- Email: [email protected]
The High Altitude water Cherenkov (HAWC) Observatory
Available online at www.sciencedirect.com
Nuclear and Particle Physics Proceedings 279–281 (2016) 87–94 www.elsevier.com/locate/nppp
The High Altitude water Cherenkov (HAWC) Observatory R.W.Springer a, , for the HAWC Collaboration† a
Department of Physics and Astronomy , University of Utah, 115 S 1400 E, Salt Lake City, UT 84112, USA
Abstract HAWC is a continuously operated, wide field of view detector comprised of three hundred 188,000 liter water Cherenkov detectors, each instrumented with four photomultipliers providing charge and timing information. HAWC covers approximately ~22,000 m2 at an altitude of 4100m and reliably estimates the energy and arrival direction of gamma and cosmic rays with significant sensitivity over energies from several hundred GeV to several hundred TeV. With an instantaneous field of view of 2 steradians, HAWC observes 2/3 of the sky in 24 hours. HAWC has been optimized to study transient and steady emission from both galactic and extragalactic sources of gamma rays and serves as a survey instrument for multi-wavelength studies. HAWC has significant discovery potential, including the possibility of indirect detection of dark matter through the observation of gamma rays produced via dark-matter particle annihilation. HAWC has been making observations since summer 2012 and officially commenced data-taking operations with the completed detector on March 20, 2015. This paper will describe the detector design, science capabilities, first scientific results and future plans of the HAWC observatory. Keywords: gamma-ray astronomy, instrumentation
1. Design of the HAWC Observatory
Figure 1 Aerial photograph of HAWC observatory
1.1. Observatory Site The High Altitude Water Cherenkov (HAWC) observatory, shown in figure 1, is a continuously operated, wide field of view detector principally designed to observe astrophysical sources of gamma rays. HAWC, is based upon the technology developed by the Milagro detector [1] that uses the Earth’s atmosphere as a calorimeter to sample extensive air showers (EAS) produced by the primary gamma or cosmic ray. The measurement of the energy and arrival direction of gamma rays, as well as cosmic rays, impinging on the Earth’s atmosphere is achieved by sampling the resulting extensive air
———
R.W. Springer.; e-mail: [email protected]. †
http://www.hawc-observatory.org/collaboration/
http://dx.doi.org/10.1016/j.nuclphysbps.2016.10.013 2405-6014/© 2016 Elsevier B.V. All rights reserved.
showing the three hundred 7.3m diameter 4.5m deep water tanks and the electronics hut and water treatment facility.
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showers using an array of 300 closely spaced water tanks. This array covers an area of approximately 22,000m2 and is located at an altitude of 4100 m on Sierra Negra Mountain in Mexico (180 59’ N, 970 18’ W) resulting in a detector with significant sensitivity to gamma rays and cosmic rays with energies from several hundred GeV up to several hundred TeV. Milagro was located at an altitude of 2600m and had an effective area approximately 10 times smaller. The increased area and higher altitude of HAWC results in more than an order of magnitude increase in sensitivity of HAWC over Milagro. The higher altitude of 4100m allows HAWC to sample vertical showers at an atmospheric slant depth of slightly less than 600 g/cm2, closer to the depth of shower maximum for lower energy extensive air showers.
1.2. Water Cherenkov Detectors
approximately 2 steradians. As the Earth rotates over one day, HAWC observes a swath of the sky from 260 to 640 in Declination and 0 to 24 hours in Right Ascension, roughly 2/3 of the sky. 1.3. Readout Electronics and Data Acquisition HAWC reuses the 900 Milagro Hamamatsu 8” Hamamatsu R5912 PMTs, and added 300 larger and higher quantum eciency PMTs, Hamamatsu R7081HQE 10” PMT, to further increase HAWC’s sensitivity to lower energy gamma rays. A block diagram of the recycled Milagro Time-OverThreshold (ToT) discriminator based front end electronics is shown in figure 3. In addition to providing precise timing information for the shower front from the leading edge of the discriminated signal, the width or time-over-threshold of the discriminated signal is proportional to the charge. A VME-based high speed data acquisition system using CAEN VX1190 TDCs with a precision of 0.1ns is used to record time stamps of the pulse edges. Each PMT is connected to the central electronics building through equal length, ~145m, Belden 8241 RG-59 cable through which high-voltage is provided and signals are read. A laser-based system providing light to each WCD through a network of optical fibers is used to perform timing and charge calibration.
Figure 2 A HAWC Water Cherenkov Detector indicating the layout of the 4 PMTs that measure the Cherenkov light produced by charged particles from extensive air showers.
