- Email: [email protected]
Status and updates from the High Altitude Water Cherenkov (HAWC) Observatory
Available online at www.sciencedirect.com
Nuclear Physics B (Proc. Suppl.) 239–240 (2013) 220–224 www.elsevier.com/locate/npbps
Status and updates from the High Altitude Water Cherenkov (HAWC) Observatory B.M. Baughman University of Maryland, Department of Physics, College Park MD 20742-4111 USA
for the HAWC Collaboration
Abstract The High Altitude Water Cherenkov Observatory (HAWC) is currently being deployed on the slopes of Volcan Sierra Negra, Puebla, Mexico. The HAWC observatory will consist of 300 Water Cherenkov Detectors totaling approximately 22,000 m2 of instrumented area. The water Cherenkov technique allows HAWC to have a nearly 100% duty cycle and large field of view, making the HAWC observatory an ideal instrument for the study of transient phenomena. With its large effective area, excellent angular and energy resolutions, and efficient gamma-hadron separation, HAWC will survey the TeV gamma ray sky, measure spectra of galactic sources from 1 TeV to beyond 100 TeV, and map galactic diffuse gamma ray emission. The science goals and performance of the HAWC observatory as well as how it will complement contemporaneous space and ground-based detectors will be presented. Keywords: gamma-rays, hawc, water cherenkov
1. Introduction The High Altitude Water Cherenkov Observatory (HAWC) is currently under construction near Puebla, Mexico with a total of 29 fully functional waterCherenkov detectors deployed as of September, 2012. When construction is completed in 2014 it will consist of 300 optically–isolated galvanized steel tanks, each 7.3 m in diameter and 4.5 m deep. The HAWC design is built upon the successful water Cherenkov technique pioneered with Milagro [1–8]. With about a factor of 10 increase in densely instrumented deep–water area over Milagro, the HAWC design provides a dramatic improvement in low–energy effective area, angular resolution, and energy resolution. The HAWC site, at an elevation of 4100 m compared to 2649 m for Milagro, extends the effective area of HAWC well below the minimum energy threshold of Milagro. The segmentation
Email address: [email protected] (B.M. Baughman)
0920-5632/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysbps.2013.05.033
of the deep water into optically isolated tanks improves the hadron rejection efficiency over that of Milagro by a factor of about 10 at high energies. HAWC, with its nearly 100% duty cycle, large field of view, and large effective area, will complement the capabilities of the Fermi-LAT[9–13].
2. Status HAWC is well into the deployment phase with 30 water-Cherenkov detectors already deployed and operational and more tanks being constructed regularly. This was accomplished by refining the logistics needed to deploy HAWC: • Procurement and transportation of water. • Construction of tanks. • Deployment of PMTs.
B.M. Baughman / Nuclear Physics B (Proc. Suppl.) 239–240 (2013) 220–224
The speed with which deployment proceeds depends critically on streamlining these procedures. This was accomplished through the deployment of HAWC-30, the first stage of deployment of the full array completed September, 2012. 2.1. Water Procurement and Transportation Water has been procured from a well at the base of the mountain. The water from the well needs to be softened before it can be used in HAWC. We have installed a water softening system capable of handling all the water demands at the well to accomplish the softening. Transportation of the water from the well to the HAWC site is done via water trucks. We have contracted with the well owner for a consistent supply of water at the HAWC site. The water arriving at the site is deposited within a “dirty” water storage array. “Dirty” water is then filtered through an extensive water filtration system designed to remove particulates and eliminate organic which could cause degradation of the water clarity over time. Secondary sources of water are being examined closer to the HAWC site. These secondary sites would reduce the need for trucking water from the base of the mountain to the site. Furthermore, these secondary sources do not need to be softened. 2.2. Tank Construction The safe construction of over 300 metal tanks at high elevation requires a simple and efficient procedure. During the construction of VAMOS, the engineering array built prior to HAWC-30, many different construction techniques were experimented with. The final procedure is extremely efficient and allows for the construction of each tank to proceed at the same height (that is those constructing the tank do not need to climb ladders) making it extremely safe. Each ring of a tank is bolted together then raised to a comfortable working height by multiple jacks which have been thoroughly stabilized. 2.3. PMT Deployment During the construction of VAMOS, PMTs were deployed before filling each tank. This method was found to be time consuming (as it required multiple people enter the tank after inflation) and less than ideal in terms of keeping the tank clean. After this experience the bladder (plastic container used to contain the water and ensure light tightness) team at CSU developed a method for wet deployment. This method is fast and effective and was refined over the course of the deployment of HAWC-30.
