HAWC Project Summary High energy gamma rays probe the most extreme astrophysical environments including those that produce the highest energy

Transcription

1 HAWC Project Summary High energy gamma rays probe the most extreme astrophysical environments including those that produce the highest energy cosmic-ray particles. Most of the discoveries in the TeV energy range have been made by imaging atmospheric Cherenkov telescopes (IACTs) which have a few milli-steradian field of view and ~10% duty cycle. However, the Milagro observatory has demonstrated that a detector with a wide field of view (2sr) and nearly 100% duty cycle can discover new sources of TeV gamma rays at energies between 10 and 100 TeV, and map the diffuse emission from the plane of our Galaxy. The HAWC (High Altitude Water Cherenkov) observatory builds on the experience and technology of Milagro to make a second-generation high-sensitivity detector. This unique detector will be capable of continuously surveying the TeV sky for steady and transient sources from 100 GeV to 100 TeV. HAWC will be built by a collaboration of scientists from the US and Mexico with joint support. The HAWC site is Sierra Negra, Mexico, which is a very high altitude (4100m) site near existing infrastructure and collaborating universities. The HAWC observatory will utilize water Cherenkov technology (as proven by Milagro) and many of the Milagro components. The first phase of HAWC can be operational quickly, surpassing Milagro s sensitivity within two years of the onset of funding. Because of the increased altitude, the increased physical area, and optimized design, HAWC will have an improved angular resolution, larger effective area, lower energy threshold and better background rejection. These improvements will result in a sensitivity of 10-15x (depending on source spectrum) that of Milagro and can be accomplished without any new technology, but only a modest upgrade to the existing electronics. We have used the existing Milagro data and simulations to verify these calculations. HAWC will enable very high energy gamma-ray studies that are unattainable with the current suite of instruments: 1) HAWC will map the Galactic diffuse gamma-ray emission above 1 TeV and thereby measure the cosmic-ray flux and spectrum throughout the Galaxy. This map will allow us to look for regions of strong emission above that expected from correlations with matter: a signature of cosmic-ray acceleration. 2) HAWC with its improved angular and energy resolution plus enhanced background rejection will discover the highest energy gamma-ray sources in the Galaxy. Milagro has already observed gamma rays from one source, MGROJ , above 100 TeV. HAWC s measurement of high-energy spectra will allow us to determine whether these sources are also sources of the galactic cosmic rays. 3) HAWC will perform an unbiased sky survey with a detection threshold of ~30 mcrab in two years, enabling the monitoring of known sources and the discovery of new classes of diffuse and point-like TeV gamma ray sources. HAWC, in one year, will be more sensitive at energies above ~6 TeV in its entire field of view than IACTs with 50 hours of observation on a point source. 4) With the sensitivity to detect a flux of 5 times that of the Crab in just 10 minutes over the entire overhead sky, HAWC will observe AGN flares that are unobservable by other instruments, including TeV orphan flares. Multi-wavelength observations of AGN flares from radio to TeV probe the environment up to within a few hundred AU of the super-massive black hole constraining models of gamma ray production and acceleration of charged particles. 5) HAWC s low energy sensitivity and continuous operation are unique and essential to measure the prompt emission from gamma-ray bursts. HAWC can detect GRBs out to z~1 if, as predicted, their TeV fluence is comparable to their kev fluence, while for closer GRBs much lower fluences can be detected. If GLAST sees a single GRB photon above 100 GeV, HAWC will see hundreds, revealing the high energy behavior of GRBs and allowing us to probe the bulk Lorentz factor and size of the emitting region. Broader Impacts HAWC is an all-sky instrument that will serve as a TeV finder telescope for IACTs and IceCube. Beyond astrophysics, HAWC will establish a preeminent international scientific collaboration with particle physicists and astrophysicists in Mexico. HAWC will utilize this unique opportunity to reach out to Hispanics as well as maintain the Milagro tradition of providing strong scientific and educational opportunities for undergraduate students, graduate students, and post-doctoral fellows. HAWC will train the next generation of scientists in techniques of particle astrophysics, high energy physics and the mining of large datasets. HAWC scientists will also continue their extensive work bringing the excitement of particle astrophysics to high school students and the general public in the US and in Mexico. 0

2 1 Introduction This is a proposal to fund the US share of the construction of a High Altitude Water Cherenkov observatory called HAWC. An international collaboration consisting of scientists from the US and Mexico is proposing joint US/Mexican funding of this detector to be built at a very high altitude (4100m) on Sierra Negra, Mexico. Milagro was the first large-area water-cherenkov EAS detector to be used for gamma-ray astrophysics. Results from Milagro have demonstrated that this type of detector is a powerful tool for looking at the TeV sky which both complements existing TeV instruments and extends gammaray measurements to the highest energies. Using the experience we have gained from Milagro, we have designed Figure 1: HAWC site at Sierra Negra. An array of 900 tanks is a new modular detector in an optimized overlaid on the photo to show the location of HAWC. Large configuration that will allow us to make Millimeter Telescope is visible on the top of the mountain. precision measurements of the highest energy gamma rays ever detected from known sources, and to simultaneously survey the entire northern sky with high sensitivity. HAWC, IACTs and GLAST will map out the universe from 30 MeV to 100 TeV, which is the key to understanding particle acceleration. In addition, this wide field of view detector will probe the sources of extragalactic cosmic rays by studying the transient sources: such as active galactic nuclei and gamma-ray bursts. Furthermore, a host of questions in particle and space physics are accessible with this instrument. The Milagro detector was built in an existing pond, which dictated its size and altitude. The pond itself was too small to contain an entire extensive air shower, and the outrigger array was added in order to help locate the shower core. As a pioneering effort to bring water-cherenkov techniques to EAS detection, operating Milagro has pointed out obvious ways to improve the technique. To truly discriminate gamma-ray showers from hadronic showers, we must have as large a sensitive area as possible to identify the penetrating particles, such as muons, in a hadronic air shower. Seeing even a single muon isolated from the shower core indicates that the event is likely hadronic. In order to maximize the likelihood of identifying a muon in a hadronic EAS, the HAWC observatory will utilize Milagro's watercherenkov technology, but with a muon-sensitive area that is a factor of 10 larger than Milagro's. In addition, the higher altitude of HAWC relative to Milagro increases the number of particles in an EAS by a factor of >5, which will improve both HAWC's sensitivity and energy resolution. The HAWC design consists of a uniform array of 900 tightly-packed, 5m diameter by 4.3m high water tanks with one 8 photomultiplier tube (PMT) at the bottom of each tank. Tanks were chosen, rather than a pond like Milagro has, following a detailed engineering design by LANL showing the costs of tanks to be less than a covered pond. Additionally, tanks naturally provide optical isolation of the PMTs, which improves the triggering capabilities as well as the angular and energy resolution of HAWC. The tanks will be deployed incrementally and as quickly as funding allows with the goal of beginning data taking within two years. We will reuse the existing 900 PMTs and electronics from Milagro. Because of the increased altitude, the increased physical area, and optimized design, HAWC will have an improved angular resolution, larger effective area, lower energy threshold and better background rejection. These improvements will result in a sensitivity of times (depending on source spectrum) that of Milagro and can be accomplished without any new technology. These calculations have been verified using existing Milagro data and simulations. 1

3 HAWC will enable very-high-energy gamma-ray studies that are unattainable with the current suite of instruments: 1) HAWC with its improved angular and energy resolution plus enhanced background rejection will discover the highest energy gamma-ray sources in the Galaxy. Milagro has already observed one source, MGROJ , up to nearly 100 TeV (See details in Section 3.2.2). HAWC s measurement of high energy spectra will allow us to determine whether these gamma rays come from the source of the Galactic cosmic rays helping answer one of the key questions of the field. 2) HAWC will map the multi-tev diffuse gamma-ray emission to measure the cosmic ray flux throughout the Galaxy. This map will allow us to look for regions of strong emission above that expected from correlations with matter. This will be a signature of regions of cosmic ray acceleration. 3) HAWC will perform an unbiased sky survey with a detection threshold of ~30 mcrab in two years, enabling the monitoring of known sources and the discovery of new classes of diffuse and point-like TeV gamma ray sources. As seen in Figure 2, HAWC, in one year, will be more sensitive at energies above ~6 TeV in its entire field of view than IACTs with 50 hours of observation on a point source. (Typically, IACTs only have a total annual observation time of <1000 hours). 4) With the sensitivity to detect a flux of 5 times that of the Crab in just 10 minutes over the entire overhead sky, HAWC will observe many flares from AGN unobservable by other instruments including TeV orphan flares. Multi-wavelength observations of AGN flares from radio to TeV probe the environment up to within a few hundred AU of the super-massive black hole constraining models of gamma ray production and acceleration of charged particles. 5) HAWC s low energy sensitivity and continuous operation are unique and essential to measure the prompt emission from gamma-ray bursts. HAWC can detect GRBs out to z~1 if, as predicted, their TeV fluence is comparable to their kev fluence, while for closer GRBs much lower fluences can be detected. If GLAST sees a single GRB photon above 100 GeV, HAWC will see hundreds, revealing the high energy behavior of GRBs. GRB measurements of the evolution of the flux at different energies provide strong constraints on the magnetic fields, the circumburst medium, and the bulk Lorentz factors. HAWC s background rejection at the highest energies (>50 TeV) is more than an order of magnitude better than Milagro s and will make a nearly background-free measurement. This background rejection, combined with HAWC s vastly superior energy resolution and angular resolution, will allow us to make a precision measurement of the highest energy gamma rays ever seen. The background rejection and angular resolution are key. At the highest energies, the effective area for gamma-ray events to trigger HAWC is similar to Milagro s, but our ability to tell gamma rays from hadrons gives HAWC a vast improvement in sensitivity. The improvements of HAWC over Milagro are summarized in Table 1. Milagro HAWC Detector Area 3500 m 2 (surface) 22,500 m m 2 (muon) Time to 5σ on the Crab 120 days 5hrs Median Energy 4 TeV 1 TeV Angular Resolution o 0.25 o 0.50 o Energy Resolution at 5 TeV 140% 72% Energy Resolution at 50 TeV 85% 35% Hadron Rejection efficiency at 10 TeV 90% >99.5% Q for gamma/hadron rejection Time to detect 5 Crab flare at 5σ 5 days 10 minutes Eff. Area at 100 GeV 5 m m 2 Eff. Area at 1 TeV 10 3 m 2 20x10 3 m 2 Eff Area at 10 TeV 20x10 3 m 2 50x10 3 m 2 Eff Area at 50 TeV 70x10 3 m 2 70x10 3 m 2 Volume of Universe where 3x10-6 erg/cm 2 GRB is 7 Gpc 3 47 Gpc 3 detectable Flux Sensitivity to a Crab-like source (1 year) (5σ detection) 625 mcrab 45 mcrab Table 1- A comparison of Milagro and HAWC. Note that comparisons are generally made for a Crab-like spectrum of differential photon spectral index However, with a lower threshold some comparisons are between events at different energies. In some cases, the HAWC values will improve when we optimize our reconstruction for angular resolution and background rejection. 2