Each water Cherenkov detector (WCD), as shown in figure 2, consists of a commercially available corrugated galvanized steel water storage tank, 7.3m in diameter and 5.4 m high. A light-tight plastic bladder custom manufactured by HAWC collaborators contains approximately 188,000 liters of purified water inside each tank. Four upwardlooking photomultiplier tubes provide timing and charge information by observing Cherenkov light produced by charged particles as they traverse each 4.5m deep pool of water. HAWC can reliably estimate the energy and arrival direction of cosmic and gamma rays arriving from zenith angles of up to 450 , resulting in an instantaneous field of view of
Figure 3 Block diagram of Time-Over-Threshold (TOT) front-end electronics used to provide timing and charge information from the WCDs. A schematic diagram of the DAQ and online processing is shown in figure 4. The rate of signals from each PMT is typically between 10 to 20 kHz. At the TDC level there is no hardware trigger and the online processing farm treats all incoming hits. Approximately 24 kHz of events are identified as air showers, mainly from cosmic rays, and stored on disk at a raw data rate of ~20 MB/sec or 700 TB/year. Reconstruction and analysis is done within a few seconds of triggering with an online processing farm of ~ 200 cores. The raw data is transported via disk
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Figure 4 Schematic of Readout Electronics, Experiment Control and online processing.
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Figure 5 Shower front and axis superimposed above the array of HAWC WCDs illustrating the relationship between timing delay among PMT hits and the arrival direction of the air shower.
Figure 6 Upper left panel shows charge deposit vs. tank position for a likely cosmic ray event. Note the clumpiness and large energy deposits far from the core of the shower. The upper-right panel shows the energy deposition form a likely gamma ray event from actual HAWC data. The lower left panel shows the lateral shower distribution from a likely cosmic-ray event. The lower right panel shows the lateral shower distribution for a likely gamma ray event whose hits fit better to a smooth distribution function.
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array to an offline processing center located at UNAM in Mexico City. The data is also copied to the University of Maryland via internet. More thorough re-processing with improved calibrations and algorithms are performed at these offline processing centers. 1.4. Software Reconstruction and Simulation The reconstruction of events proceeds in three steps: charge calibration and selection of hits; identication of the shower core position; and reconstruction of the shower direction. The charge and timing calibration are used to convert Time-overThreshold data from the TDCs to hits proportional to the charge deposited in each PMT. Once the hits are calibrated, we identify the shower core and reconstruct the direction from the timing and charge information as schematically indicated in figure 5. In addition to determining arrival direction and energy the reconstruction software must also discriminate gamma rays from the hadronic cosmic ray background. This gamma/hadron separation is achieved by measuring the clumpiness of the shower for hadron induced events as indicated in figure 6. This can be done by looking for high energy deposits away from the shower core. The goodness of fit of the lateral shower distribution to the NishimuraKamata-Greisen (NKG) function [2,3] can also be used for discrimination. The same reconstruction algorithms are used for both simulation and data [4]. Using CORSIKA [5], we simulate air showers produced by gamma rays, protons, and seven heavier nuclei (helium, carbon, oxygen, neon, silicon, magnesium, and iron) which represent the major mass groups of the Galactic cosmic rays. The simulated events are generated to sample the parameter space of geometry and energies to estimate the sensitivity of HAWC to various particles as a function of energy and arrival direction. These simulated events are also used to evaluate the resolution of HAWC for reconstructing arrival directions and energies and gamma/hadron separation. 2. HAWC Science Capabilities The HAWC Observatory is designed to record air showers produced by cosmic rays and gamma rays between 100 GeV and 100 TeV. Because of its large eld of view and high live-time, HAWC is wellsuited to measure gamma rays from point sources,
diuse emission, transient sources as well as extended sources. HAWC is sensitive to gamma rays coming from a large fraction of the visible sky at any given moment. Its continuous operation, while the earth rotates, will enable it to view a large fraction of the entire sky enabling it to perform a complete, unbiased survey for TeV gamma-ray sources over a large fraction of the sky with a source detection threshold of 50 mCrab in a single year as shown as a function of declination of point source in figure 7. 2.1. All-Sky Survey Sensitivity Estimates
Figure 7 Sensitivity to integrated flux above 2 TeV as a function of declination of point source of gamma rays.
Figure 8 HAWC-300 Sensitivity to gamma rays for a 1 year exposure super-imposed on sources from the TeVcat catalog as
The sensitivity contours in galactic coordinates of HAWC-300 for a 1 year exposure with sources from the TeVcat catalog [6] as of April 2013 superimposed is shown in gure 8. The differential sensitivity as a function of energy with a comparison to other observatories is shown in figure 9. A more complete description of the sensitivity of HAWC to sources of gamma rays can be found in [7].