221
3. Instrument Response The expected instrument response function of HAWC have been predicted using Monte Carlo simulations. The atmospheric shower simulations for HAWC are performed in two stages: 1. Each primary particle and resulting shower are simulated from the top of the atmosphere to an elevation of 4100 m with CORSIKA. 2. Shower particles which survive to 4100 m are used as input for a detailed GEANT4 simulation of the HAWC detector. CORSIKA [14] v6970 is used with QGSJET–II for high–energy hadronic interactions and FLUKA 2008.3d.1 for low–energy hadronic interactions. The GEANT4 [15, 16] portion of our simulations, which was originally developed for Milagro [17], includes detailed descriptions of the geometrical and optical properties of the HAWC tanks and photomultiplier tubes (PMTs). Particular attention has been paid to ensure that the simulation reproduces the wavelength response of the PMTs. HAWC will use the same PMTs as Milagro, and so the simulations benefit from previously constructed GEANT4 models [18]. 3.1. Hadron Efficiency Hadronic cosmic–ray induced air showers differ in their morphology to gamma–ray induced air showers, in that they are far more likely to contain multiple sub– showers. These sub–showers are induced by early interactions in the shower with high transverse momentum. Such sub–showers can deposit high concentrations of energy at large distances (tens of meters) from the core of the shower. An example of this phenomenon can be seen in Fig. 1 where two showers with very similar number of hit tubes (similar size) have very different morphologies. We use these differences to differentiate between hadron and gamma induced showers. Specifically, we identify the PMT with the highest measured number of PEs at a radial distance of at least 40 m from the reconstructed core (denoted CxPE40 ). We scale this to the size of the shower using Eqn. 1: S =
Nhit CxPE40
(1)
which scales with the number of PMTs with at least one measured PE (Nhit ). For any given fixed Nhit , large values of S implies a more gamma–like shower, while a smaller values are more hadron–like. During analysis we bin in Nhit and determine the optimal value on which to cut S by maximizing the signal
222
B.M. Baughman / Nuclear Physics B (Proc. Suppl.) 239–240 (2013) 220–224
Figure 2: The fraction of hadrons remaining in a gamma selection after gamma/hadron separation for both HAWC (solid blue) and Milagro (dot-dashed red) Gamma–ray efficiency is always ≥ 50%
3.2. Effective Area The effective area of any gamma–ray detector depends greatly on its background rejection capability. As shown above (Sec. 3.1) HAWC’s gamma/hadron separation is excellent allowing us to preserve a larger fraction of its effective area at lower energies when compared with Milagro as can be seen in Fig. 3. The figure
Figure 1: Upper plot shows a shower induced by a 8 TeV γ-ray with 838 hit PMTs. Lower plot shows a 24 TeV proton induced shower with 837 hit PMTs.
to noise ratio while keeping at least 50% of the gamma rays. Events with S above the optimal value are treated as gamma rays while those below are treated as hadrons. Fig. 2 shows the resulting efficiency for hadrons to pass the optimized cut on S for a given energy. At the highest energies the requirement to keep 50% of gamma rays begins to dominate and thus a sub optimal cut value is used. Gamma–ray air showers at these energies often deposit large energies outside the 40 m radius. Such showers become indistinguishable using S as a gamma/hadron separator. Development is ongoing for a separator without this defect.
Figure 3: Effective area for HAWC before (solid blue) and after (dashed blue) applying gamma/hadron separation cuts. For comparison the effective area of Milagro before (solid red) and after (dashed red) applying gamma/hadron separation cuts is also shown.
