTA is a perfect project to develop technology and promote knowledge tranfer in Brazil. The instrumentation needed for CTA is well within the reach of the companies in Brazil during reasearch in the timeline forseen by CTA. On the other hand, none of the parts needed for CTA are off the shelf products which force the development of new ideas and solutions. All the above mentioned items: mechanical structure, mirrors, eletronics and solid state detectors could be projected and produced in Brazil by a joint effort of groups from the University and interested companies. These development scheme is already taking part in the CTA framework. Two companies have already developed products for CTA in Brazil: Opto Eletronica (São Carlos) and Orbital Engenharia (São José dos Campos). Opto Eletronica is collaboration with the IFSC-USP group in CTA in order to produce special coating surfaces for the CTA mirrors. Several techniques and surface layers have been tested. Recently a large sample of mirrors have been produced by Opto Electronica and tested in IFSC-USP and Max- Planck-Heidelberg. The mirrors have been approved in all tests performed. At the same time, Orbital Engenharia has projected a challenging mechanical part of the CTA telescopes called quadrupod. The quadrupod is the large (16 m) arm that hold the photomultiplier camera in position. The quadrupod is a important piece of the telescope. It must place the photomultiplier with a precision of a few millimetres (< 5 mm) in order to keep the optical resolution of the telescopes within the requirements for operation. At the same time, the quadrupod must be flexible in order to contemplate vibrations due to the movement of the telescope and wind load. The quadrupod projected by Orbital Engenharia has been incorporated in the official development of CTA telescopes after the approval of the desing by the chief group. Besides these two projects already taking place others have been studied and are still within our scope. For example, the carbon fibre industry is a first obvious target. Many parts of the telescopes are constructed in carbon fibre. The Brazilian industries of carbon fibre have reached a high level of quality due to the needs of the airplanes companies (Embraer). We are in contact with some companies in these field and tring to develop a partnership.

13 7 - Estimate of Total Annual Budget for five years The budget presented below is an estimation of the total ammount to the invested by Brazil along the next 5 years in order to contribute in the contruction of the Observatory: 1) Equipament: R$ ,00 per year 2) Infrastructure: R$ 0,00 3) Travel: R$ ,00 per year. Participation in meeting 4) Maintenance and operation costs: R$ ,00 per year to start when the construction of the Observatory begins ( ). Other sources: 1) One project approved by FAPESP R$ ,00 - Responsable: Prof. Dr. Luiz Vitor de Souza Filho 2) One project requested to CNPq R$ ,00 - Responsable: Prof. Dr. Luiz Vitor de Souza Filho

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