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2.5. Active galactic Nuclei (AGN) Extensive unbiased monitoring of flaring AGN will be possible using the wide field of view and large live-time capabilities of HAWC. These observations will further ongoing studies of flux variability in these objects. Measurements of the Extragalactic Background Light (EBL) [9] may also be possible by studying the features of the measured energy spectra from the AGN. 2.6. Gamma Ray Bursts (GRB)
Figure 9 Differential Sensitivity vs. Energy for a 50 hour pointed exposure (IACT) or 1(5) year exposure for all-sky observatory. Various. The required sensitivity to detect various levels of flux from the Crab Nebula are also plotted to serve as a guide.
2.2. Point Sources of Gamma Rays After a one-year exposure, HAWC will have a sensitivity to point sources with uxes of less than 5% of the Crab ux in energies around 10 TeV as shown in Figure 7. Due to their distance, most extragalactic sources of will appear point-like when observed from the Earth. There are also sources of gamma-rays in our galaxy that appear point-like such as the Crab Nebula. One of the goals of the HAWC collaboration is to achieve the best possible angular resolution to resolve point sources of gamma-rays in crowded regions of the sky such as the galactic plane and center.
In addition to the TDC-based data acquisition system, HAWC has a scaler-based DAQ system that records the rate on each PMT in 10 ms windows. By looking for statistical excesses over noise, this system has sensitivity to lower energy fluxes of gamma rays from GRBs. The sensitivity of HAWC to GRBs is further described in [10]. 2.7. Energy Spectra Measurements HAWC will perform measurements of energy spectra of gamma ray sources using spectral fitting techniques developed in Milagro. Distributions of observables that serve as energy proxies will be fit using simulation for different spectral assumptions. The fitted energy spectrum of the Crab Nebula [11] from Milagro is shown in figure 10.
2.3. Extended Sources of Gamma Rays Another capability of HAWC is to extended sources due to its sensitivity to angular region of the sky. HAWC will be perform morphological studies on extended such as Geminga.
observe a large able to sources
2.4. Transient Sources of Gamma Rays/Variability HAWC will monitor the ux from sources as a function of time due to the daily observations of objects in a large fraction of the sky. It has recently been established that the ux of gamma-rays from the Crab Nebula experience temporal variability [8]. Other transient sources such as AGN and GRB will be briefly discussed below.
Figure 10 Crab Nebula energy spectrum fit as performed by Milagro.
2.8. Dark Matter Searches HAWC will search for gamma rays produced in the annihilation and decay of TeV-scale astrophysical
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WIMPs [12]. Dark matter sources are expected to be found in many locations spread across the sky and for nearby sources spatially extended. There is a possibility that gamma rays from dark matter could have a hard energy spectrum that can peak at high energies, thereby being visible to HAWC. 2.9. Cosmic Ray Studies
3.2. Gamma Ray All-Sky Map The significance map from the whole sky in equatorial coordinates from 283 days of HAWC-111 combined with 105 days of HAWC-250 data [15] is shown in figure 12. Many sources of TeV-scale gamma rays such as the Crab Nebula, Markarian 421 and Markarian 501 AGNs as well as many sources in the galactic plane are clearly visible.
HAWC will also perform studies and measurements using its voluminous set of cosmic ray data. High precision measurements of the anisotropies in arrival directions at various angular scales will be made. Measurements of the energy spectrum and its directional dependence will also be performed. 3. First Results The following results, some of which are preliminary, have been obtained from the early data from HAWC and reported in the 2015 ICRC conference [13] and elsewhere.
Figure 12 HAWC All-Sky Significance map.
3.1. Crab Nebula Signal The significance map in the vicinity of the Crab Nebula is shown in figure 11. An excess of 38 V was observed for HAWC-250 (250 tanks operational) from 150 days of data as reported [14] at the 2015 ICRC conference. Subsequent improvements in the data analysis has resulted in an observed excess of 80 sigma for the same data.
Figure 11 Significance map from 150 days of HAWC 250 data in region of Crab Nebula shows an excess of 38 V.
3.3. Geminga Extended Source Analysis Using an extended source smoothing analysis technique extended sources such as Geminga, shown in figure 13, have been observed in the HAWC data as reported [16] at the 2015 ICRC conference.
Figure 13 Significance map obtained using a 1 degree top-hat smoothing analysis for an extended region about Geminga.
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3.4. Blazar Light Curves The weekly integrated flux above 1 TeV for Markarian 421 [17] is shown in figure 14. This is an example of the Transient monitoring capability of HAWC and its utility in multi-wavelength monitoring campaigns.
Figure 16 Significance of the cosmic ray flux after fit and subtraction of the dipole, quadrupole and octupole terms. The map has also had d10 degree smoothing applied.
3.7. Solar Physics Figure 14 Markarian 421 weekly flux light curve as observed by HAWC between July 13,2013 and July 9,2014.