4 HAWC will be located at Sierra Negra (18.9 N, 97.4 W) in Mexico, about a 2 hour drive from Puebla, a town of population ~ 2 million. Puebla has an international airport with flights to Houston and is ~ 2 hour drive from the international airport in Mexico City. The Sierra Negra site has been developed by the Mexican Instituto Nacional de Astrofísica Óptica y Electrónica (INAOE) for astrophysics research. In particular the largest project is the >$100M Large Millimeter Telescope (LMT), a joint project between Mexico and the US. Several other astrophysics projects are located near the LMT, and a consortium has been founded to share resources amongst these projects. The resources shared by the consortium include roads, security, power and, in the near future, high bandwidth communications. The LMT also currently has an office and will have a dormitory in Atzitzintla (pop. 3000, 2680m elev.) that HAWC as part of the consortium can share. Additional costs would be required to extend electricity and communications for approximately 1km to the location of HAWC and are included in the budget. The HAWC power and communications requirements can be met with the existing or planned capacity. The permit to use the site for HAWC has been approved and the documents are in the supplemental documents section. The cost of the HAWC detector is ~$7 M. This proposal requests $5.5M in equipment costs from the US over four years. Our Mexican collaborators will contribute $1.5M towards HAWC. * The first year will largely be devoted to infrastructure development at the site. After that, our ability to buy tanks at ~$4k each will control the schedule. Our plan is to deploy all 900 tanks with a single Milagro PMT as quickly as we can. While we are deploying the first 300 tanks we will put three PMTs in each enhancing our sensitivity while we are expanding and allowing us to fully utilize the PMTs and debug all electronics. Under this scenario, we will be four times more sensitive than Milagro after the first 300 tanks are finished. Under the proposed budget HAWC will be completed after about 3.5 years. Any upgrades will be driven by the availability of funds. The flexibility of the tank design gives us the ability to adapt our upgrade path to what we have learned during the construction. For example, if we see GRBs during the construction we might decide it is more important to keep the low-energy response we gain with three PMTs in the tanks so we could choose to buy PMTs and tanks at the same time, keeping the array densely populated. There will be a further discussion of various upgrade scenarios in Section 5.6 National Academy studies have repeatedly recognized the need for gamma-ray observatories. This interdisciplinary field combines the knowledge of astrophysics with the detectors of particle physics and the theories of extreme gravity and cosmology. The report of the Committee on Elementary Particle Physics in the 21 st Century, Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics (EPP 2010) [1], lists the field of gamma-ray astrophysics in the chapter on The Experimental Opportunities, and emphasizes the importance of astroparticle physics projects where the intellectual and technological capabilities of particle physicists can make unique contributions. In the National Academy study, Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century [2], understanding the sources of cosmic rays was one of the eleven questions for the next century: How do cosmic accelerators work and what are they accelerating? Following this report, the President s Office of Science and Technology Policy (OSTP) assembled a road map, A 21 st Century Frontier for Discovery: The Physics of the Universe [3], of how the various federal agencies would implement the recommendations. Earlier, in the decadal astronomy study report, Decade of Discovery in Astronomy and Astrophysics [4] the panel noted that None of the existing or planned observatories have the sensitivity to detect very many sources they are what astronomers call starved for photons. This was written before this proposal. The HAWC observatory, with its large field of view, a sensitivity comparable to the current generation of imaging air Cherenkov Telescopes (IACTs), and high duty cycle has the capability to radically alter this situation. This was recognized in a recent document by the Steering Committee of ApPEC (Astroparticle Physics European Coordination) which is producing a roadmap for astroparticle physics in Europe which covers the next decade. This report says The MILAGRO collaboration is planning a larger array called HAWC which is being designed to allow for a * In addition, the Mexican collaboration submitted a separate proposal to CONACyT in Mexico for $5.5M USD. This mega project proposal was accepted as part of a High Energy Physics Network which has received $1M this year. The division of these funds amongst the network and the funding for following years is still being discussed. Any additional Mexican funds beyond the $1.5 M would be used to pay for upgrades (2 possible upgrade paths are discussed in Section 5.6) of HAWC as well as enabling a faster construction. 3

5 survey of an important part of the sky in the gamma-energy range above few hundred GeV and which would therefore have an important overlap/complementarity with the physics goals of CTA +. The outline for the remainder of this proposal is as follows: In section 2 we detail the scientific motivation for HAWC. In section 3, we give results from previous support - concentrating on the results from Milagro and briefly describing other projects in which we have participated. In section 4, we present the technical design of HAWC. In section 5, we give details of the Sierra Negra site. In section 6, we present the expected performance of HAWC. In section 7, we present our plan for deployment, institutional responsibilities and management plan. In section 8, we describe our current education and outreach plus our plans for HAWC. Section 9 is a proposal summary. Further details on HAWC and on each of these sections and be found on a dedicated website 2 Scientific Motivation What astrophysical sources accelerate cosmic rays? This nearly 100-year-old question is a primary objective for the field of high-energy particle astrophysics. This question is not just important because of its age, but also because of the broad impact of cosmic rays on many scientific fields. Cosmic rays have lead to, and may in the future lead to, a new understanding of particle physics. The accelerators of cosmic rays produce particles of energies far exceeding human capabilities. Black holes and intense gravitational and electromagnetic fields power these accelerators providing unique laboratories that cannot be replicated on Earth. Cosmic rays propagate throughout the Universe and serve as unique probes of the distant universe and dark matter. Cosmic rays influence and probe the dynamics of our Galaxy, where their energy density is comparable to that of starlight and electromagnetic fields. Outside of Physics and Astronomy, Biologists and climate scientists have even explored the effects of cosmic rays on evolution and the Earth s climate. The study of cosmic rays is truly multi-disciplinary. High-energy gamma-ray observations are an essential probe of cosmic rays, because gamma rays are created by cosmic rays interacting near their origin. The resulting gamma rays travel in straight lines, unperturbed by the Galactic and extra-galactic magnetic fields, and, unlike the charged cosmic rays, point back to their sources, providing the direction to the cosmic ray accelerator. In addition, the characteristics of the gamma-ray flux variability and spectra constrain the acceleration mechanisms and the environment of the accelerator. The highest energy gamma rays and the shortest timescales of variability provide the strongest constraints on the acceleration mechanisms at work in these sources. These two objectives high energy and transient observations are the primary scientific motivation for HAWC and are described here. In addition, HAWC s wide field of view provides a unique discovery potential. History has shown that astronomical surveys of new wavebands produce unexpected and amazing observations. 2.1 HAWC observations up to 100 TeV probe Galactic cosmic rays Cosmic rays up to at least 10 3 TeV, and perhaps as high as 10 6 TeV, are of Galactic origin. The observed spectral break at 10 3 TeV -- the knee -- may be due to the spectrum of cosmic ray sources, the escape of the cosmic rays from the Galaxy, or a combination of the two effects. It has even been speculated that a single nearby Figure 2: HAWC sensitivity compared to source is the cause of the knee. Protons with energies of other experiments. The solid (dotted) line is ~10 3 TeV colliding with molecular clouds or other matter 1 (5) years of HAWC exposure vs 50 hours will produce ~100 TeV gamma rays. These 100 TeV for the IACTs. Note: HAWC sensitivity is for gamma rays will be observable by HAWC from individual point and extended sources as well as from the sea of 2π sr vs a single source for IACTs, including the planned CTA. cosmic rays interacting with matter in the Galactic plane. Supernova remnants (SNRs) have been postulated as the origin of Galactic cosmic rays largely because + CTA (Cherenkov Telescope Array) is an ~150M Euro IACT array being planned by the European community. 4

6 they have sufficient energy to provide the observed local cosmic ray energy density. SNRs also have sufficiently strong magnetic fields to trap particles long enough to accelerate them up to at least ev. TeV gamma rays have been observed from SNRs, but also from other Galactic sources--pulsar winds and compact binaries. Which of these sources accelerate hadronic cosmic rays? What is the total power output of these Galactic accelerators? HAWC s observations up to the highest energies of many sources from different classes are essential to answer these questions. The highest energy observations are key to distinguishing gamma rays produced by electrons from those produced by hadrons. There are observational differences in the TeV gamma-ray spectrum from electron accelerators and proton accelerators accessible to HAWC. Electrons lose their energy more quickly than protons due to synchrotron emission and are therefore more difficult to accelerate to the highest energies. Also, the cross section for inverse Compton scattering decreases at higher energies, resulting in a break in the gamma-ray spectrum by at least TeV. Gamma rays from hadronic cascades in the accelerating region, on the other hand, follow the power law spectrum of the particles initiating the cascades up to the highest energies Discovering and Understanding Extreme Galactic Accelerators The flux of gamma rays decreases with increasing energy, thus a large effective area and long integration times are required to detect the highest energy gamma-rays. HAWC s effective area is comparable to that of IACTs; however, HAWC observes every source in half of the sky for 1500 hours per year. Atmospheric Cherenkov telescopes typically observe sources for ~<50 hours and survey observations are ~10 hours. As seen in Figure 2, at energies above ~6 TeV, the HAWC one year sensitivity is better than the sensitivity of a 50 hour VERITAS or HESS observation of a single source. An IACT can only spend up to ~200 hours per year observing a single interesting source due to solar and lunar constraints. However, even with maximum exposure, an IACT still wouldn t be able to match HAWC sensitivity above 10 TeV. Consequently, most of the HESS sources in the Galactic plane survey are not detected above 10 TeV. Figure 3 shows the spectra of one of the HESS sources and the capability of HAWC to clearly discern the difference between a continuation of this spectra and an exponential cut off of 40 TeV. The spectra of the known TeV Galactic sources are hard with an average differential spectrum of index -2.3 as compared to the steeper Crab spectrum of index HESS detects the source in Figure 3 with 0.2 times the Crab flux above 200 GeV, but if the measured spectra continue to higher energies, then Figure 3: HESS data is shown (in red) for J The two lines show a spectrum with slope one (solid) unbroken and one with an exponential cutoff at 40 TeV. A simulation of 1 yr of HAWC data is shown in green (blue) with (no) cutoff demonstrating that HAWC will distinguish between these spectra. Figure 4 Comparison of γ-ray sensitivity between the IACT and HAWC observations as a function of source extent and various observation times. The HESS detected Galactic sources are shown as well as the Milagro sources with their error bars. 5

7 this source is as bright as the Crab above 100 TeV. HAWC would detect ~20 gamma rays > 100 TeV per year from such a source in its field of view Extended TeV Sources HESS has observed that most Galactic sources are extended. Figure 4 shows the sensitivity as a function of source size for HAWC and IACTs. When the source size is much larger than the point spread function, the sensitivity of the detector worsens because the background increases. The current observations of HESS are clustered around their sensitivity limit, implying the existence of even more extended sources. For example, nearby sources would have a larger angular extent. Milagro already sees evidence of TeV emission from the nearby pulsar wind nebula, Geminga, with an angular extent of 2.9 degrees, corresponding to a diameter of ~8 pc [5] (See section 3.22). HAWC will detect Geminga with a significance > 50 σ and will be able map out the spectrum of this source vs. the distance from the pulsar. The highest energy electrons should lose energy quickly as they propagate away from the source; this is not true for protons. Therefore, if Geminga is an electron accelerator we will see a clear change in the spectral index of the resultant gamma rays the farther from the source we look Diffuse Emission from the Galactic Plane EGRET and Milagro have shown that the Galactic plane is the brightest feature in the GeV and TeV sky, respectively. While some of the emission is likely due to unresolved point sources, a large fraction is due to cosmic-ray interactions with the matter in the Galaxy. Gamma-ray observations are the most direct probe of the flux and spectrum of cosmic rays outside our solar neighborhood. Hadronic cosmic rays interacting with matter produce neutral pions that decay to give gamma rays, whereas electrons create high-energy gamma rays through inverse Compton scattering with infrared photons and the cosmic microwave background [6]. In addition, processes not directly related to cosmic-ray production may also contribute to the diffuse emission. For example, self-annihilating super-symmetric dark matter could play a significant role as an additional emission component with a distinct spectral signature for HAWC to find [7]. HAWC will map the diffuse emission in the Galaxy at multiple energies to be able to distinguish both the energy and spatial differences between the leptonic and hadronic emission mechanisms. The HAWC site is closer to the equator and can observe the inner Galaxy all the way to the Galactic center. HAWC will thus be able to study nearby regions such as Cygnus at a distance of 1-2 kpc as well as the more distant inner Galaxy at ~10 kpc. The Cygnus region could be dominated by a very few cosmic-ray accelerators whereas the cosmic rays from the inner Galaxy are from a large collection of sources and will reflect the cosmic-ray spectrum after propagation farther from their origins. These regions are hundreds of square degrees and require the large field of view of HAWC. The diffuse GeV and TeV gamma-ray flux recorded with EGRET and Milagro respectively are above predictions based upon the assumption that local cosmic rays are representative of those elsewhere in the Galaxy. In order to match the EGRET data, the cosmic-ray density in the rest of the Galaxy must be two times higher than measured locally[8]. Increasing the cosmic-ray density enough to match the Milagro data would violate the measured limits on the anti-proton flux. However, unresolved TeV sources may be contributing to Milagro s measurement of the flux from the Galactic plane. For example in the Cygnus region, Milagro detects an excess, MGRO J , coincident with the largest matter density. This Milagro source is also coincident with TeV J , but the Milagro source is both brighter by a factor of 3 and more extended than the HEGRA source. Deeper HAWC and VERITAS observations of both the spatial and spectral morphology will determine whether other sources exist in this region and whether the more localized TeV source could be the accelerator of protons which illuminate the entire region. The combination of the diffuse sensitivity of HAWC with the deeper, higher angular resolution IACT follow-up observations provides the most efficient way to map the entire Galactic plane over all angular scales. 2.2 HAWC Observations of Transient Sources probe Extragalactic Cosmic Rays The origin of extragalactic cosmic rays is unknown. Very few sources are capable of accelerating particles up to ev. The source size is either too small or the magnetic field too weak to contain the particles long enough for them to be accelerated to extreme energies. Two classes of sources are likely candidates active galactic nuclei (AGN) and gamma-ray bursts (GRBs). 6