shows the average vertical equivalent effective area for gamma rays incident on the detector within 30◦ from zenith. The effective area for HAWC were obtained using an expected background trigger rate of 16 kHz and
223
B.M. Baughman / Nuclear Physics B (Proc. Suppl.) 239–240 (2013) 220–224
3.3. Angular Resolution We characterize the angular resolution of HAWC using two measures: the 68% containment angle (PSF68% ) and the optimal bin size. PSF68% is the 68th percentile of the distribution of opening angles between the true and reconstructed directions of simulated events. The optimal bin size describes the angular scale on which the signal to noise ratio is maximized for a given source and background. For each we assume a source with a Crab–like spectrum transiting at 35◦ from zenith and an isotropic cosmic–ray background entering the atmosphere. The optimal bin size is calculated in three steps: 1. Sort the events according to the opening angle between the true and reconstructed arrival directions. 2. Calculate the cumulative distribution of the events as a function of opening angle. 3. Divide the cumulative event sum by the square root of the solid angle contained within each opening angle bin. The maximum of this function, shown in Eqn. 2, corresponds to the angular scale on which the signal to noise ratio is optimal: S /N ∝ √
N(< Δθ) 2π(1. − cos Δθ)
(2)
Where Δθ is the angular difference the true and reconstructed incident directions. Fig. 4 shows both PSF68% and optimal bin size as well as the optimal bin size for the Milagro Observatory. The HAWC optimal bin size falls below 1◦ at about 500 GeV while Milagro attained a similar angular resolution only for showers at about 3 TeV. 3.4. Energy Resolution HAWC uses the number of PEs recorded in an event to estimate the energy of electromagnetic showers at ground level. This is then used to estimate the energy of the primary particle. The resulting resolution is dominated by fluctuations in the fraction of energy reaching the ground. Fig. 5 shows the energy resolution for gamma–ray induced air showers arriving well within the instrumented area of the detector and less than 45◦ from zenith for both HAWC and Milagro.
Angular Resolution
2.5 2.0
Optimal Bin PSF68% Milagro
Angle [degrees]
optimal bin sizes (defined in Sec. 3.3). The low–energy response will allow HAWC to place competitive limits on transient events including GRBs [19].
1.5
1.0
0.5 103
104
Observed Energy [GeV]
105
Figure 4: The optimal bin size as a function of energy is shown for both HAWC (dashed cyan) and Milagro (dot-dashed red). The PSF68% for HAWC (solid blue) is also shown.
4. Conclusion The deployment of HAWC is well underway with 29 active WCDs as of September, 2012 and an expected completion date in Fall of 2014. HAWC-30 has, as planed, served as an excellent refinement of the deployment procedures. We have streamlined our procurement and processing of water to ensure a steady flow to the site. All water treatment apparati have been installed and are running smoothly. Construction techniques used for deploying tanks has been formalized and construction proceeds smoothly in the hands of local workers. PMT deployment has been greatly improved over that used in VAMOS allowing for safe, easy deployment of PMTs. HAWC’s instrument response functions have been probed using extensive Monte Carlo simulations. We have shown that HAWC can expect to be an excellent always-on gamma-ray telescope and an excellent platform for the study of transient gamma-ray phenomena such as GRBs and flaring AGNs and galactic sources. With its competitive angular resolution, large effective area, excellent gamma/hadron separation, and near constant uptime, HAWC will be an high–energy complement to the Fermi satellite. References [1] Milagro Collaboration, Observation of TeV Gamma Rays from the Crab Nebula with Milagro Using a New Background Rejection Technique, Astrophys. J. 595 (2003) 803–811. arXiv:astroph/0305308, doi:10.1086/377498. URL http://dx.doi.org/10.1086/377498
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B.M. Baughman / Nuclear Physics B (Proc. Suppl.) 239–240 (2013) 220–224
Energy Resolution
Energy Resolution[%]
200
150
Milagro Primary At Ground
100
50
103
104
Observed Energy [GeV]
105
Figure 5: The energy spread for HAWC to reconstruct the energy reaching the ground (dashed cyan) and of incident primary (solid blue). Milagro’s energy resolution to the incident primary’s energy is also shown (dot-dashed red).