3.5. GRB Searches In addition to detecting or searching for GRBs, shown in figure 15, those GRBs with hard energy spectra at high redshift could be studied with HAWC to derive limits on possible Lorentz Invariance violation [18].
Figure 15 The significance distribution of 18 Swift-triggered GRBs as seen in HAWC (shaded blue area) [XX]. Each of these events is consistent with background.
The HAWC scaler DAQ system also has the capability to contribute to Solar Physics by monitoring the variability in the flux of low energy galactic cosmic rays due to the influence of large magnetic structures propagating in the heliosphere such as distubances associated with Coronal Mass Ejections (CME). Figure 17 shows the effect on the mean rate of the HAWC PMT scaler system [22] in association with a Forbush decrease.
Figure 17 Forbush decrease observed on September 14, 2014 by HAWC and a neutron monitor at the site.
3.6. Cosmic Ray Anisotropy Cosmic ray events comprise the vast bulk of data recorded by HAWC and serve as a background to gamma ray studies. Studies of the cosmic ray events themselves, however, present scientific opportunities such as the observation of the small-scale anisotropy shown in figure 16. The observed anisotropy reported by HAWC [19] is consistent with observations from IceCube [20] and ARGO-YBJ [21].
3.8. Additional Physics Topics HAWC is addressing additional topics not specifically mentioned above as indicated by the 22 scientific talk presentations to the ICRC 2015 in the areas of galactic and extragalactic gamma ray, cosmic ray, dark matter and fundamental physics listed in [13].
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4. Summary and Future Plans References The HAWC detector was officially completed on March 20, 2015. The detector is operating well and first science results are being produced in many topics as have been mentioned in this paper and reported more extensively elsewhere[13]. An outrigger array expansion of smaller WCDs, as indicated in figure 18, to extend sensitivity to higher energies by sampling air showers footprints that extend beyond the current array of large water tanks has begun. Plans for a similar detector to HAWC to be deployed in the Southern Hemisphere are being developed [23].
Figure 18 Diagram indicating proposed placement position of outrigger array WCDs about the main HAWC WCD array.
Acknowledgements We acknowledge the support from: the US National Science Foundation (NSF); the US Department of Energy Office of High-Energy Physics; the Laboratory Directed Research and Development (LDRD) program of Los Alamos National Laboratory; Consejo Nacional de Ciencia y Tecnología (CONACyT), Mexico (grants 260378, 55155, 105666, 122331, 132197, 167281, 167733); Red de Física de Altas Energías, Mexico; DGAPAUNAM (grants IG100414-3, IN108713, IN121309, IN115409, IN111315); VIEP-BUAP (grant 161EXC-2011); the University of Wisconsin Alumni Research Foundation; the Institute of Geophysics, Planetary Physics, and Signatures at Los Alamos National Laboratory; the Luc Binette Foundation UNAM Postdoctoral Fellowship program.
[1] [2] [3] [4]
R. Atkins et al., NIM A, 449, p 478, 2000 J. Nishimura and K. Kamata, Progr. Theor. Phys. 6 (1958) 93 Greisen K.Prog. Cosmic Ray Phys. 1956;III:1. A. J. Smith , for the HAWC Collaboration, HAWC Design, Operation, Reconstruction and Analysis, International Cosmic Ray Conference (2015) [arXiv:1508.05826]. [5] D. Heck et al., CORSIKA: a Monte Carlo code to simulate extensive air showers.Forschungszentrum Karlsruhe GmbH, Karlsruhe (Germany).1998 [adsabs entry] [6] S. P. Wakely, D. Horan, in: Rogelio Caballero, Juan Carlos D’Olivo, Gustavo Medina-Tanco, Lukas Nellen, Federico A. Snchez, Jos F. Valds-Galicia (Ed.), Proc. 30th International Cosmic Ray Conference, Merida, Yucatan, Mexico, 3 (2008) http://tevcat.uchicago.edu. [7] A. U. Abeysekara et al., Sensitivity of the high altitude water Cherenkov detector to sources of multi-TeV gamma rays, Astroparticle Physics 50 (2013) 26–32, [ arXiv:1306.5800] [8] M. Tavani et al., Discovery of Powerful Gamma-Ray Flares from the Crab Nebula, [arXiv.org:1101.2311] [9] E. Dwek, F.Krennich, The Extragalactic Background Light and the Gamma-ray Opacity of the Universe, [arXiv:1209.4661] [10] A. U. Abeysekara et al., On the Sensitivity of the HAWC Observatory to Gamma-Ray Bursts,[ arXiv:1108.6034] [11] A. A. Abdo et al., Observation and Spectral Measurements