8 While the energy in Galactic SNR is well matched to the measured flux of Galactic cosmic rays, it is unclear what sources have sufficient energy to produce the extragalactic cosmic rays. Both AGN and GRBs are variable and relativistically beamed with unknown opening angles. Do AGN have a quiescent flux or only flares? What fraction of AGN and GRBs emit GeV or TeV gamma rays? Because these sources are transient, to accurately estimate the energy available to accelerate cosmic rays it is essential to have unbiased observations of these sources. Large field of view detectors with continuous observations, such as HAWC and GLAST, are required to answer these questions. HAWC will monitor the TeV sky and GLAST will monitor the GeV sky. Figure 5 illustrates the complementary ability of HAWC to observe shorter time scale variations than GLAST and extend the energy range of observations beyond those of GLAST. If a GRB or an AGN flare is detected by HAWC and GLAST, the overlap will yield observations over 7 orders of magnitude in energy, all in the gamma-ray band. The low energy sensitivity of HAWC is essential for observing extragalactic sources. Gamma rays of energy E γ emitted at a redshift z are attenuated by pair production on the extragalactic background light and the optical depth is τ ~ z 4/3 (E γ /90 GeV) 3/2 for 0.1<z<2. 9 So for a source at z = 0.1, 0.5 or 1, the gamma ray flux is reduced by a factor of 1/e = 0.37 at E γ = 700, 170, or 90 GeV, respectively. HAWC has a large effective area at low energies with ~100 m 2 at 100 GeV. The effective area increases with energy as a power law with index 2.6 as can be understood from basic electromagnetic shower theory or detailed simulations (see Section 5.2). HAWC s sensitivity for different redshifts is shown in Figure 5 where the y-axis is the flux required prior to pair absorption in order to have a significant HAWC detection. As also seen in Figure 5, HAWC s sensitivity in units of energy fluence is better than that of GLAST > 10 GeV even for moderate redshift objects, resulting in many complementary overlapping observations with GLAST Active Galactic Nuclei Figure 5: The flux required for a GLAST (blue) detection of 5 γ-rays above 10 GeV and for a HAWC (red) 5σ detection threshold for a source differential photon flux of spectral index -2 that is absorbed by the extragalactic background light. The gap between the lines on the left and right is due to the Earth blocking the view of the source. Active galactic nuclei (AGN) are supermassive black holes (~10 8 times the mass of the Sun) with luminosities that far outshine the rest of the galaxy in which they are located. Blazars are a subset of AGN with jets of particles that are pointed towards the Earth, and these objects are highly variable and emit much of their energy in gamma rays. Different classes of blazers exist, and the different observational properties are not yet understood, but are likely to be tied to fundamental properties of these objects. Gamma rays are produced by particles accelerated in shocks that propagate along the jets. If the accelerated particles are protons, then gamma rays are most efficiently produced by hadronic cascades originating with a p+γ interaction. The protons must have energies exceeding ~10 18 ev making AGN possible sources of ultra high energy cosmic rays (UHECR). However, electrons can also be accelerated and will radiate gamma rays via inverse Compton scattering. In general, rapid variability favors electron acceleration while higher energies favor proton acceleration. HAWC s continuous observation of the TeV sky will detect many flaring AGN at the highest possible energies. Every day, without solar or lunar constraints, HAWC provides unbiased monitoring of all blazers in the Northern sky, resulting in a unique ability to study the properties of the TeV blazar population. The long HAWC observations will determine the average flux as well as the duty factor of flares of different luminosities. The power at different timescales is indicative of the size of the emission region. Long periods could be the signature of a precessing jet caused by a binary black hole system. Such binaries are excellent candidates for gravitational wave detection. HAWC s improved sensitivity will better measure the fluxes of flares and detect shorter duration flares than Milagro (see Figure 15 for Milagro s 7

9 observation of Mrk 421). HAWC will promptly notify IACTs to allow even deeper observations of the flaring states resulting in even shorter transient observations. To date TeV emission has been observed from 19 AGN [10]. EGRET observations showed that 70% of AGN were variable. [11] Since energy loss is more rapid with increasing electron energy, the TeV observations are predicted to exhibit even greater variability, which has been the case for Mrk421 and Mrk501. However, few flares have been observed from the newly discovered TeV AGN. The lack of TeV variability may simply be due to the lack of long time scale, continuous observations. HAWC will provide these long-time-scale continuous observations by observing every AGN in its field of view everyday even when the AGN are up during the day and IACTs can t look. The notable exception to the lack of observed TeV variability is the recent flare observed by HESS of PKS J [12]. This source was monitored with multiple short observations by HESS and was observed to flare to ~50 times its quiescent flux for one hour. This source is at a redshift of and is detected up to ~5 TeV with a differential photon spectral index -3.5, which does not vary with intensity. Even with such a steep spectrum, HAWC will detect such a one hour flare source with >6σ. TeV variability constrains both the acceleration process and the environment near the acceleration sites. Since the variability timescale cannot be shorter than the light travel time across the emitting region, Γt var > R e /c = (R e /R s )x(2gm/c 3 ), (where Γ is the bulk Lorentz factor of the emitting region, t var is the timescale of variability, R e is the size of the emitting region, R s is the Schwarzschild radius of the blackhole and M is the mass of the black hole), measurements of the fastest variability can probe the bulk Lorentz factor of the emitting region and the size of the emitting region. For example, in the case of PKS J , if the emitting region is comparable in size to the Schwarzschild radius of ~20 AU for a 10 9 solar mass black hole, then the bulk Lorentz factor of the emission region must be ~100. This would mean that a region the size of our solar system has been accelerated to % the speed of light by the black hole. Such Lorentz factors are more typically associated with gamma-ray bursts, and about an order of magnitude larger than those normally associated with AGN. If AGN accelerate electrons that upscatter synchrotron photons, then the TeV emission should be correlated with x-ray observations. While several TeV flares follow this pattern, there have been orphan TeV flares that are easily detectable by HAWC where there is no commensurate change in the x-ray flux. HAWC naturally provides a mechanism for obtaining many such multi-wavelength datasets and will allow us to study orphan flares and the correlations between TeV gamma ray, x-ray, optical, and neutrino emission in detail Gamma Ray Bursts (GRBs) Emitting over ergs in gamma rays, gamma ray bursts are the most energetic phenomena known in the universe. Like AGN, the emission is thought to be collimated in jets however the bulk Lorentz factor of the particle flows may be as large as 1000 in GRBs whereas in AGN Lorentz factors are typically believed to be GRBs are transient, lasting from fractions of a second to ~1000 seconds. The duration distribution is bi-modal with the break between short and long bursts at ~2 seconds. The progenitors of short and long bursts are different. The prevailing model of short bursts is the coalescence of binary neutron star systems and for long bursts the collapse of a supermassive star. In both cases the energy source is the gravitational potential energy released by the accretion of matter onto a compact object. The highest energy gamma ray conclusively detected from a GRB is an 18 GeV photon detected by EGRET roughly 90 minutes after the onset of the burst [13]. In addition, a high-energy component in GRB that extended to 200 MeV with a spectral index of -1 [14] has been cited as evidence for proton acceleration in GRBs, with the implication that GRBs accelerate protons to energies above ev. However, detection of TeV gamma rays has proven difficult. The best such evidence comes from the Milagrito detector [15]. Theoretical considerations argue for the creation of >100 GeV gamma rays in GRBs [16], [17], [18]. Gamma rays are produced by either leptonic or hadronic processes and yield multiwavelength spectra similar to AGN. Similar to AGN, GRB measurements of the evolution of the flux at different energies provide strong constraints on the magnetic fields, the circumburst medium, and the bulk Lorentz factors. HAWC will detect multiple GRBs, if, as predicted, the TeV energy fluence is the same as the sub- MeV fluence. GRBs occur roughly twice per day throughout the universe, so about 20 GRBs per year are within 20 degrees of HAWC s zenith. For this zenith angle cut, HAWC is sensitive to 10 second duration bursts greater than 1x10-5 ergs/cm 2 out to z~1 as seen in Figure 6. Approximately ¼ of all bursts are 8

10 brighter than 1x10-5 ergs/cm 2 and dimmer bursts that are more nearby are also detectable. Therefore, HAWC will detect several GRBs per year under the assumption of equal fluence from TeV and sub MeV Figure 6 Fluence sensitivity as emitted at the source for a 5σ detection of a 10 second GRB vs. redshift for HAWC (left). The different color lines indicate the sensitivity for GRBs at different zenith angles. The superimposed triangles indicate the kev-mev fluence and redshift of satellite detected GRBs. Also shown on the HAWC plot is the effect of adding an additional 2 PMTs per tank (one possible upgrade). On the right is shown the signal in HAWC for a GRB with a fluence of 1x10-4 ergs/cm 2 for three cut-off energies. This signal will scale with fluence, so even a burst 10 times dimmer will easily be seen. gamma rays. Even if no TeV emission is detected by HAWC from GRBs, stringent upper limits will be placed that constrain emission models. It is important to note that because the effective are for HAWC at 100 GeV is over 100m 2, if GLAST sees a single GRB photon above 100 GeV, HAWC will see hundreds. Observations at the highest photon energies for a large number of gamma-ray bursts is necessary for a complete understanding of the acceleration processes and energy budgets of these extraordinary phenomena. This is crucial for our understanding of the astrophysics of gamma-ray bursts and also on whether they are the sources of ultra high energy cosmic-rays. Only a wide field of view, high duty factor observatory can make these observations as the key to success is the ability to catch a burst (i.e. have it within the field of view of an operating detector). There is significant complementarity between IACTs and HAWC for the observation of gamma-ray bursts; the IACT s low duty cycle of 10%, small field of view and the several 10s of second slew time after notification of a GRB, drastically limits the number of GRB that they could observe in the prompt phase. However, HAWC will alert the IACTs of extreme high-energy transient events, so that they can follow with sensitive TeV afterglow observations. 2.3 HAWC Survey Observations have Discovery Potential When new wavelength bands are explored in astronomy, previously unknown sources and unknown types of sources are discovered. For example, the EGRET catalog [19] contains over 150 previously unidentified sources, HESS has discovered several sources with no known counterparts, and Milagro has detected at least 3 new Galactic sources with no obvious counterpart. The discovery of new classes of objects unobserved at other wavelengths, is a major strength of all-sky monitors. These serendipitous discoveries, while not possible to predict a priori, are frequently the most important scientifically. Examples of investigations that will be done with HAWC are described here. Cosmic Ray Anisotropy: Milagro observes an excess on scale of ~10 o in the highest energy cosmic rays with nearly 15 σ significance as described in Section 3.4. HAWC will be able to measure the energy spectrum of this anisotropy as well as search for smaller fractional excesses. This more detailed information is needed to explain how charged cosmic rays in the interstellar magnetic field can produce such small scale anisotropy. Galaxy Clusters: Some fraction of the immense gravitational energy in a cluster of galaxies is predicted 9

