- Email: [email protected]
The HAWC observatory
Nuclear Instruments and Methods in Physics Research A 692 (2012) 72–76
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The HAWC observatory Tyce DeYoung Department of Physics, Pennsylvania State University, University Park, PA 16802, USA
For the HAWC Collaboration a r t i c l e i n f o
a b s t r a c t
Available online 21 January 2012
The High Altitude Water Cherenkov (HAWC) observatory is a new very high energy water Cherenkov gamma ray telescope, now under construction at 4100 m altitude at Sierra Negra, Mexico. Due to its increased altitude, larger surface area and improved design, HAWC will be about 15 times more sensitive than its predecessor, Milagro. With its wide field of view and high duty factor, HAWC will be an excellent instrument for the studies of diffuse gamma ray emission, the high energy spectra of Galactic gamma ray sources, and transient emission from extragalactic objects such as GRBs and AGN, as well as surveying a large fraction of the VHE sky. & 2012 Elsevier B.V. All rights reserved.
Keyword: Gamma ray astronomy
1. Introduction The last decade has seen considerable advances in the field of very high energy (VHE, Eg \ 100 GeV) gamma ray astronomy, as imaging air Cherenkov telescopes such as HESS, MAGIC, and VERITAS have provided ever more detailed views of sources such as supernova remnants, pulsar wind nebulae, and Active Galactic Nuclei (AGN). New satellite instruments such as the Fermi LAT have contributed to our view of these objects at lower energies, with a wide field of view that enhances our understanding of the variability of these objects. Wide field of view instruments are also useful for measuring diffuse fluxes and for observing transient emitters such as gamma ray bursts (GRBs). In the VHE band, observatories such as Milagro, Tibet AS-g, and ARGO have provided useful observations but with a lower sensitivity than the current generation of air Cherenkov telescopes. The High Altitude Water Cherenkov (HAWC) observatory, now under construction at Sierra Negra, Mexico, will be the world’s most sensitive wide field of view VHE gamma ray detector. The detector will consist of 300 water-filled tanks, each instrumented with four photomultiplier tubes (PMTs) sensitive to the Cherenkov radiation emitted by relativistic particles in the tanks. The tanks are 7.2 m in diameter and 4.3 m tall, and cover a total area of around 20,000 m2, of which approximately 60% is active Cherenkov medium. The planned layout of HAWC is shown in Fig. 1. When VHE gamma rays strike the atmosphere above the detector, some particles from the resulting extensive air shower will survive to ground level and be detected in these tanks. The
E-mail addresses: [email protected], [email protected] 0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2012.01.026
incident gamma ray direction can be reconstructed from the timing of the air shower front sweeping across the detector, and the background of cosmic ray air showers can be reduced by rejecting showers with evidence of muons, which are rarely produced in gamma ray showers. At 4100 m elevation, the Sierra Negra site allows the detection of significantly lower energy air showers than HAWC’s predecessor, Milagro, which substantially enhances HAWC’s sensitivity to gamma ray sources.
2. Detector performance The Sierra Negra site lies on a saddle between the peaks of Sierra Negra and Pico de Orizaba in the Parque Nacional Pico de Orizaba, near the city of Puebla, Mexico. The site, which also hosts the Large Millimeter Telescope, provides a large flat area at 4100 m above sea level, approximately 1500 m higher than the site of the Milagro telescope. The site is located at 181590 4100 N, 971180 2800 W, providing a view of most of the Northern Hemisphere sky and somewhat more of the southern sky than was visible to Milagro. The effective area of HAWC as a function of energy is shown in Fig. 2, in comparison to Milagro. The effect of the higher altitude of the HAWC site is clearly visible in the order of magnitude increase in the effective area at trigger level for gamma rays with energies below several TeV. This improvement is further reinforced by the effect of a typical cut to reject background events produced by hadronic cosmic ray air showers, as indicated by the dashed lines. The combination will substantially extend the sensitivity of HAWC to extragalactic sources such as GRBs and AGN, which are observable primarily at low energies due to the attenuation of VHE gamma rays through interactions with the
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Fig. 1. Planned layout of the 300 HAWC water tanks, along with the VAMOS engineering array of seven tanks at left. The center of the HAWC array will contain the electronics shack, and the gaps between the rows of tanks permit physical access as well as providing space for cable runs and water drainage. Fig. 3. Angular resolution of HAWC as compared to Milagro. The solid line indicates 1s resolution: the angular error within which 68% of gamma rays are reconstructed. The bin size which optimizes sensitivity to point sources is slightly smaller, and is shown with a dashed line. For comparison, the angular resolution of Milagro is shown with the dot-dashed line.