of the Crab Nebula with Milagro,ApJ,750,63,2012 [12] A. U. Abeysekara et al.,The Sensitivity of HAWC to HighMass Dark Matter Annihilations,[ Phys. Rev. D 90 (2014), 122002] [13] A. U. Abeysekara et al , HAWC Contributions to the 34th International Cosmic Ray Conference (ICRC2015), [arXiv:1508.03327] [14] F. Salesa Greus, for the HAWC Collaboration, Observations of the Crab Nebula with Early HAWC Data, International Cosmic Ray Conference (2015) [arXiv:1508.04274]. [15] J. Pretz, for the HAWC Collaboration, Highlights from the High Altitude Water Cherenkov Observatory, International Cosmic Ray Conference (2015) [arXiv:1509.07851]. [16] B. Baughman, J. Wood, for the HAWC Collaboration, TeV Gamma-Ray Emission Observed from Geminga with HAWC, International Cosmic Ray Conference (2015) [arXiv:1508.03497] [17] R. Lauer, P.W. Younk, for the HAWC Collaboration, Monitoring TeV blazars with HAWC, [arXiv:1508.04479] [18] D. Lennarz, I.Taboada, First Results from HAWC on GRBs, International Cosmic Ray Conference (2015) [arXiv:1508.07325] [19] S.Y. BenZvi, D.W. Fiorino, S. Westerhoff, for the HAWC Collaboration, Observation of Anisotropy in the Arrival Direction Distribution of TeV Cosmic Rays with HAWC, International Cosmic Ray Conference (2015) [arXiv:1508.04781] [20] IceCube Collaboration, R. Abbasi et al., Observation of Anisotropy in the Arrival Directions of Galactic Cosmic Rays at Multiple Angular Scales with IceCube, [arXiv:1105.2326] [21] ARGO-YBJ Collaboration, B. Bartoli et al., Medium scale anisotropy in the TeV cosmic ray flux observed by ARGOYBJ, Phys.Rev. D88 (2013) 082001, [arXiv:1309.6182] [22] O. Enriquez-Rivera, A. Lara, R. Caballero-Lopez, for the HAWC Collaboration, Solar Event Simulations using the HAWC Scaler System, [arXiv:1508.07285] [23] A. Sandoval, for the HAWC Collaboration, HAWC Upgrade with a Sparse Outrigger Array, [arXiv:1509.04269]
Nuclear and Particle Physics Proceedings 279–281 (2016) 87–94 www.elsevier.com/locate/nppp
The High Altitude water Cherenkov (HAWC) Observatory R.W.Springer a, , for the HAWC Collaboration† a
Department of Physics and Astronomy , University of Utah, 115 S 1400 E, Salt Lake City, UT 84112, USA
Abstract HAWC is a continuously operated, wide field of view detector comprised of three hundred 188,000 liter water Cherenkov detectors, each instrumented with four photomultipliers providing charge and timing information. HAWC covers approximately ~22,000 m2 at an altitude of 4100m and reliably estimates the energy and arrival direction of gamma and cosmic rays with significant sensitivity over energies from several hundred GeV to several hundred TeV. With an instantaneous field of view of 2 steradians, HAWC observes 2/3 of the sky in 24 hours. HAWC has been optimized to study transient and steady emission from both galactic and extragalactic sources of gamma rays and serves as a survey instrument for multi-wavelength studies. HAWC has significant discovery potential, including the possibility of indirect detection of dark matter through the observation of gamma rays produced via dark-matter particle annihilation. HAWC has been making observations since summer 2012 and officially commenced data-taking operations with the completed detector on March 20, 2015. This paper will describe the detector design, science capabilities, first scientific results and future plans of the HAWC observatory. Keywords: gamma-ray astronomy, instrumentation
1. Design of the HAWC Observatory
Figure 1 Aerial photograph of HAWC observatory
1.1. Observatory Site The High Altitude Water Cherenkov (HAWC) observatory, shown in figure 1, is a continuously operated, wide field of view detector principally designed to observe astrophysical sources of gamma rays. HAWC, is based upon the technology developed by the Milagro detector [1] that uses the Earth’s atmosphere as a calorimeter to sample extensive air showers (EAS) produced by the primary gamma or cosmic ray. The measurement of the energy and arrival direction of gamma rays, as well as cosmic rays, impinging on the Earth’s atmosphere is achieved by sampling the resulting extensive air
———
R.W. Springer.; e-mail: [email protected]. †
http://www.hawc-observatory.org/collaboration/
http://dx.doi.org/10.1016/j.nuclphysbps.2016.10.013 2405-6014/© 2016 Elsevier B.V. All rights reserved.
showing the three hundred 7.3m diameter 4.5m deep water tanks and the electronics hut and water treatment facility.