[2] R. Atkins et al., Constraints on Very High Energy Gamma-Ray Emission from Gamma-Ray Bursts, Astrophys. J. 630 (2005) 996–1002. arXiv:astro-ph/0503270, doi:10.1086/432501. URL http://dx.doi.org/10.1086/432501 [3] Milagro Collaboration, Evidence for TeV Gamma-Ray Emission from a Region of the Galactic Plane, Physical Review Letters 95 (25). arXiv:astro-ph/0502303, doi:10.1103/PhysRevLett.95.251103. URL http://dx.doi.org/10.1103/PhysRevLett.95. 251103 [4] A. A. Abdo et al., Milagro Constraints on Very High Energy Emission from Short-Duration Gamma-Ray Bursts, Astrophys. J. 666 (2007) 361–367. arXiv:0705.1554, doi:10.1086/519763. URL http://dx.doi.org/10.1086/519763 [5] A. A. Abdo et al., Discovery of TeV Gamma-Ray Emission from the Cygnus Region of the Galaxy, Astrophys. J. Lett. 658 (1) (2007) L33–L36. arXiv:astro-ph/0611691, doi:10.1086/513696. URL http://dx.doi.org/10.1086/513696 [6] A. A. Abdo et al., Discovery of Localized Regions of Excess 10-TeV Cosmic Rays, Physical Review Letters 101 (22) (2008) 221101+. arXiv:0801.3827, doi:10.1103/PhysRevLett.101.221101. URL http://dx.doi.org/10.1103/PhysRevLett.101. 221101 [7] A. A. Abdo et al., A Measurement of the Spatial Distribution of Diffuse TeV Gamma-Ray Emission from the Galactic Plane with Milagro, Astrophys. J. 688 (2008) 1078–1083. arXiv:0805.0417, doi:10.1086/592213. URL http://dx.doi.org/10.1086/592213 [8] A. A. Abdo et al., The Large-Scale Cosmic-Ray Anisotropy as Observed with Milagro, The Astrophysical Journal 698 (2) (2009) 2121+. doi:10.1088/0004-637X/698/2/2121. URL http://dx.doi.org/10.1088/0004-637X/698/2/ 2121 [9] A. A. Abdo et al., Fermi/Large Area Telescope Bright GammaRay Source List, The Astrophysical Journal Supplement Series 183 (1) (2009) 46–66. arXiv:0902.1340, doi:10.1088/00670049/183/1/46. URL http://dx.doi.org/10.1088/0067-0049/183/1/
46 [10] A. A. Abdo, M. Ackermann, M. Ajello, J. Ampe, B. Anderson, W. B. Atwood, M. Axelsson, R. Bagagli, L. Baldini, J. Ballet, and E. Al, The on-orbit calibration of the Fermi Large Area Telescope, Astroparticle Physics 32 (2009) 193–219. arXiv:0904.2226, doi:10.1016/j.astropartphys.2009.08.002. URL http://dx.doi.org/10.1016/j.astropartphys. 2009.08.002 [11] A. A. Abdo, M. Ackermann, M. Ajello, W. B. Atwood, M. Axelsson, L. Baldini, J. Ballet, G. Barbiellini, M. G. Baring, D. Bastieri, and E. Al, The First Fermi Large Area Telescope Catalog of Gamma-ray Pulsars, Astrophys. J. Supp. 187 (2010) 460–494. arXiv:0910.1608, doi:10.1088/0067-0049/187/2/460. URL http://dx.doi.org/10.1088/0067-0049/187/2/ 460 [12] A. A. Abdo et al., The First Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope, Astrophys. J. 715 (2010) 429–457. arXiv:1002.0150, doi:10.1088/0004637X/715/1/429. URL http://dx.doi.org/10.1088/0004-637X/715/1/ 429 [13] A. A. Abdo, M. Ackermann, M. Ajello, A. Allafort, E. Antolini, W. B. Atwood, M. Axelsson, L. Baldini, J. Ballet, G. Barbiellini, and E. Al, Fermi Large Area Telescope First Source Catalog, Astrophys. J. Supp. 188 (2010) 405–436. arXiv:1002.2280, doi:10.1088/0067-0049/188/2/405. URL http://dx.doi.org/10.1088/0067-0049/188/2/ 405 [14] D. Heck, J. Knapp, J. N. Capdevielle, G. Schatz, and T. Thouw, CORSIKA: A Monte Carlo Code to Simulate Extensive Air Showers, Tech. rep. (1998). URL http://www-ik.fzk.de/corsika/physics\ _description/corsika\_phys.html [15] Geant4 Collaboration et al., Geant4-a simulation toolkit, Nuclear Instruments and Methods in Physics Research A 506 (2003) 250–303. doi:10.1016/S0168-9002(03)01368-8. URL http://dx.doi.org/10.1016/S0168-9002(03) 01368-8 [16] J. Allison et al., Geant4 developments and applications, IEEE Transactions