11 to result in shocks that will accelerate electrons and protons up to ~10 18 ev. Both accretion [20] and merger shocks [21] will accelerate particles with the former being more efficient at producing the highest energy particles. While individual galaxy clusters can be observed, the unknown history and masses of the clusters makes the prediction of gamma-ray fluxes difficult. There are ~600 galaxy clusters nearer than z=0.1. The angular extent of the emission is expected to be up to 1 degree in some cases. HAWC will observe all nearby clusters and determine which clusters emit VHE gamma rays. Galactic Pair Halos: AGN will have gamma-ray halos extending to nearly one degree that are produced by pair production of even higher energy gamma rays near the source [22]. These halos can come from AGN that do not have jets pointed towards Earth. A HAWC detection of galactic pair halos would measure the extragalactic background light at different redshifts, probing the cosmological evolution of the Universe. Nearby Galaxies: TeV gamma rays should be produced by cosmic ray interactions with matter in other galaxies just as in the Milky Way. However, some galaxies may have an enhanced cosmic ray flux, such as starburst galaxies, or different relative electron and hadron fluxes and spectra than our galaxy [23]. HAWC s large field of view allows many potential sources to be studied. Galactic Center: The center of our Galaxy is a known TeV source, yet the origin of these gamma rays is unknown. While the Galactic center transits at only 49 degrees, HAWC will still have sensitivity to detect this source in the highest energy gamma-rays, extending the spectrum to 100 TeV, and to search for variability. A HAWC observation of variability would rule out a dark matter origin and would be very difficult for hadronic models. Molecular Clouds: Gamma rays are produced by cosmic ray interactions with matter which is concentrated in molecular clouds. Because they are nearby the cosmic-ray flux is presumed to be the same as at Earth, so the gamma-ray flux uniquely determines the only free parameter, which is the ratio of CO to molecular hydrogen. While some of these clouds have large angular extent, smaller but dense clouds may still be undiscovered at high Galactic latitudes [24]. The sky survey capability of HAWC is required to observe both the large angular extent of known molecular clouds and to discover new ones. Compact Binaries: Black holes or neutron stars orbiting a massive star likely accelerate particles by shocks produced in the accretion process. Three binaries have been observed to have TeV emission modulated by the orbital period. The variability implies a small source region and hence a high optical depth for gamma rays, yet the TeV emission extends to high energies with a hard spectral index. Over 100 x-ray binaries have been cataloged, with orbital periods ranging from hours to years. HAWC s daily observations are essential to observe all phases. For example, HAWC would be able to distinguish the differing models of TeV emission at periastron for PSR B , as shown in Figure 7, which could not be tested due to the full moon interfering with HESS observations. Full Moon Figure 7: TeV lightcurve obtained by HESS for the binary pulsar which has a highly eccentric orbit of 3.4 years that emits TeV gamma-rays near periastron a. Three models for the TeV emission are shown, but the TeV observations at periastron were not possible due to moonlight. If this source were within HAWC s field of view, the blue error bars show the capability of HAWC to detect different model. Microquasars: A binary that exhibits jet-like behavior is referred to as a micro-quasar and provides a test of jet physics on shorter time and size scales than AGN. These objects have been known to flare at radio and x-ray wavelengths. At the 2008 International Cosmic Ray Conference, the MAGIC 10

12 collaboration announced such a TeV flare for the microquasar and black hole Cyg X-1. This flare preceded an x-ray flare, but the statistical significance was weak. Nearly a dozen microquasars are known and HAWC will search for TeV flares both coincident and independent of other wavelengths. Other Transient Galactic Sources: The surveys of the Galactic plane by Imaging Atmosperic Cherenkov telescopes can miss transient sources because these telescopes scan their few square degree field of view with short duration observations at various locations. A large fraction of the EGRET unidentified sources at low Galactic latitudes are variable [25] indicating new classes of gamma-ray emitters which could extend to higher energies. Solar Energetic Particles: Our Sun is the nearest astrophysical particle accelerator. Solar particles in excess of 10 GeV have been detected by Milagro associated with coronal mass ejections [26]. HAWC provides diagnostic and discovery potential in the area of energetic solar particles and the dynamics of the inner heliosphere. HAWC with its greater sensitivity will be able to detect the weakest flux of protons. Because of the low geomagnetic latitude of Mexico, measuring the tails of the high-energy proton and neutron distributions will provide new diagnostic capabilities for investigating coronal shock acceleration. HAWC will work in concert with ground level neutron monitors only a km away at Sierra Negra, as Milagro has done with the Climax station, in order to extend observations of solar energetic particles to the highest energies. Solar Weather: Large-scale magnetic structures inside the inner heliosphere modulate the Galactic cosmic ray flux at Earth. HAWC measurements of the cosmic-ray flux and anisotropy will provide detailed information about these phenomena. Conversely, measurements of time-dependent cosmic-ray anisotropies are telltale signs of approaching coronal mass ejections not visible by other means. Indirect Detection of Dark Matter: Supersymmetric models in high energy physics have provided a candidate particle for the dark matter in the lightest supersymmetric particle, the neutralino. LHC experiments may be able to determine if this particle exists, but gamma-ray observations are required to know whether this particle is the dark matter. Depending on the mass of the neutralino, space-based detectors such as GLAST or ground-based gamma-ray detectors such as HAWC may be the most sensitive to search for the neutralino signature [27]. While the Galactic center should have a large concentration of dark matter, there will also be clumps throughout the Galaxy of varying masses. A nearby clump of sufficient mass could be detectable by HAWC. The spectrum of the gamma-ray flux and the spatial extent of ~ 1 degree provide unique signatures to distinguish nearby dark matter clumps from other gamma-ray sources [28]. These objects will be bright only at gamma-ray energies and thus can only be found by survey the sky this is ideal match to the capabilities of HAWC. Lorentz Invariance: The combination of cosmological distances and rapid variability make short duration transients, such as gamma ray bursts, a unique laboratory to study the dependence of the speed of light on the energy of the photon. Theories of quantum gravity predict a time delay Δt for photons of energy E 1 and E 2 traveling a distance L of Δt~L(E 1 -E 2 )/E QG =40zE TeV sec. E QG is an energy scale at which Lorentz invariance would be non-negligible. E TeV is the energy in TeV of the highest energy photons detected, and z is the redshift of the GRB. HAWC detections at the maximum energy allowed by the absorption on the extragalactic background light of a one second time delay relative to kev-mev lightcurve will probe E QG above the Planck mass (10 19 GeV). Recent observations of minute time scale flaring of Mrk501 at z=0.034 by MAGIC [29] show evidence of such time delays which could be due to Lorentz invariance. However, a single measurement can only set a stringent limit. The multiple flares or bursts that HAWC will observe from sources at various redshifts would allow differentiation of source effects from a violation of Lorentz invariance. 2.4 Synergy with other High Energy Astrophysics Projects Multi-wavelength and multi-messenger observations are essential to understanding the gamma-ray sky. HAWC will search the TeV sky in real time for flaring sources and notify the community within seconds of short-duration flares. This capability enables observations at other wavelengths or with more sensitive IACT observations. For steady sources, HAWC will provide a TeV flux or upper limit for all sources within >2π sr. GLAST is expected to detect thousands of gamma-ray sources [30], and many of these will not have obvious counterparts. HAWC will provide a natural extension of the energy reach of GLAST to the TeV scale and beyond for the half of these sources within HAWC s field of view. 11

13 HAWC will discover new TeV sources and monitor known sources. Follow up IACT observations will reduce the duration of the shortest time scale variability observed, map the spatial morphology, and constrain the spectrum to lower energies. At the highest energies of TeV, HAWC will extend the spectra of the IACT observations. HAWC and IceCube, a TeV-PeV neutrino observatory, will observe the same range of energies and the same Northern hemisphere sky. Because proton cascades produce comparable fluxes of photons and neutrinos at similar energies, HAWC s sources are excellent IceCube candidates as seen in Figure 8. HAWC s observations of flaring sources are thus very useful to select the direction and time interval from which to search for neutrino emission. Such a selection can improve the sensitivity of IceCube by more than a factor of 2 by reducing the search trials. Such triggering is particularly probing should HAWC observe an orphan AGN flare since these flares suggest hadron acceleration and neutrino production. The ultra high energy cosmic ray (UHECR) observatories Auger and HiRes have observed the GZK cutoff in the spectrum which implies that the highest energy Figure 8: Beacom & Kistler 07 prediction for IceCube neutrino detection from MGRO J compared to atmospheric background cosmic ray origins are within ~100 Mpc of earth. An anisotropy in the UHECRs might be detectable by Auger but would be blurred by the deflection of magnetic fields. However, some of the UHECRs will interact near their sources and produce gamma rays. HAWC can search for TeV emission from potential classes of sources, such as nearby AGN or galaxy clusters, and with its angular resolution can determine which of these AGN emit TeV gamma rays. These TeV sources are likely the sources of UHECRs as well. 3 Results from Previous Support Members of the HAWC collaboration have a broad base in particle astrophysics and high energy physics experiments (D0). The majority of the collaboration is actively involved in Milagro. Others have joined with experience in GLAST, VERITAS, IceCube, Auger, and HiRes. Having these strong connections to other experiments in Particle Astrophysics will help us realize the many synergies between HAWC and these other instruments. In this section we will concentrate on the results from Milagro as it is most closely related to the proposed work in HAWC. However, elsewhere in this proposal, we point out the impact this wide-field instrument will have on the rest of astrophysics and particularly where we already have connections. Los Alamos, UCI, and Maryland were all supported by the NSF to work on Milagro. 3.1 Milagro Milagro is the first large area, continuously operating, water Cherenkov detector used for gamma-ray astronomy. As such Milagro is a prototype for the technology and, with the detection of the Crab Nebula and other sources, its sensitivity relative to other techniques can be calibrated and (most importantly for this proposal) the veracity of the Monte Carlo simulation of the response of such an instrument can be demonstrated. The original Milagro proposal was submitted in 1990, just after the initial Whipple results on the Crab Nebula. Milagro was designed to utilize an existing 60m X 80m X 8m reservoir at 2650m near Los Alamos and was projected to see the Crab at 4-5σ/yr. The proposal included an array of Figure 9 : The Crab spectrum as observed by Milagro (preliminary). 12