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showers using an array of 300 closely spaced water tanks. This array covers an area of approximately 22,000m2 and is located at an altitude of 4100 m on Sierra Negra Mountain in Mexico (180 59’ N, 970 18’ W) resulting in a detector with significant sensitivity to gamma rays and cosmic rays with energies from several hundred GeV up to several hundred TeV. Milagro was located at an altitude of 2600m and had an effective area approximately 10 times smaller. The increased area and higher altitude of HAWC results in more than an order of magnitude increase in sensitivity of HAWC over Milagro. The higher altitude of 4100m allows HAWC to sample vertical showers at an atmospheric slant depth of slightly less than 600 g/cm2, closer to the depth of shower maximum for lower energy extensive air showers.
1.2. Water Cherenkov Detectors
approximately 2 steradians. As the Earth rotates over one day, HAWC observes a swath of the sky from 260 to 640 in Declination and 0 to 24 hours in Right Ascension, roughly 2/3 of the sky. 1.3. Readout Electronics and Data Acquisition HAWC reuses the 900 Milagro Hamamatsu 8” Hamamatsu R5912 PMTs, and added 300 larger and higher quantum eciency PMTs, Hamamatsu R7081HQE 10” PMT, to further increase HAWC’s sensitivity to lower energy gamma rays. A block diagram of the recycled Milagro Time-OverThreshold (ToT) discriminator based front end electronics is shown in figure 3. In addition to providing precise timing information for the shower front from the leading edge of the discriminated signal, the width or time-over-threshold of the discriminated signal is proportional to the charge. A VME-based high speed data acquisition system using CAEN VX1190 TDCs with a precision of 0.1ns is used to record time stamps of the pulse edges. Each PMT is connected to the central electronics building through equal length, ~145m, Belden 8241 RG-59 cable through which high-voltage is provided and signals are read. A laser-based system providing light to each WCD through a network of optical fibers is used to perform timing and charge calibration.
Figure 2 A HAWC Water Cherenkov Detector indicating the layout of the 4 PMTs that measure the Cherenkov light produced by charged particles from extensive air showers.
Each water Cherenkov detector (WCD), as shown in figure 2, consists of a commercially available corrugated galvanized steel water storage tank, 7.3m in diameter and 5.4 m high. A light-tight plastic bladder custom manufactured by HAWC collaborators contains approximately 188,000 liters of purified water inside each tank. Four upwardlooking photomultiplier tubes provide timing and charge information by observing Cherenkov light produced by charged particles as they traverse each 4.5m deep pool of water. HAWC can reliably estimate the energy and arrival direction of cosmic and gamma rays arriving from zenith angles of up to 450 , resulting in an instantaneous field of view of
Figure 3 Block diagram of Time-Over-Threshold (TOT) front-end electronics used to provide timing and charge information from the WCDs. A schematic diagram of the DAQ and online processing is shown in figure 4. The rate of signals from each PMT is typically between 10 to 20 kHz. At the TDC level there is no hardware trigger and the online processing farm treats all incoming hits. Approximately 24 kHz of events are identified as air showers, mainly from cosmic rays, and stored on disk at a raw data rate of ~20 MB/sec or 700 TB/year. Reconstruction and analysis is done within a few seconds of triggering with an online processing farm of ~ 200 cores. The raw data is transported via disk
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Figure 4 Schematic of Readout Electronics, Experiment Control and online processing.
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Figure 5 Shower front and axis superimposed above the array of HAWC WCDs illustrating the relationship between timing delay among PMT hits and the arrival direction of the air shower.
Figure 6 Upper left panel shows charge deposit vs. tank position for a likely cosmic ray event. Note the clumpiness and large energy deposits far from the core of the shower. The upper-right panel shows the energy deposition form a likely gamma ray event from actual HAWC data. The lower left panel shows the lateral shower distribution from a likely cosmic-ray event. The lower right panel shows the lateral shower distribution for a likely gamma ray event whose hits fit better to a smooth distribution function.