Fig. 2. Effective area of HAWC for the detection of gamma rays, as compared to Milagro. The effective area is averaged over the solid angle within 301 of zenith. Solid lines indicate the effective area at trigger level, while dashed lines include the effect of a typical cut against hadronic showers. HAWC provides a substantial increase at energies below 10 TeV due to its higher altitude.
extragalactic background light (EBL). The array of outriggers surrounding Milagro was sufficient to detect showers at the highest energies; HAWC’s instrumented area is comparable to that of the Milagro outrigger array, so the effective area for gamma rays above 10 TeV is comparable. HAWC’s advantage over Milagro at those energies is in its superior angular resolution and energy resolution, as shown below. The higher altitude and greater active Cherenkov area of HAWC mean that substantially higher numbers of particles will be detected, as compared to Milagro. This allows considerably higher accuracy in the reconstruction of the shower front, leading to much better angular resolution than was possible with Milagro. The improvement is shown in Fig. 3. At energies above 10 TeV, HAWC’s angular resolution will be 0.1–0.21, with a resolution better than 1.01 for energies above roughly 500 GeV. By contrast, Milagro was limited to 0.51 even for the highest energy air showers. The higher efficiency for detecting particles also leads to better energy resolution than was possible with Milagro, as shown in Fig. 4. The energy resolution is limited by two effects: the accuracy with which the amount of energy reaching the detector can be reconstructed, and the stochastic history of the shower development in the atmosphere. The latter element is dominated
Fig. 4. Energy resolution of HAWC, compared to Milagro. The dashed line indicates the precision of reconstruction of the total energy of the particles reaching ground level, which is the experimental observable. The solid line indicates the resolution on the primary gamma ray energy; the difference between solid and dashed lines is due to the intrinsically stochastic nature of air shower development in the atmosphere, which is an irreducible uncertainty for a ground array such as HAWC.
by the depth to which the primary gamma ray penetrated into the atmosphere before first interacting, which cannot be determined by an air shower array such as HAWC. As shown in the figure, the error in the determination of the ground-level energy is substantially smaller than the total energy resolution, indicating that HAWC approaches the intrinsic limit of energy resolution for an extensive air shower array at this altitude. As is the case for the angular resolution, the higher altitude of the HAWC site leads to substantially better energy resolution than that of Milagro. Finally, the higher altitude and larger detector footprint of HAWC lead to substantial improvements in the accuracy of hadronic background rejection, as compared to Milagro. Air showers produced by hadronic cosmic rays are far more numerous than those
74
T. DeYoung / Nuclear Instruments and Methods in Physics Research A 692 (2012) 72–76
Fig. 6. Sensitivity of HAWC vs. energy for one year (dashed) and five years (solid) observation time. For atmospheric Cherenkov telescopes, 50 h of observation is assumed, while for Fermi and the air shower detectors the observation duration corresponds to the integrated time the source is present in the field of view. The thin dashed lines indicate the spectrum of the Crab nebula, scaled to 100%, 10%, and 1% of its actual flux. Fig. 5. Fraction of hadronic air showers surviving an energy-dependent cut which preserves 50% of the gamma rays at that estimated energy. Above 10 TeV, the hadronic background can be reduced by a factor of nearly 300, although the efficiency for detecting muons falls off with decreasing shower energy. At all energies, background rejection is considerably more effective than with Milagro, due to the much larger instrumented area of HAWC.
produced by gamma rays, so background rejection is extremely important for determining the sensitivity of a VHE gamma ray detector. The primary discriminant used by HAWC is the presence of muons in hadronic air showers. Muons striking a tank significantly removed from the central axis of the air shower will produce a high amplitude signal in one or more PMTs, much higher in amplitude than the signals produced by the electromagnetic particles (electrons, positrons, and photons) which fall off in number with distance from the shower axis. Searching for large signals at a distance of tens of meters from the reconstructed shower core thus allows us to reject a substantial fraction of the hadronic background, as shown in Fig. 5, especially for the higher energy showers where muon production is common. The same technique was used with Milagro, but due to the relatively small size of the central Milagro pond, muons were typically only seen if they were near the shower axis (and thus relatively difficult to detect).