on Nuclear Science 53 (1) (2006) 270–278. doi:10.1109/TNS.2006.869826. URL http://dx.doi.org/10.1109/TNS.2006.869826 [17] V. Vasileiou, Monte Carlo Simulation of the Milagro Gammaray Observatory, in: International Cosmic Ray Conference, Vol. 3 of International Cosmic Ray Conference, 2008, pp. 1377–1380. URL http://adsabs.harvard.edu/cgi-bin/nph-bib\ _query?bibcode=2008ICRC....3.1377V [18] V. Vasileiou, R. W. Ellsworth, and A. Smith, PhotocathodeUniformity Tests of the Hamamatsu R5912 Photomultiplier Tubes Used in the Milagro Experiment, in: International Cosmic Ray Conference, Vol. 4 of International Cosmic Ray Conference, 2008, pp. 813–816. arXiv:0711.1910. URL http://arxiv.org/abs/0711.1910 [19] H. A. W. C. Collaboration et al., On the sensitivity of the HAWC observatory to gamma-ray burstsarXiv:1108.6034. URL http://arxiv.org/abs/1108.6034
Nuclear Physics B (Proc. Suppl.) 239–240 (2013) 220–224 www.elsevier.com/locate/npbps
Status and updates from the High Altitude Water Cherenkov (HAWC) Observatory B.M. Baughman University of Maryland, Department of Physics, College Park MD 20742-4111 USA
for the HAWC Collaboration
Abstract The High Altitude Water Cherenkov Observatory (HAWC) is currently being deployed on the slopes of Volcan Sierra Negra, Puebla, Mexico. The HAWC observatory will consist of 300 Water Cherenkov Detectors totaling approximately 22,000 m2 of instrumented area. The water Cherenkov technique allows HAWC to have a nearly 100% duty cycle and large field of view, making the HAWC observatory an ideal instrument for the study of transient phenomena. With its large effective area, excellent angular and energy resolutions, and efficient gamma-hadron separation, HAWC will survey the TeV gamma ray sky, measure spectra of galactic sources from 1 TeV to beyond 100 TeV, and map galactic diffuse gamma ray emission. The science goals and performance of the HAWC observatory as well as how it will complement contemporaneous space and ground-based detectors will be presented. Keywords: gamma-rays, hawc, water cherenkov
1. Introduction The High Altitude Water Cherenkov Observatory (HAWC) is currently under construction near Puebla, Mexico with a total of 29 fully functional waterCherenkov detectors deployed as of September, 2012. When construction is completed in 2014 it will consist of 300 optically–isolated galvanized steel tanks, each 7.3 m in diameter and 4.5 m deep. The HAWC design is built upon the successful water Cherenkov technique pioneered with Milagro [1–8]. With about a factor of 10 increase in densely instrumented deep–water area over Milagro, the HAWC design provides a dramatic improvement in low–energy effective area, angular resolution, and energy resolution. The HAWC site, at an elevation of 4100 m compared to 2649 m for Milagro, extends the effective area of HAWC well below the minimum energy threshold of Milagro. The segmentation
Email address: [email protected] (B.M. Baughman)
0920-5632/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysbps.2013.05.033
of the deep water into optically isolated tanks improves the hadron rejection efficiency over that of Milagro by a factor of about 10 at high energies. HAWC, with its nearly 100% duty cycle, large field of view, and large effective area, will complement the capabilities of the Fermi-LAT[9–13].
2. Status HAWC is well into the deployment phase with 30 water-Cherenkov detectors already deployed and operational and more tanks being constructed regularly. This was accomplished by refining the logistics needed to deploy HAWC: • Procurement and transportation of water. • Construction of tanks. • Deployment of PMTs.