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array to an offline processing center located at UNAM in Mexico City. The data is also copied to the University of Maryland via internet. More thorough re-processing with improved calibrations and algorithms are performed at these offline processing centers. 1.4. Software Reconstruction and Simulation The reconstruction of events proceeds in three steps: charge calibration and selection of hits; identication of the shower core position; and reconstruction of the shower direction. The charge and timing calibration are used to convert Time-overThreshold data from the TDCs to hits proportional to the charge deposited in each PMT. Once the hits are calibrated, we identify the shower core and reconstruct the direction from the timing and charge information as schematically indicated in figure 5. In addition to determining arrival direction and energy the reconstruction software must also discriminate gamma rays from the hadronic cosmic ray background. This gamma/hadron separation is achieved by measuring the clumpiness of the shower for hadron induced events as indicated in figure 6. This can be done by looking for high energy deposits away from the shower core. The goodness of fit of the lateral shower distribution to the NishimuraKamata-Greisen (NKG) function [2,3] can also be used for discrimination. The same reconstruction algorithms are used for both simulation and data [4]. Using CORSIKA [5], we simulate air showers produced by gamma rays, protons, and seven heavier nuclei (helium, carbon, oxygen, neon, silicon, magnesium, and iron) which represent the major mass groups of the Galactic cosmic rays. The simulated events are generated to sample the parameter space of geometry and energies to estimate the sensitivity of HAWC to various particles as a function of energy and arrival direction. These simulated events are also used to evaluate the resolution of HAWC for reconstructing arrival directions and energies and gamma/hadron separation. 2. HAWC Science Capabilities The HAWC Observatory is designed to record air showers produced by cosmic rays and gamma rays between 100 GeV and 100 TeV. Because of its large eld of view and high live-time, HAWC is wellsuited to measure gamma rays from point sources,
diuse emission, transient sources as well as extended sources. HAWC is sensitive to gamma rays coming from a large fraction of the visible sky at any given moment. Its continuous operation, while the earth rotates, will enable it to view a large fraction of the entire sky enabling it to perform a complete, unbiased survey for TeV gamma-ray sources over a large fraction of the sky with a source detection threshold of 50 mCrab in a single year as shown as a function of declination of point source in figure 7. 2.1. All-Sky Survey Sensitivity Estimates
Figure 7 Sensitivity to integrated flux above 2 TeV as a function of declination of point source of gamma rays.
Figure 8 HAWC-300 Sensitivity to gamma rays for a 1 year exposure super-imposed on sources from the TeVcat catalog as
The sensitivity contours in galactic coordinates of HAWC-300 for a 1 year exposure with sources from the TeVcat catalog [6] as of April 2013 superimposed is shown in gure 8. The differential sensitivity as a function of energy with a comparison to other observatories is shown in figure 9. A more complete description of the sensitivity of HAWC to sources of gamma rays can be found in [7].
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2.5. Active galactic Nuclei (AGN) Extensive unbiased monitoring of flaring AGN will be possible using the wide field of view and large live-time capabilities of HAWC. These observations will further ongoing studies of flux variability in these objects. Measurements of the Extragalactic Background Light (EBL) [9] may also be possible by studying the features of the measured energy spectra from the AGN. 2.6. Gamma Ray Bursts (GRB)
Figure 9 Differential Sensitivity vs. Energy for a 50 hour pointed exposure (IACT) or 1(5) year exposure for all-sky observatory. Various. The required sensitivity to detect various levels of flux from the Crab Nebula are also plotted to serve as a guide.
2.2. Point Sources of Gamma Rays After a one-year exposure, HAWC will have a sensitivity to point sources with uxes of less than 5% of the Crab ux in energies around 10 TeV as shown in Figure 7. Due to their distance, most extragalactic sources of will appear point-like when observed from the Earth. There are also sources of gamma-rays in our galaxy that appear point-like such as the Crab Nebula. One of the goals of the HAWC collaboration is to achieve the best possible angular resolution to resolve point sources of gamma-rays in crowded regions of the sky such as the galactic plane and center.
In addition to the TDC-based data acquisition system, HAWC has a scaler-based DAQ system that records the rate on each PMT in 10 ms windows. By looking for statistical excesses over noise, this system has sensitivity to lower energy fluxes of gamma rays from GRBs. The sensitivity of HAWC to GRBs is further described in [10]. 2.7. Energy Spectra Measurements HAWC will perform measurements of energy spectra of gamma ray sources using spectral fitting techniques developed in Milagro. Distributions of observables that serve as energy proxies will be fit using simulation for different spectral assumptions. The fitted energy spectrum of the Crab Nebula [11] from Milagro is shown in figure 10.
2.3. Extended Sources of Gamma Rays Another capability of HAWC is to extended sources due to its sensitivity to angular region of the sky. HAWC will be perform morphological studies on extended such as Geminga.
observe a large able to sources
2.4. Transient Sources of Gamma Rays/Variability HAWC will monitor the ux from sources as a function of time due to the daily observations of objects in a large fraction of the sky. It has recently been established that the ux of gamma-rays from the Crab Nebula experience temporal variability [8]. Other transient sources such as AGN and GRB will be briefly discussed below.
Figure 10 Crab Nebula energy spectrum fit as performed by Milagro.
2.8. Dark Matter Searches HAWC will search for gamma rays produced in the annihilation and decay of TeV-scale astrophysical
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WIMPs [12]. Dark matter sources are expected to be found in many locations spread across the sky and for nearby sources spatially extended. There is a possibility that gamma rays from dark matter could have a hard energy spectrum that can peak at high energies, thereby being visible to HAWC. 2.9. Cosmic Ray Studies
3.2. Gamma Ray All-Sky Map The significance map from the whole sky in equatorial coordinates from 283 days of HAWC-111 combined with 105 days of HAWC-250 data [15] is shown in figure 12. Many sources of TeV-scale gamma rays such as the Crab Nebula, Markarian 421 and Markarian 501 AGNs as well as many sources in the galactic plane are clearly visible.