14 outrigger counters to allow us to locate shower cores and to correct for shower front curvature. The project was approved in 1991 and funded in The original three-year $3M proposal was stretched to five years, but only $2.7M was provided. This funding break prevented construction of the outrigger array. We began taking data with the pond alone in By 2002, we had seen the Crab at 4σ/yr with the incomplete detector. In 2004, after an appeal to SAGENAP and an additional $600k, the outrigger array was finished. We began data taking with the full experiment in The value of the outrigger array was quickly seen. We now have about 8 σ/yr on the Crab and have discovered new TeV sources and diffuse flux from the Galactic plane. The results described in this section include data since 2000, but the recent post-outrigger data dominate the results. In evaluating the success of the Milagro collaboration, it is important to note that the completed detector has only been available for about three years, despite being proposed and approved almost seventeen years ago. 3.2 Milagro Results Milagro [31] has provided a new and unique look at the Northern sky. While HESS has surveyed the galactic center, Milagro has surveyed the region of the galaxy visible from the Northern Hemisphere including the Cygnus Region. This survey has revealed both new TeV sources and diffuse emission in the galaxy. This wide-field look at the TeV sky allows us to compare what we see to what was seen at GeV energies by EGRET. EGRET saw 14 GeV sources in the region surveyed by Milagro. Milagro observed six of these same sources in the TeV. One of these was the Crab and one was spatially coincident with a previously reported HEGRA source. The other four sources are new TeV detections by Milagro. One of these new sources (MGRO ) was later confirmed by HESS. In addition, three of these sources overlap with excesses observed by the Tibet Air Shower Experiment (EAS-γ). The chance probability that the 5 excesses (excluding the Crab) we identify as sources overlap with the 13 EGRET GeV sources is estimated to be ~1 in Many of these EGRET sources have extensively observed by IACTs, but not detected. This nondetection combined with the Milagro detection implies that these sources are hard spectrum (α>-2.3) and/or diffuse. These features make them attractive targets for wide field instruments with good high energy sensitivity like EAS detectors, but make them difficult targets for IACTs. The synergy between the two techniques is well demonstrated by the Milagro and HESS observations of MGRO , where the HESS observation measures the spatial morphology and low energy spectrum and Milagro extends the spectrum to higher energies. Another intriguing Milagro source is Geminga. Geminga was the brightest GeV source in the Northern sky seen by EGRET even brighter than the Crab. While many hours of IACT time has been dedicated to it, Geminga has not yet been detected by IACTs. In addition to the TeV sources, Milagro has observed large scale diffuse emission from the Galaxy. The Cygnus region of the Galaxy is seen by EGRET to be the brightest region of Galactic plane aside from the inner Galaxy. Milagro data reveals that the Cygnus region is also bright at TeV energies. Understanding the nature of this diffuse emission, whether it is due to synchrotron emission or neutral pions is a key to identifying sources of Galactic cosmic rays. 3.3 The Galactic Plane The results of ~6 years of Milagro data show that an excess of TeV gamma rays exists along the Galactic plane. Most of the activity occurs in the Cygnus region of the Galaxy, as seen in Figure 10. In the last two years, the sensitivity of the Milagro experiment has been substantially increased through improvements in gamma/hadron separation (that utilizes information from the outrigger array) and new analysis methods. We now use a gamma/hadron separation parameter, A4, which is capable of rejecting background with high efficiency at high energies. The analysis technique was furthered improve by weighting events based on the likelihood that they are due to gamma rays. This method is equivalent to the Likelihood ratio method in the large N limit. These two improvements result in an increase in sensitivity of ~2.5x, as confirmed by observations of the Crab nebula, and an increase in median energy from ~3.5 TeV to ~12 TeV, well above the energy of the peak sensitivity of IACTs. 13

15 Milagro discovered that the Cygnus Region is the brightest region of the Northern TeV sky. In this region, the total TeV gamma flux is seen with a significance of > 10σ above the background outside the Galactic Plane. The γ- ray flux from this region measured by Milagro at a median energy of 12 TeV was compared to predictions from the GALPPROP model. This model predicts the gamma ray emission in every spatial grid point using the propagated spectra of cosmic rays (both electrons and nucleons), the interstellar radiation field, and the gas densities [32]. The diffuse Galactic flux measured by Milagro is more than Figure 10: Galactic Plane profile showing the Milagro data and the GALPROP model optimized to fit the EGRET excess in the GeV region. The significant excess can be seen in the Cygnus region. 3 times greater than the flux predicted by GALPROP. This is true even for the optimized GALPROP model, which is tuned to match the EGRET diffuse emission data for the whole sky and reproduces the GeV excess. This can be seen in Figure 10, which shows the measured flux profile by Milagro after source subtraction and the predictions from the GALPROP model versus Galactic longitude for a region of latitude within 2 o of the Galactic plane. The excess in the data may be an indication of unresolved sources. Figure 11: Significance map of the inner Galactic Plane, using ~6 years of Milagro data. The strongest emission comes from the Cygnus Region, which runs from 65 to 85 latitude. The color scale has been set to a maximum of 7σ. The inset shows the Crab Nebula with the full color scale as an indication of the PSF. Boxes (crosses) indicate the locations of the EGRET 3EG (GeV) sources New TeV Sources Figure 11 shows a PSF-smoothed map of the Galactic plane, with the color scale indicating the statistical significance of the Milagro excess or deficit at each point. Eight source candidates are identified with a pretrial significance in the PSF-smoothed map of >4.5 σ. The PSF of Milagro is inset in the figure. A Monte Carlo simulation is used to account for the trials involved in searching this 3800 deg 2 region. This simulation predicts that 4% of such searches would result in at least one source with >4.5 σ pretrial 14

16 significance anywhere in the region due to background fluctuation. There are eight source candidates including the Crab. Excluding the Crab, Milagro has identified seven new candidates. The three most significant of these, MRGO , MGRO J and MGRO J , exceed 4.5 σ after accounting for trials. Therefore, the four most significant sources in Table 1 are considered definitive TeV gamma-ray source detections. The remaining four source candidates, labeled as C1 C4, have post trial significances of less than 4.5 σ and are regarded as lower confidence detections. Object Location (l, b) Counterpart Pre(Post)-Trial Significance TeV (x10-15 ) (/TeV/cm 2 /s) Crab 184.5, σ (14.3σ) 10.9±1.2 stat MGRO 75.0, 0.2 PWN G σ (9.3σ) 8.7±1.4 stat J GeV J MGRO 40.4, -1.0 GeV J σ (6.9σ) 8.8±2.4 stat J SNR G MGRO 80.3, 1.1 GeV J σ (4.9σ) 9.8±2.9 stat J C1 77.5, σ (3.9σ) 2.8±0.6 stat J C2 76.1, σ (2.8σ) 3.4±0.8 stat J C , 4.1 Geminga 5.1σ (2.8σ) 6.5±1.5 stat J C4 J , 2.0 GeV J Boomerang PWN SNR G σ (2.7σ) 3.5±1.2 stat Table 2: Milagro observations of Sources and Candidates in the Galactic Plane. Three new sources are discovered with >4.5 s significance after accounting for trials for searching 3800 square degrees. Four new candidates are also observed at low significance. One of these candidates is Geminga, the brightest GeV source in the Northern sky. MGRO J , is far enough south (6 o N) that it could be observed by HESS. They observed the region for 10 hours and found a 9σ excess consistent with the Milagro location. The energy spectrum measured by H.E.S.S. is shown in Figure 12 and the differential photon spectral index is Also, shown is the quoted Milagro flux at 20 TeV where we reported all our measurements. Since the source is at a large zenith angle Milagro s median energy for this source is ~50 TeV. With hard cuts spectral measurements can be made with median energies 60 and 90 TeV Figure 12 The spectrum for MGRO J as measured by HESS and Milagro. The points at a median energy of 60 and 90 TeV are preliminary results. as shown in the figure. These measurements (while preliminary) show that the spectrum of this source continues above 90 TeV without a substantial cut-off. They also represent some of the highest energy gamma-rays ever observed. 15

17 The Cygnus Region Several of these sources and candidates are in the Cygnus region. The Milagro location of MGRO J coincides with the EGRET source 3EG J as well as HEGRA TeV (recently also reported by VERITAS). The HEGRA source has a flux approximately 1/3 of the Milagro source as measured by HEGRA up to ~10 TeV. This discrepancy between Milagro and HEGRA may be understood as a possible diffuse component underlying the HEGRA identified point source that makes a greater contribution to the Milagro detection due to Milagro s angular resolution of ~0.5 degrees as compared to HEGRA s of ~0.1. Recently the Tibet air shower experiment reported three hot spots in the Cygnus region which also coincide with the Milagro excesses. These are shown as the black dots in Figure 13 along with their significances. Geminga Geminga is nearby pulsar at only 170 pc from Earth that may be responsible for the Local Bubble -- a cavity of hot, low density gas on the edge of which the solar system is located. Geminga Figure 14 Geminga Milagro sees 5σ at the spot at the EGRET source location. was produced in a supernova explosion approximately 300,000 years ago when it was located 60 pc from Earth. The pulsar's wind produces a nebula that is resolved in X- rays clearly showing two tails due to the bow shock of the pulsar's motion. Milagro's detection is coincident with the pulsar location, but is extended primarily in the direction of the pulsar's past motion; however, more statistics are required to clarify the shape of the TeV emission. (See Figure 14) Due to the large extent, this will be a difficult target for atmospheric Cherenkov telescopes. HESS has resolved several pulsar wind nebulae that also show the feature of a larger extent in TeV gamma-rays than in X-rays. This extent is used to argue that different energy electrons produce different energy photons. MRK 421 Mrk 421 is the closest TeV blazar with a red shift of Numerous multi-wavelengths campaigns have been conducted where TeV, X-ray and longer wavelength instruments have simultaneously observed Mrk 421 for periods from hours to weeks. These observations have established a fairly loose correlation [33] between the X-ray and TeV flux of these sources. However, orphan TeV flares (flares with no X-ray counterpart) have also been observed from AGN [ 34 ]. These orphan flares are scientifically interesting because they hint at hadronic acceleration in the source because TeV photons from electrons should be correlated with synchrotron emission of lower energy photons. For durations longer than 2 weeks, ACT s are not well suited because they can only operate during moon-less nights and during the half of the year when the source is up at night. Milagro operates continuously during the day and the night and is not affected Figure 13: Milagro s observation of ~200 square degrees in the Cygnus region of the Galaxy. The color scale indicates pretrial significance. Milagro sources and source candidates are named, while the black circles and significances indicate hot spots from the Tibet Air Shower Array. Figure 15: Light curve for Markarian 421 as seen by Milagro (in black) and the RXTE ASM (in red) using a 64-day time scale. Good correlation is seen between the X-ray and TeV fluxes, though a possible orphan TeV flare is seen at JD

18 by weather. While the sensitivity of Milagro for short duration observations is considerably less than IACTs such as VERITAS or HESS, it is well suited to study long duration variability. The Milagro detector can also measure the long duration bolometric fluxes of AGN and study the correlation with X-rays by combining the Milagro observations with those of the All Sky Monitor (ASM) on board the RXTE satellite. The results are shown in the Figure 15. One exception to this is the 2.5 Crab TeV flare that occurred at JD The feature is the largest excess for the entire 6 year observation period, but the X-ray flux is measured to be in a low state. This single point has an excess that corresponds to 5 σ not accounting for trials. This flare occurred during the summer of 2005 when Imaging atmospheric telescopes were not able to observe this target, because it was not up at night. 3.4 Cosmic-Ray Anisotropies Milagro has observed cosmic-ray anisotropies on both large (~60 ) and intermediate (~10 ) scales with high significance. On large scales, Milagro has seen a deficit centered at RA 180 with a fractional strength of ~3x10-3, which is in agreement with results from other experiments [35]. On intermediate scales, Milagro has observed two unexpected regions of excess with a fractional strength of ~5x10-4 at RA 70 and at RA 125, as seen in Figure 16. Diagnostics for the region at RA 70 show that the excess is strongly inconsistent with gamma rays as well as with the normal cosmic-ray background. Instead, it appears to be due to cosmic rays with a spectrum harder than -2.75, yet it is difficult to explain how charged particles can produce such a localized excess due to the deflection by interstellar magnetic fields. Potential systematic causes such as seasonal variation, changes to the detector, and underestimation of the background have been studied and excluded. Figure 16: Significance map in standard deviations of the background of the sky made with no gamma/hadron cut and 10 smoothing. The Cygnus Region is visible at RA 305, Dec 40. The dominant features at RA 70 and RA 125 are unexpected cosmic-ray anisotropies of an intermediate scale. The excess at RA 70 appears to have a harder spectrum than the background. 3.5 Other Physics Results from Milagro Milagro made a number of important observations. Space limitations require that we only give a very brief description here, but more information on each of these subjects can be found in the references and on the web at The Crab o Milagro was the first water Cherenkov EAS detector to observe the Crab [36] The Galactic Plane o Milagro made the first detection of TeV emission from the Galactic plane [37] GRBs o Milagrito made an observation of GRB a[38] with a post-trial probability of 1.2x10-3 o Milagro set many limits on GRB emission including strong limits on GRB [39] Solar Physics o Milagro has made high precision risetime measurements of energetic solar particles for a number of CME events including the one on 20 Jan [40] Dark matter o Milagro has set limits on WIMP annihilation near the Sun [41] 17