B.M. Baughman / Nuclear Physics B (Proc. Suppl.) 239–240 (2013) 220–224
The speed with which deployment proceeds depends critically on streamlining these procedures. This was accomplished through the deployment of HAWC-30, the first stage of deployment of the full array completed September, 2012. 2.1. Water Procurement and Transportation Water has been procured from a well at the base of the mountain. The water from the well needs to be softened before it can be used in HAWC. We have installed a water softening system capable of handling all the water demands at the well to accomplish the softening. Transportation of the water from the well to the HAWC site is done via water trucks. We have contracted with the well owner for a consistent supply of water at the HAWC site. The water arriving at the site is deposited within a “dirty” water storage array. “Dirty” water is then filtered through an extensive water filtration system designed to remove particulates and eliminate organic which could cause degradation of the water clarity over time. Secondary sources of water are being examined closer to the HAWC site. These secondary sites would reduce the need for trucking water from the base of the mountain to the site. Furthermore, these secondary sources do not need to be softened. 2.2. Tank Construction The safe construction of over 300 metal tanks at high elevation requires a simple and efficient procedure. During the construction of VAMOS, the engineering array built prior to HAWC-30, many different construction techniques were experimented with. The final procedure is extremely efficient and allows for the construction of each tank to proceed at the same height (that is those constructing the tank do not need to climb ladders) making it extremely safe. Each ring of a tank is bolted together then raised to a comfortable working height by multiple jacks which have been thoroughly stabilized. 2.3. PMT Deployment During the construction of VAMOS, PMTs were deployed before filling each tank. This method was found to be time consuming (as it required multiple people enter the tank after inflation) and less than ideal in terms of keeping the tank clean. After this experience the bladder (plastic container used to contain the water and ensure light tightness) team at CSU developed a method for wet deployment. This method is fast and effective and was refined over the course of the deployment of HAWC-30.
221
3. Instrument Response The expected instrument response function of HAWC have been predicted using Monte Carlo simulations. The atmospheric shower simulations for HAWC are performed in two stages: 1. Each primary particle and resulting shower are simulated from the top of the atmosphere to an elevation of 4100 m with CORSIKA. 2. Shower particles which survive to 4100 m are used as input for a detailed GEANT4 simulation of the HAWC detector. CORSIKA [14] v6970 is used with QGSJET–II for high–energy hadronic interactions and FLUKA 2008.3d.1 for low–energy hadronic interactions. The GEANT4 [15, 16] portion of our simulations, which was originally developed for Milagro [17], includes detailed descriptions of the geometrical and optical properties of the HAWC tanks and photomultiplier tubes (PMTs). Particular attention has been paid to ensure that the simulation reproduces the wavelength response of the PMTs. HAWC will use the same PMTs as Milagro, and so the simulations benefit from previously constructed GEANT4 models [18]. 3.1. Hadron Efficiency Hadronic cosmic–ray induced air showers differ in their morphology to gamma–ray induced air showers, in that they are far more likely to contain multiple sub– showers. These sub–showers are induced by early interactions in the shower with high transverse momentum. Such sub–showers can deposit high concentrations of energy at large distances (tens of meters) from the core of the shower. An example of this phenomenon can be seen in Fig. 1 where two showers with very similar number of hit tubes (similar size) have very different morphologies. We use these differences to differentiate between hadron and gamma induced showers. Specifically, we identify the PMT with the highest measured number of PEs at a radial distance of at least 40 m from the reconstructed core (denoted CxPE40 ). We scale this to the size of the shower using Eqn. 1: S =
Nhit CxPE40
(1)
which scales with the number of PMTs with at least one measured PE (Nhit ). For any given fixed Nhit , large values of S implies a more gamma–like shower, while a smaller values are more hadron–like. During analysis we bin in Nhit and determine the optimal value on which to cut S by maximizing the signal
222
B.M. Baughman / Nuclear Physics B (Proc. Suppl.) 239–240 (2013) 220–224
Figure 2: The fraction of hadrons remaining in a gamma selection after gamma/hadron separation for both HAWC (solid blue) and Milagro (dot-dashed red) Gamma–ray efficiency is always ≥ 50%
3.2. Effective Area The effective area of any gamma–ray detector depends greatly on its background rejection capability. As shown above (Sec. 3.1) HAWC’s gamma/hadron separation is excellent allowing us to preserve a larger fraction of its effective area at lower energies when compared with Milagro as can be seen in Fig. 3. The figure
Figure 1: Upper plot shows a shower induced by a 8 TeV γ-ray with 838 hit PMTs. Lower plot shows a 24 TeV proton induced shower with 837 hit PMTs.
to noise ratio while keeping at least 50% of the gamma rays. Events with S above the optimal value are treated as gamma rays while those below are treated as hadrons. Fig. 2 shows the resulting efficiency for hadrons to pass the optimized cut on S for a given energy. At the highest energies the requirement to keep 50% of gamma rays begins to dominate and thus a sub optimal cut value is used. Gamma–ray air showers at these energies often deposit large energies outside the 40 m radius. Such showers become indistinguishable using S as a gamma/hadron separator. Development is ongoing for a separator without this defect.