HAWC will also perform studies and measurements using its voluminous set of cosmic ray data. High precision measurements of the anisotropies in arrival directions at various angular scales will be made. Measurements of the energy spectrum and its directional dependence will also be performed. 3. First Results The following results, some of which are preliminary, have been obtained from the early data from HAWC and reported in the 2015 ICRC conference [13] and elsewhere.
Figure 12 HAWC All-Sky Significance map.
3.1. Crab Nebula Signal The significance map in the vicinity of the Crab Nebula is shown in figure 11. An excess of 38 V was observed for HAWC-250 (250 tanks operational) from 150 days of data as reported [14] at the 2015 ICRC conference. Subsequent improvements in the data analysis has resulted in an observed excess of 80 sigma for the same data.
Figure 11 Significance map from 150 days of HAWC 250 data in region of Crab Nebula shows an excess of 38 V.
3.3. Geminga Extended Source Analysis Using an extended source smoothing analysis technique extended sources such as Geminga, shown in figure 13, have been observed in the HAWC data as reported [16] at the 2015 ICRC conference.
Figure 13 Significance map obtained using a 1 degree top-hat smoothing analysis for an extended region about Geminga.
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3.4. Blazar Light Curves The weekly integrated flux above 1 TeV for Markarian 421 [17] is shown in figure 14. This is an example of the Transient monitoring capability of HAWC and its utility in multi-wavelength monitoring campaigns.
Figure 16 Significance of the cosmic ray flux after fit and subtraction of the dipole, quadrupole and octupole terms. The map has also had d10 degree smoothing applied.
3.7. Solar Physics Figure 14 Markarian 421 weekly flux light curve as observed by HAWC between July 13,2013 and July 9,2014.
3.5. GRB Searches In addition to detecting or searching for GRBs, shown in figure 15, those GRBs with hard energy spectra at high redshift could be studied with HAWC to derive limits on possible Lorentz Invariance violation [18].
Figure 15 The significance distribution of 18 Swift-triggered GRBs as seen in HAWC (shaded blue area) [XX]. Each of these events is consistent with background.
The HAWC scaler DAQ system also has the capability to contribute to Solar Physics by monitoring the variability in the flux of low energy galactic cosmic rays due to the influence of large magnetic structures propagating in the heliosphere such as distubances associated with Coronal Mass Ejections (CME). Figure 17 shows the effect on the mean rate of the HAWC PMT scaler system [22] in association with a Forbush decrease.
Figure 17 Forbush decrease observed on September 14, 2014 by HAWC and a neutron monitor at the site.
3.6. Cosmic Ray Anisotropy Cosmic ray events comprise the vast bulk of data recorded by HAWC and serve as a background to gamma ray studies. Studies of the cosmic ray events themselves, however, present scientific opportunities such as the observation of the small-scale anisotropy shown in figure 16. The observed anisotropy reported by HAWC [19] is consistent with observations from IceCube [20] and ARGO-YBJ [21].
3.8. Additional Physics Topics HAWC is addressing additional topics not specifically mentioned above as indicated by the 22 scientific talk presentations to the ICRC 2015 in the areas of galactic and extragalactic gamma ray, cosmic ray, dark matter and fundamental physics listed in [13].
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4. Summary and Future Plans References The HAWC detector was officially completed on March 20, 2015. The detector is operating well and first science results are being produced in many topics as have been mentioned in this paper and reported more extensively elsewhere[13]. An outrigger array expansion of smaller WCDs, as indicated in figure 18, to extend sensitivity to higher energies by sampling air showers footprints that extend beyond the current array of large water tanks has begun. Plans for a similar detector to HAWC to be deployed in the Southern Hemisphere are being developed [23].
Figure 18 Diagram indicating proposed placement position of outrigger array WCDs about the main HAWC WCD array.
Acknowledgements We acknowledge the support from: the US National Science Foundation (NSF); the US Department of Energy Office of High-Energy Physics; the Laboratory Directed Research and Development (LDRD) program of Los Alamos National Laboratory; Consejo Nacional de Ciencia y Tecnología (CONACyT), Mexico (grants 260378, 55155, 105666, 122331, 132197, 167281, 167733); Red de Física de Altas Energías, Mexico; DGAPAUNAM (grants IG100414-3, IN108713, IN121309, IN115409, IN111315); VIEP-BUAP (grant 161EXC-2011); the University of Wisconsin Alumni Research Foundation; the Institute of Geophysics, Planetary Physics, and Signatures at Los Alamos National Laboratory; the Luc Binette Foundation UNAM Postdoctoral Fellowship program.
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