19 3.6 Previous Activities of US HAWC Collaborators Name Previous Experiments Role in Previous Experiments University of Maryland J. Goodman Milagro/IceCube/Super-K PI/Spokesman, Project Management, DAQ, /Software, Deployment / DAQ A. Smith Milagro Analysis, Software, Simulation, Data handling G. Sullivan IceCube/Milagro/Super-K Level II man., Software / Triggering / Atmospheric Analysis D. Berley Milagro Analysis R. Ellsworth Milagro/IceCube/Super-K Analysis, Simulations, IceTop V. Vasileiou Milagro/CAST Student, Simulations, GRB Analysis Los Alamos National Laboratory G. Sinnis Milagro/HIRES Spokesman, Project Management, Analysis, DAQ / Operations B. Dingus MILAGRO/GLAST/EGRET Project Management, Operations, Analysis Pennsylvania State University T. DeYoung IceCube/Milagro/AMANDA Online software, Simulation, AMANDA-IceCube integration, Analysis Michigan State University J. Linnemann Milagro/D0 Gamma/hadron separation; D0 Level 2 and Level 3 Trigger leader; D0 databases; statistical methodology University of California Irvine G. Yodh Milagro Analysis/Energy Reconstruction/Hadron studies/outriggers University of Utah D. Kieda VERITAS/Whipple/HIRES Construction, Electronics University of New Hampshire J. Ryan Milagro/COMPTEL Solar Physics University of New Mexico J. Matthews Auger Calibration NASA, Goddard Space Flight Center J. McEnery Milagro, GLAST, Whipple GRB Analysis, Analysis Software Table 3 Previous experience of US Collaborators 4 HAWC Technical Design The HAWC design builds upon our experience with the Milagro detector. Milagro is the first large, uniformly instrumented, air shower array using water Cherenkov technology. The Milagro pond is instrumented with 2 layers of PMTs, a shallow layer for triggering and shower angle reconstruction and a deep calorimetric layer used for hadron rejection. Surrounding the central pond is an array of plastic, outrigger tanks (1 m deep by 3 m diameter) used for core position and shower angle reconstruction. In contrast, the HAWC design utilizes a single deep layer of PMTs with wider separation than used in Milagro. This configuration gives HAWC a much larger active area than Milagro for the same photocathode area. HAWC will re-use the 900 8" Hamamatsu PMTs from Milagro, and deploy each PMT in a 4.6 m deep by 5.0 m diameter commercial plastic water tank. The tanks will be deployed in a dense pattern that provides more than 75% coverage of the 150m x 150m instrumented area. Each tank will contain an 8" baffled upward-facing PMT anchored to the bottom. Figure 17 shows the proposed deployment pattern. Figure 18 shows a single tank cross-section diagram as visualized in GEANT4. 18

20 A critical improvement between the Milagro pond and the HAWC design is the optical isolation of the PMTs. In an open design such as Milagro's, triggering at low thresholds is complicated by the high rate of single muon events, which penetrate the detector and illuminate the entire reservoir with Cherenkov radiation. The Cherenkov photons from single muons are difficult to separate at the trigger level from low energy air showers. The two-layer design reduces the problem of single muons by positioning a layer of baffled PMTs near the surface that only observes the volume of water directly above and is relatively insensitive to muons. However, this shallow layer is a poor calorimeter, as it is close to the surface of the water and thus receives different light intensity depending on the EAS particle s distance from the PMT. Therefore in the Milagro design, an additional deep layer is required for calorimetry. This calorimetry is essential for distinguishing the brighter penetrating particles in a hadronic shower from the electromagnetic particles in a gamma-ray initiated shower. Rather than using two layers of PMTs, we can have a calorimetric Figure 17: Proposed layout for HAWC tanks. The detector by counting house is in the center of the array. Row pairs optically isolating are slightly separated to allow for cabling and drainage. the PMTs. This has a number of advantages. First, a single deep layer can be used for triggering, angular reconstruction, and calorimetry because single particles can at most illuminate a few PMTs. Also, because the PMTs are deeper, they can view a larger volume of water since the observable water volume is roughly o defined by the 41 Cherenkov cone above the PMT. This larger spacing between PMTs gives a greater total detector area. HAWC is designed with a 5m spacing compared with a 2.8 m spacing for Milagro. The HAWC 2 single layer forms a high efficiency detector with an area of 22,500 m compared to the deep and shallow layers of Milagro which cover only m and 3528m respectively. Finally, optically isolating the PMTs Figure 18: 5m diameter tank as simulated in Geant4 for a attenuates scattered light. Photons that are not promptly detected are single vertical muon. The # of efficiently absorbed by the tank. This reduces the late tails in the PMT photons are reduced by a photon timing distributions and reduces the noise rate in the PMTs. factor of 50 for visualization. Optical isolation will reduce the noise rate in HAWC PMTs to that of the Milagro top layer, ~20 khz, despite the higher elevation. The interior of the tanks is black plastic to minimize late light from reflections. In contrast, the Milagro outriggers are lined with white Tyvek to maximize light collection. We have found that while lining the interior of the tanks increases the photon count, it compromises the time-over-threshold method for pulse amplitude measurement because additional delayed hits lengthen the PMT pulses. Also, the timing of the shower front is not as accurately measured because the pulse amplitude is used to correct the timing for slewing effects. We previously considered a HAWC design more similar to Milagro with a lined reservoir and curtains separating the PMTs. The reservoir could be made light tight by using a floating cover or enclosing it with a building. With LANL funds, we commissioned an engineering study that examined the complete costs and risks of each option. The complete Figure 19: HAWC prototype at the Milagro site with technician, Scott Delay. 19

21 report can be found on the HAWC proposal web site The conclusion was that a floating cover is more robust and less expensive than a building by about 10%, but the installation and repair of PMTs would be difficult. A building was the most expensive and riskiest option, requiring a completely light tight environment despite daily thermal changes and corrosion resistant materials to prevent contaminating the water. The option of using 900 individual tanks was considered as an addendum to the engineering study when we learned of the additional costs and risks associated with the pond. The engineers found no difficulties with the array of tanks and calculated the costs associated with flattening the required area, providing a drainage system, and installing the tanks. These costs, including the tank costs, are less than that of a covered pond. Additionally, the tank based design was found (via Monte Carlo simulations) to achieve approximately the same sensitivity as a function of energy as the pond. Furthermore, the tank design has the following advantages: 1) Science. Deployment and operation can begin immediately after the site is prepared. HAWC will achieve ~4 times greater sensitivity than Milagro with the deployment of only 1/3 of the array (2nd year). 2) Water. The single-pond design would have us fill the pond after construction is complete. With the tank design, we can add water incrementally, as we add tanks. This flexibility allows us to explore more economical ways to provide water to the site. 3) Risk. The solution is scalable, so cost risks can be absorbed by adjusting the number of tanks. 4) Flexibility. The tanks allow us to reconfigure the detector to attack different scientific goals. A larger array with a less dense core would increase the area at the highest energies, or more than 1 PMT could be placed in the tanks in a central area to increase the sensitivity at low energies. 4.1 Tank Design The cylindrical tanks are constructed from opaque polypropylene, similar to Milagro s outrigger tanks, and are used for commercial and residential water storage. The tank walls are ~1 thick. Each tank weighs ~2000 pounds and can be deployed easily with a forklift. The tanks are 4.6 m high and 5 m in diameter. The PMTs with the encapsulated base are 34 cm high, so the total amount of water covering each PMT is ~4 m or ~10.5 radiation lengths. When full, the tanks will each contain ~84,000 liters of purified water. A test tank has been installed at the Milagro detector as seen in Figure 19. The test tank has a diameter of 3.6 m and is slightly shorter than the tanks specified in the design, but serves as a suitable prototype. The single photo-electron rate of the PMT is ~10 khz, consistent with predictions from Monte Carlo simulations. The single muons that self trigger the PMT in the tank have an average of ~30 photoelectrons, also consistent with Monte Carlo simulations. We do not anticipate problems with the HAWC tanks freezing. The top few inches of the Milagro outriggers do freeze each winter. The ice changes the reflective properties of the surface, which causes more internal reflection and increased single photoelectron rates, but has a negligible effect on the detection or reconstruction of extensive air showers. Furthermore, the Milagro site is much colder than the HAWC site. We will monitor the test tank in the next year to understand how these large tanks behave in cold weather. 4.2 Water Delivery System The project will require ~80 million liters of fresh filtered water. We have budgeted for the water to be supplied from a well located 12km from the Sierra Negra site. A series of pump stations will bring the water up the mountain. We will also pursue the possibility of obtaining the water from a well adjacent to the HAWC site. An aquifer that is fed from glacial runoff has been identified in a geo-electric study performed in We plan to dig a test well to verify the availability of water. Should the test well prove productive, the cost of providing the water needed for the project could be substantially reduced. We will also investigate the collection of runoff from the ~100 cm/yr of annual rainfall at the site and in nearby drainages. The water will be filtered and staged in undeployed tanks and then transferred to empty tanks after they have been positioned and instrumented. The Milagro detector currently uses smaller tanks that are 1 m high and 3 m diameter that are also made of polypropylene as outrigger detectors. The water in the outriggers was filtered on the initial fill, but not subsequently. The cold and dark environment of the outriggers has resulted in no significant degradation of the water clarity in the outriggers over 3 years of operation. However, the larger tanks require longer attenuation lengths and we have budgeted for the 20

22 HAWC tanks to be filtered. The water clarity in the prototype tank at the Milagro site will be monitored using a UV LED to understand the degradation of the water with time. Elements of the Milagro water purification system will be used for HAWC with the same filtration setup and a capacity of 760 liter per minute. A carbon filter precedes a series of progressively smaller filtration stages from 10 microns to 1 micron to.3 microns. The water is sterilized using a UV light source. This system will be used to fill the tanks initially as well as allow us to filter 60 tanks at a time with 12 liter per minute for 5 days before switching to the next tank array (total of 15). In addition, the filtration system provides cooling for the electronics. A 40 liter per minute pump will be used by two heat exchangers with a combined cooling capacity of 140kW. 4.3 Deployment and Cabling Prior to deployment, the entire 150m x 150m instrumented area will be graded and prepared, removing large rocks and any other obstructions. An over-excavation of 0.5 meters is planned to allow burying cables, and a storm sewer drainage system. The tanks will be placed in rows with a 1 m gap between every other row. Beneath the 1 m gap will be the cables and water filtration pipes. The counting house will be located at the center of the array to minimize the cable lengths and reduce signal dispersion. The cables will all be the same length of ~200 meters which is also the same length as those in the Milagro pond. The cables will be buried in PVC cable trays to prevent daily heating and cooling of the cables to reduce the effect on the signal timing. As in Milagro, the HV and signal are carried by a single cable that will run from each PMT to a service box in the ground next to the tank. From there on it will run under ground to the lightning protection service box next to the counting house. The HV / signal passes from here through an individual grounded spark gap array. These lightning protection boxes have been successfully used for the Milagro outrigger array over many years and have prevented observed lightning strikes from damaging the front end electronics. 4.4 Instrumentation HAWC will reuse the Hamamatsu R5912 photomultiplier tubes, the bases and encapsulations from Milagro. A single RG-59 cable provides high voltage to each PMT and carries the high frequency signal back to the front-end electronics. In HAWC, the cable will be permanently attached to the PMT housing as is now done in Milagro, thus avoiding the problems that Milagro initially encountered with underwater connectors. The Milagro front-end electronics will be reused with minor modifications. These modules isolate and process the high frequency signals from the PMTs. The pulses are shaped and analog edges are generated at two discriminator levels, ~1/4 PE and ~5 PEs. These analog edges are subsequently digitized with multi-hit TDCs. The Time-Over-Threshold (TOT) method is used to measure both pulse arrival time and amplitude with a single multi-hit TDC channel. Additionally, the front-end boards provide summed trigger signals and direct access to the analog pulses for debugging and calibration. The high voltage for the entire experiment will be provided by a single supply with multiple channels. HAWC will trigger when ~30 or more PMTs are hit (~1/4 PE threshold) within 50ns. Simulations show that this gives a rate of ~8kHz. In Milagro the trigger window is 190ns, which we have found to be longer than optimal, so we will shorten the window for HAWC. The trigger pulses are generated in the front-end boards, which output an analog sum of the signals from 16 channels. In Milagro, the signals from each board are then combined using NIM electronics and this sum is discriminated to form a trigger. New trigger electronics will be built for HAWC that feed the trigger pulses from the front end boards into a custom VME trigger module that uses a programmed FPGA to provide the trigger logic. The new trigger will allow us to develop more complex triggering algorithms, to push the trigger threshold lower, and possibly perform gamma/hadron separation at the trigger level. For HAWC we plan to use the new CAEN model V1190A VME TDCs. These modules can handle the increased HAWC data rate as well as simultaneous digitization and read-out, thus eliminating the principal source of dead time that Milagro experienced with Fastbus TDCs. A number of these units have been purchased and will be evaluated in Milagro. A time stamp for each event is provided by a GPS clock that is latched by the trigger. The VME TDCs, GPS clock and trigger module will be housed in a single VME crate and read out into a single computer. As in Milagro, we will have a separate monitoring system to measure various parameters, such as voltage, weather, water depths and temperatures, and rates of individual PMTs. These parameters 21