Figure 3: Effective area for HAWC before (solid blue) and after (dashed blue) applying gamma/hadron separation cuts. For comparison the effective area of Milagro before (solid red) and after (dashed red) applying gamma/hadron separation cuts is also shown.
shows the average vertical equivalent effective area for gamma rays incident on the detector within 30◦ from zenith. The effective area for HAWC were obtained using an expected background trigger rate of 16 kHz and
223
B.M. Baughman / Nuclear Physics B (Proc. Suppl.) 239–240 (2013) 220–224
3.3. Angular Resolution We characterize the angular resolution of HAWC using two measures: the 68% containment angle (PSF68% ) and the optimal bin size. PSF68% is the 68th percentile of the distribution of opening angles between the true and reconstructed directions of simulated events. The optimal bin size describes the angular scale on which the signal to noise ratio is maximized for a given source and background. For each we assume a source with a Crab–like spectrum transiting at 35◦ from zenith and an isotropic cosmic–ray background entering the atmosphere. The optimal bin size is calculated in three steps: 1. Sort the events according to the opening angle between the true and reconstructed arrival directions. 2. Calculate the cumulative distribution of the events as a function of opening angle. 3. Divide the cumulative event sum by the square root of the solid angle contained within each opening angle bin. The maximum of this function, shown in Eqn. 2, corresponds to the angular scale on which the signal to noise ratio is optimal: S /N ∝ √
N(< Δθ) 2π(1. − cos Δθ)
(2)
Where Δθ is the angular difference the true and reconstructed incident directions. Fig. 4 shows both PSF68% and optimal bin size as well as the optimal bin size for the Milagro Observatory. The HAWC optimal bin size falls below 1◦ at about 500 GeV while Milagro attained a similar angular resolution only for showers at about 3 TeV. 3.4. Energy Resolution HAWC uses the number of PEs recorded in an event to estimate the energy of electromagnetic showers at ground level. This is then used to estimate the energy of the primary particle. The resulting resolution is dominated by fluctuations in the fraction of energy reaching the ground. Fig. 5 shows the energy resolution for gamma–ray induced air showers arriving well within the instrumented area of the detector and less than 45◦ from zenith for both HAWC and Milagro.
Angular Resolution
2.5 2.0
Optimal Bin PSF68% Milagro
Angle [degrees]
optimal bin sizes (defined in Sec. 3.3). The low–energy response will allow HAWC to place competitive limits on transient events including GRBs [19].
1.5
1.0
0.5 103
104
Observed Energy [GeV]
105
Figure 4: The optimal bin size as a function of energy is shown for both HAWC (dashed cyan) and Milagro (dot-dashed red). The PSF68% for HAWC (solid blue) is also shown.
4. Conclusion The deployment of HAWC is well underway with 29 active WCDs as of September, 2012 and an expected completion date in Fall of 2014. HAWC-30 has, as planed, served as an excellent refinement of the deployment procedures. We have streamlined our procurement and processing of water to ensure a steady flow to the site. All water treatment apparati have been installed and are running smoothly. Construction techniques used for deploying tanks has been formalized and construction proceeds smoothly in the hands of local workers. PMT deployment has been greatly improved over that used in VAMOS allowing for safe, easy deployment of PMTs. HAWC’s instrument response functions have been probed using extensive Monte Carlo simulations. We have shown that HAWC can expect to be an excellent always-on gamma-ray telescope and an excellent platform for the study of transient gamma-ray phenomena such as GRBs and flaring AGNs and galactic sources. With its competitive angular resolution, large effective area, excellent gamma/hadron separation, and near constant uptime, HAWC will be an high–energy complement to the Fermi satellite. References [1] Milagro Collaboration, Observation of TeV Gamma Rays from the Crab Nebula with Milagro Using a New Background Rejection Technique, Astrophys. J. 595 (2003) 803–811. arXiv:astroph/0305308, doi:10.1086/377498. URL http://dx.doi.org/10.1086/377498
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Energy Resolution
Energy Resolution[%]
200
150
Milagro Primary At Ground
100
50
103
104
Observed Energy [GeV]
105
Figure 5: The energy spread for HAWC to reconstruct the energy reaching the ground (dashed cyan) and of incident primary (solid blue). Milagro’s energy resolution to the incident primary’s energy is also shown (dot-dashed red).
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