23 will be read out every second and stored in a database. We will upgrade the Milagro system by using scalers in a VME based system, rather than CAMAC. The laser calibration system for HAWC is designed to provide the relative timing and pulse-height calibrations for the PMTs in the HAWC detector. The absolute energy scale will be determined using single muons passing through the detector which produce ~30 photoelectrons. The laser system is patterned after the calibration system in Milagro, but each tank requires a separate optical fiber. The proposed light source design builds on the present Milagro calibration system including the JDS Uniphase PowerChip NanoLaser operating at 532nm. The ~1ns light pulses pass through a variable neutral density filter to allow control of the light intensity over 4 orders of magnitude. The laser beam will be directed through a series of optical fan-outs to illuminate one half of the PMTs at any given time. This system will run continuously at a low rate and will be controlled remotely. 4.5 Online Computing and Operation The HAWC hardware will be read out using a single data acquisition computer. Software has been developed for Milagro to reconstruct the data in near real time by distributing it to an array (~4 computers) of client nodes. The stream of reconstructed events is then made available to online analysis programs that search the sky for transients, which can be detected within 5-10s of their occurrence. These clients also monitor the quality of the data. The design for the Milagro online software was utilized by the IceCube experiment for their online data processing and filtering system. The HAWC online systems will be based on the Milagro design, as extended and improved by IceCube. Data will be buffered locally on disk arrays. After initial filtering to remove obvious background events, the raw and reconstructed data will be transmitted via the Internet to two archive sites, one in the United States and one in Mexico. This operational model demands a high bandwidth network connection to the site of the experiment. Although we will archive the data at two sites, all collaborators will have complete access to both sites and will work together on a single data product. HAWC will utilize the robust and flexible C++ data analysis framework developed by IceCube for both online and offline data analysis. This common framework facilitates the rapid deployment of new reconstruction and filtering techniques for online processing at the site. Reuse of the IceCube software systems will leverage the considerable software investment made in the course of IceCube construction, permitting the deployment of a robust, well-tested system with considerably less effort and cost than would be required to develop and test a new system. Computers and disk arrays will be tested for operational reliability at the ~4000m elevation. (Most commercially available computers are certified to operate up to ~3000m.) At higher elevations, CPU cooling becomes an issue. This problem can be mitigated by providing either cooler air or more air flow. Hard disks can fail at high elevations because the read head uses the pressure of the atmosphere to float over the platter. Sealed pressurized drives are readily available for high altitude operation and will be used if necessary. HAWC is designed to be completely remote controllable. This is critical for reliable 24/7 operation of the detector. This operational model requires reliable power and a high bandwidth internet connection. All scientific collaborators are expected to take remote shifts, monitoring and maintaining the experiment. Shift-takers are required to review the operation of the experiment and the data quality daily and respond immediately to electronic pages (text messages) indicating problems. Repairs that require travel to the site will be performed by on-site technicians who maintain the experiment. This operational model is used by Milagro where ~95% uptime has been achieved. 5 The HAWC Site The HAWC site is inside the Parque Nacional Pico de Orizaba, a Mexican national park comprising Citlaltepetl or Pico de Orizaba, the highest peak in Mexico at 5610m, and Sierra Negra, a 4600m volcano 7km SW Citlaltepetl. The Large Millimeter Telescope (LMT) is located on top of Sierra Negra, and HAWC will be located on a 200m x 450m plateau near the saddle between the two peaks. The exact geographical coordinates of the site are latitude 18º59 41 N, longitude 97º18 28, altitude 4100 meters above sea level. The latitude of the Sierra Negra site provides an excellent visibility of celestial objects: HAWC will see 15% more of the celestial sphere within a 45º field of view compared to Milagro. When considering a cone of 45º the survey solid angle reaches 8.4sr, or 2/3 of the entire sky. The Crab culminates at 3º from the zenith and will be visible for slightly more than 6 hours each day; Cygnus 22

24 reaches 20º zenith angle and the Galactic plane will be covered such as to include over a dozen of the VHE sources observed by HESS. Even the Galactic Center, at 48º from zenith, will be observable at the highest energies. The coverage of HAWC will have a 90% overlap with that of IceCube. The longitude of the site is also favorable, as its visibility has good overlap with observatories in the US, Mexico and Chile, which can be promptly alerted of any interesting activity and can aim to simultaneous observations of objects in the field of view of HAWC. 5.1 Climate High sites with tolerable conditions are scarce. The site is located close enough to the equator to have weather conditions as benign as could be wished for its altitude. Weather conditions have been monitored for over six years at the summit of Sierra Negra, 500 meters above the HAWC site with a horizontal distance of 1km. The median temperature (adding the 6.5º thermal gradient to the 4600m measurements) becomes 4.3ºC for the site, with sub-zero temperatures only 5% of the time (specifically 10% of the time during winter). Water freezing inside the detector will not be an issue. Wind velocities are generally mild, with a median of 4 m/s for the recorded data. Occurrence of wind above 10 m/s is rare. Still, the recent passage of a hurricane Dean some 100 km North of the site provided winds up to 150 km/h, the largest measured in the 6 years of meteorological monitoring. The Large Millimeter Telescope has been designed to withstand winds up to 250 km/h, equivalent to a hurricane of category 5, which is not considered possible for a site over 4km high and 100 km inland. 5.2 Site accessibility The coordinates of the HAWC observatory are defined tentatively, with over 200m of freedom in approximately the EW direction, as the northern base of the Sierra Negra can fit a square of (200m) 2 in more than one location. An even a larger rectangle of 450m 200m can be placed between two 20m topographic contour levels. The HAWC site is located just over 2 hours from Puebla, a city of 2 million inhabitants with a relatively small international airport currently in expansion. Puebla itself is 2 hours by road from either Mexico City (to its East) or the HAWC site (to its West). Most of the distance between Puebla and the LMT HAWC site is through the Puebla-Veracruz motorway, with the last 40 minutes on minor roads. Veracruz is a major international port within 2.5 hours drive to the site. The LMT project required an access road wide enough to allow transportation of items up to 6m wide. Electricity and internet have been installed up to the top of the mountain. The road and infrastructure would need to be extended 1 km to reach the HAWC site over mostly flat terrain. 5.3 The Large Millimeter Telescope The Large Millimeter Telescope (LMT) is the largest scientific project ever undertaken in Mexico(120M$) and was constructed by a joint US/Mexico Collaboration. LMT is a single dish 50 meter telescope for millimeter-wave astronomy located at 4600 meter and due to operate in the frequency range of 80 to 350 GHz. Sierra Negra was the highest of close to twenty candidate sites monitored for water vapor content in the atmosphere and was selected as the LMT site in February Construction of the telescope began in 2000, with the antenna inaugurated by President Fox in November The surface of the telescope is presently being completed, set and tested while the LMT scientific instruments prove their performance in other telescopes. The construction of the LMT required the development of the site infrastructure. The electric grid was extended 13 km to reach the LMT site to supply up to 1 MegaWatt of power during operation, with a potential peak supply of 5 MW. The road was constructed mostly during to be able to allow pieces up to 6m wide and has been continuously improved. A fiber optic Internet line running parallel to the electric power has been set and is currently kept to just 2 Mb/s, but will be expanded once LMT enters the operation phase. The LMT installations are already able to lodge scientists in oxygen enriched areas. However, staying at the site is not encouraged and a base camp will be set at a lower altitude. 5.4 Sierra Negra Consortium The construction of the LMT and the development of the Sierra Negra site brought the opportunity for other instruments to benefit from the high altitude site. Nine such facilities are in different stages: - the Telescopio de Neutrones Solares in a solar neutron telescope installed by the Instituto de Geofísica 23

25 of UNAM and in operation since It detected a major solar event in September RT5 is a 5m radio telescope in construction, due to perform daily monitoring of the Sun at 43 GHz during daytime and astronomical observations during nighttime. It can also function as test-bed for LMT instrumentation. RT5 is a joint project of INAOE and the Institutos de Astronomía and Geofísica of UNAM. - two Cerenkov telescopes formerly part of the HEGRA array will be installed at the top of Sierra Negra, at about 1 km horizontal distance from HAWC. These will monitor blazars during the GLAST era and can also complement HAWC observations. - the Instituto de Física of UNAM is to build an antineutron detector. - the University of Puebla (BUAP) has set an array of cosmic ray detectors on the top of the mountain. These are small water tanks with individual PMTs. BUAP is also setting an array of larger tanks in the slope of Citlaltepetl to be complemented with a fluorescence detector at Sierra Negra. - non astrophysical facilities include a seismological station from BUAP already operational, a greenhouse gas monitor from the Climate Institute and a geo-reference point from INEGI, the latter two in planning stage. Figure 26: at left, Google Earth image of the Parque Nacional Pico de Orizaba showing the Citlaltepetl, with snow, and Sierra Negra SW of it. At right we show a map from INEGI, with 20m topographic contours and the UTM grid indicated in blue. The blue square indicates the predetermined location of HAWC and its 2 dimensions, with the dotted rectangle covering 90,000 m. The red dot is the nearest point to the LMT road, electricity and internet. Together with HAWC and LMT, these facilities are members of the Sierra Negra Consortium (CSN, for its abbreviation in Spanish), a non-profit organization charged with organizing the joint operation of the site. The CSN will act as a provider of common services like site access, electricity, Internet, communications (with special consideration to preventing RFI to the LMT), water supply, security (there is a gate house with a 24 hour a day guard), etc. and the respective maintenance; in exchange the consortium members will cover their share of the operations cost, according to the location and characteristics of each experiment. 5.5 Site permission The HAWC site is inside a National Park, with the land formerly owned by the Figure 27: Diagram facing North showing the position of HAWC (yellow square), the path of descending water, mostly underground (blue) and two zones of water convergence in brown. The exploration well is to be drilled East of HAWC. Regarding the water convergence points only the Eastern one is considered for water acquisition. 24

26 Mexican Federal government. Permission for using the site can be granted by the Secretaría del Medio Ambiente y Recursos Naturales (SEMARNAT), the federal body in charge of the environment. An environmental impact declaration for the installation of HAWC in the Parque Nacional Pico de Orizaba was submitted to SEMARNAT on June 2007 and a conditioned permission was granted in September The permit allows a construction phase of three years and up to ten years operation, allowing the installation of the experiment, its peripheral infrastructure and water acquisition systems. A separate permit is required for the construction of the 1km access road and power line, which we plan to apply for by November. The conditions imposed to the project through the SEMARNAT permit are directed to minimize the environmental impact and to compensate it through a reforestation program related to the HAWC project. The permit is online at the HAWC proposal web site. (a) (b) 5.6 Water availability This proposal includes funding to pump water for HAWC from a nearby valley. However, given the location and the reasonably high precipitation in the region (100 cm/year with a marked seasonal dependence), we will likely be able to acquire the water locally, through a deep well nearer to the HAWC site or a water capture system. A 3D topographic model of the region was constructed to model water flows in the vicinity of the site and was complemented with geo-electrical studies to determine the most suitable location for an extraction well. The demands on the capacity of the well are reasonable: considering the volume of water required for the 900 tanks the demand is only 2.4 liter/second for acquiring the water within one year using this method alone. Extending the time for water acquisitions relieves the demands on the well with the same proportion. We already have defined the position for a test well and plan to perform its drilling in the next month. Depending on the water extractable, the test well can be expanded to a full extraction well. This will require widening its diameter, installing a high power pump and about 2km of pipes to transport the liquid to the HAWC observatory. A second water acquisition option is a capture system placed below a convergence point, a natural nozzle, of water running down Citlaltepetl Sierra Negra, which was identified some 7 km WNW from the the HAWC site. This capture system will be a large concrete parallelepiped, which would be particularly efficient during the rain season (May-October), while the well would provide water in a more continuous manner. (c) Figure 20: Illustration of the sensitivity of HAWC and Milagro as a function of primary gamma-ray energy. Panel (a) shows the effective area of HAWC and Milagro. HAWC and Milagro have similar effective area at high energies, but HAWC has a ~4x lower energy threshold, principally due to the higher elevation. This results in more than an order of magnitude increase in effective area below a few TeV. Panel (b) shows the angular resolution of Milagro and HAWC vs. energy. HAWC s angular resolution is about twice as good as Milagro except at the highest energies. We believe that the flattening of the HAWC angular resolution at the highest energies is limited not by the instrument, but by the systematic errors in the reconstruction software which were developed for Milagro and have not been re-optimized for HAWC. Panel (c) shows the efficacy of the γ/h rejection for Milagro and HAWC. Plotted here is the efficiency for hadron events passing a cut at a level that passes 50% of the gamma-ray events. On average, HAWC passes 10x less background. 25

27 6 Detector Performance The simulation for HAWC is an extension of the Milagro simulation [42] software package. CORSIKA [43] is used to simulate gamma-ray and hadron induced atmospheric showers. A custom detector simulation using GEANT4 [44] is used to propagate the secondary shower particles that reach the detector elevation through the HAWC detector. Cherenkov light production is simulated and individual Cherenkov photons are tracked through the detector. Detailed optical modeling of the water (absorption and scattering), reflection and absorption at surfaces, and the PMT response are included. The simulation has been thoroughly tested through comparison with Milagro data. The Milagro electronics utilize the TOT method for pulse amplitude estimation. The response of this system is simulated by generating a pulse waveform for every detected photon. These simulated pulses are then digitized and converted to amplitude and timing measurements for use by the reconstruction software. In this section, we describe how the sensitivity of the HAWC detector is determined. We describe the detector simulation and show that the sensitivity of HAWC is ~15 times that of Milagro. We describe in detail the performance of the HAWC detector with 900 5m-diameter water tanks each instrumented with a single 8 PMT as the standard HAWC design. We then show how the sensitivity of HAWC would improve with the expansion of the detector coverage or the increase in the number of PMTs in each tank. Details of the sensitivity estimation can be found on the HAWC web site: Although we are able to dead reckon the rate of hadronic background events in Milagro to within ~40% of the flux as measured by high altitude balloon experiments [45], [46], we do not depend on dead reckoning to estimate the background rates in HAWC. Instead, gamma-ray and background rates are scaled from measured values in Milagro by comparing the predictions of the HAWC and Milagro simulations. In this way we not only remove potential systematic errors internal to the simulation from the air shower modeling, optical model, and detection efficiency, but also remove systematic errors in the measurement of gamma-ray fluxes and hadronic backgrounds provided by other experiments. 6.1 Detector Optimization The depth and spacing of the PMTs was optimized for gamma-ray sensitivity from TeV. The detector must act as an effective calorimeter in order for the background rejection methods to work properly, so the PMTs need to be sufficiently deep that electro-magnetic (EM) particles are unable to pass close to the photo-cathode and produce large pulses that are not proportional to the deposited energy. We have found that this requires at least 3.5m of water (~9 radiation lengths). However, if the PMTs are too deep, the sensitivity (PEs/GeV) is significantly reduced. At the selected depth of 4m (water above the photo-cathode), HAWC detects ~20 PEs/GeV for EM particles and ~30 PEs for through-going muons. The optimal radius of the tanks is determined by the Cherenkov angle in water. The illumination of EM particles is found to be roughly uniform over an area with a radius of 0.75 x depth. This dictates that for a tank of depth 4m, a radius of ~<3m is optimal. There is no scientific advantage to making the tanks smaller, but additional sensitivity to low energy showers can be achieved by increasing the photocathode density by placing more PMTs in each tank. This is discussed in detail at the end of this section. 6.2 Event Reconstruction In HAWC, as in Milagro, the primary source of PMT noise is secondary gamma rays, electrons and muons from low-energy hadronic cosmic ray showers. Uncorrelated noise rates set the limit for the trigger threshold. In Milagro, the noise rate is ~20 khz in the top layer and ~50kHz in the bottom layer. The rate from dark noise and ambient radioactivity is only ~2kHz. We have simulated EAS showers from primary hadrons with energies as low as 5 GeV in Milagro and HAWC. Despite the higher elevation the single PMT hit rate for HAWC will be approximately equal to the top layer of Milagro, ~20kHz. This relatively low noise rate is due to the optical isolation of the tanks in HAWC, compared with the open design of Milagro. A single particle in HAWC will only illuminate a single PMT, where in Milagro many PMTs can be hit. The total non-correlated hit rate for the entire detector will be ~20MHz, or ~1 PMT hit/50ns. We anticipate triggering on showers that produce >~30 PMTs hit within a 50ns window and anticipate no difficulty achieving this threshold. We estimate the trigger rate in this regime to be ~5-10kHz, about 3-6 times the trigger rate of Milagro. However, the absence of the second layer reduces the average event multiplicity by about a factor of ~2, so the data rate in HAWC will only be ~1.5-3 times Milagro, or Mbytes/s (uncompressed). This rate will be easily accommodated with the upgraded VME TDC based DAQ. 26

28 To reconstruct the direction of atmospheric showers in HAWC we first determine the shower core by fitting the distribution of pulse amplitudes to a standard lateral distribution profile. After the core is located, the PMT hit times are adjusted to account for the curvature of the shower front. Typically, the shower front curvature correction is ~0.5 o -1.0 o, so misidentification of the core position leads to degraded angular resolution. The corrected PMT hit times are then fit to a plane to determine the incoming shower angle. The width of the distribution of timing residuals ranges from ~1-3ns depending on the pulse amplitude. Figure 20(a) shows the effective area of HAWC and Figure 20(b) shows the angular resolution of HAWC. We find that the angular resolution of HAWC reaches a minimum of ~0.25 o above 5 TeV. One would expect that the angular resolution would improve as the energy rises. The flattening is a consequence of systematic errors in the parameterization of the curvature correction as a function of energy. The curvature correction used here was optimized for Milagro and has not been re-optimized for HAWC. As we improve the reconstruction algorithms we expect the angular resolution to improve, however, for estimation of sensitivity, we conservatively characterize the angular resolution as depicted in Figure 20. Notice that HAWC has a substantial effective area well below the nominal threshold. This is a phenomenon that is not unique to HAWC, but common to all EAS gamma-ray detectors. The longitudinal Figure 21: Gamma/Hadron rejection capability of HAWC at three thresholds. The gamma-ray efficiency is shown in blue and the proton efficiency is shown in red. The capacity of HAWC to reject hadronic background increases with energy. shower profiles of electromagnetic showers of different energies have the same shape after shower maximum. In this regime, the energies of the particles in the shower have dropped below the critical energy (the energy where the cross-section for pair production and Compton scattering are equal) and the number of EM particles in the shower begins to diminish. Past shower maximum, the total energy carried by high energy particles is reduced by a factor of ~1.65 for each radiation length. Therefore, if a primary gamma ray penetrates one radiation length deeper than average prior to its first interaction, the result will be a ~1.65x increase in the energy observable at ground level. Thus showers with energies below the nominal energy threshold can be detected when the primary gamma-ray penetrates deeply into the atmosphere before interacting. In order to be detectable below the nominal threshold energy E thr, a gamma-ray of energy E must penetrate additional radiation lengths N before interacting, and N is simply N = ln(e/e thr )/ln The probability P that the gamma ray will penetrate N radiation lengths before interacting is P=exp((-9/7)N). Combining the two expressions gives P(E) ~ (E/E thr ) 2.6 Thus the effective area below the effective threshold should scale like a power-law with index 2.6 which is indeed reproduced by the detailed Monte Carlo simulation seen in Figure 20 (a) for both HAWC and Milagro. 6.3 Hadron Rejection Hadronic showers are identified through the pattern of energy deposition in the detector. While gamma-ray induced showers have compact cores with smoothly falling lateral density, hadronic showers typically deposit large amounts of energy in distinct clumps far from the shower core. This is due not only to the presence of hadrons and muons in hadronic showers, but also clumps of EM energy far from the core caused by high P T hadronic interactions in the development of the atmospheric shower. As a simple 27

29 gamma/hadron discriminator, we have extended the compactness parameter, C, developed for Milagro 47. Here C is defined as the total number of PMTs hit divided by the largest pulse amplitude that is more than 40 m from the reconstructed core position. Gamma ray induced showers have only small hits far from the core and therefore have large values of C. Hadron induced showers with muons and hadrons and multiple clumps of EM energy have low values for C. Figure 21 shows Compactness distribution for gamma ray and hadron triggers for three different energies. The background rejection capability of HAWC improves with increasing energy. Figure 20(c) shows the efficiency for protons passing the gammahadron cut for HAWC when the gamma-ray efficiency is fixed at 50%. At similar energies, HAWC can reject hadronic backgrounds ~10x better than Milagro. Figure 22: Comparison of gamma-ray and proton induced events. We show here the charge deposition profile of 4 gamma-ray events (top row) and 4 hadron events (bottom row). The color scale is set to show number of PEs divided by the total number of hit PMTs. The label on the color scale shows the inverse, (nhits/pes) for each PMT, which is the compactness variable in Milagro. Events with red hits outside the core region (indicated by the circle) are rejected by the compactness cut. Figure 22 illustrates the hadron rejection capacity of HAWC. The top four panels show typical gamma-ray induced events and the bottom four panels show proton events. The hit amplitudes in the array are indicated by the color scale. The large black circle indicates the position of the fit core and the radius of the circle defines the exclusion region for the compactness cut. All of the proton events have large hits outside the shower core where none of the gamma-ray events do. At high energies there are many independent hits outside the shower core that would lead to the redundant rejection of the proton events. All of the simulated proton events shown here would not pass the gamma-ray cut (C>6-8). Note that the area enclosed by the circle around the shower core is roughly the same as the area of the calorimetric layer of the Milagro detector, so this core exclusion method can not be applied to Milagro. It is important to point out that despite simulating hundreds of millions of atmospheric showers, the available statistics in our current simulated data set at high energies are insufficient to reliably predict the background rejection (i.e. no simulated proton events with energies above ~50 TeV survive the cut criteria). HAWC may be capable of rejecting nearly all-hadronic background above TeV, but at this time we are unable to simulate enough high-energy background events to demonstrate this. The gamma/hadron capability shown here should be regarded as conservative. Further study will likely reveal substantial improvement. 6.4 Energy Resolution The energy resolution of EAS arrays is in general poorer than that of IACTs. IACTs are able to directly sample the entire shower as it develops in the atmosphere, so the detected light at the surface is approximately proportional to the energy of the shower. For EAS arrays with nearly 100% coverage of the ground, such as Milagro and HAWC, the total energy carried by the particles reaching the ground level can be accurately estimated (as indicated by the red line in Figure 23), but because only the longitudinal 28