3. Sensitivity to astrophysical sources The combination of the improved effective area, angular resolution, and background rejection available with HAWC will lead to a sensitivity nearly 15 times greater than that of Milagro. The sensitivity of HAWC in different energy ranges is compared to that of Milagro and to the Fermi LAT and various air Cherenkov telescopes in Fig. 6. Because HAWC observes sources whenever they are overhead, the sensitivity of HAWC to the high energy end of the spectrum is considerably better than is possible with air Cherenkov telescopes, given realistic observing budgets. With five years of observation, HAWC will survey approximately half the sky to a sensitivity of around 50 mCrab. In addition to its sensitivity to high energy emission from point sources, HAWC’s wide field of view makes it ideally suited to detect extended or diffuse emission from Galactic regions which may be too large for convenient observations with air Cherenkov telescopes. Milagro observations of the Cygnus region of the Galaxy [1,2] indicated a complex region, containing several point sources as well as apparently extended emission, perhaps from the interactions of cosmic rays on dust in the Galactic plane.
HAWC will be able to improve on Milagro’s observations of this region, perhaps resolving additional point sources or measuring the energy spectrum of the diffuse emission. HAWC’s wide field of view also makes it an excellent instrument for searching for VHE emission from transient sources, such as GRBs. Recent observations by the Fermi LAT [3–6] have confirmed the existence of a second spectral component in addition to the band component for several bright GRBs. This second component is an apparent power-law emission spectrum which develops more slowly than the lower energy emission and which extends to the highest energies accessible to the LAT (although in one instance a high energy cutoff may have been observed). The highest energy photon observed by the LAT was at 33.4 GeV [4], corresponding to an emitted energy of 94 GeV after correcting for the source redshift. It is unclear whether this emission extends to the VHE band, as might be expected if this second component reflects the acceleration of hadrons to extremely high energies. For relatively nearby GRBs, HAWC will be well suited to measuring the higher end of this spectral component [7]. Fig. 7 shows the sensitivity of HAWC to GRB emission as a function of the emission duration. It is assumed that the GRB time and location are known from observations at lower energies, and that the power law component observed in the Fermi LAT data extends until it is cut off by EBL absorbtion. Because HAWC’s effective area rises sharply with energy, the detectability of a GRB depends strongly on the index of the power law component; Fig. 7 shows the sensitivity for a relatively hard index of 1.6, as observed in GRB 090510, for various redshifts ranging from 0 to 2. For short GRBs with a redshift below approximately 1.0, the nF n sensitivity of HAWC is comparable that of the Fermi LAT, so HAWC should either detect GRBs or provide meaningful constraints on their emission at energies above those detected by the LAT. Fig. 7 also shows the sensitivity of HAWC observations in scaler mode, where individual gamma ray air showers are not detected but a rise in the flux of soft gamma rays associated with a GRB is observed as a correlated increase in the hit rates of all PMTs. In this mode, HAWC is sensitive to gamma rays at energies above roughly 700 MeV. For relatively hard spectra like the E1:6 spectrum assumed in Fig. 7, scaler mode observations offer relatively little advantage over the normal observation mode. For softer spectra, however, scaler observations are more sensitive than regular observations. Additionally, the different energy ranges to which the regular and scaler observations are sensitive
T. DeYoung / Nuclear Instruments and Methods in Physics Research A 692 (2012) 72–76
75
permit a rough measurement of the energy spectrum at energies of 1–100 GeV. Alternatively, if the spectral index is known from external observations, e.g. by the Fermi LAT, comparison of the observations in scaler and normal mode would allow us to infer the presence of a spectral cutoff. If the hard spectral component is not cut off by intrinsic effects in the GRB, but extends up to the energies where it is cut off by EBL attenuation, HAWC will record a wealth of data about the high energy behavior of the GRB. Fig. 8 shows a simulation of HAWC observations of GRB 090510, assuming that it occurred overhead and that the power law spectrum observed by the Fermi LAT was cut off by EBL attenuation. The Gilmore model of the EBL spectrum [8] was used, and both its effect and the hadronic background contribution are included in Fig. 8.
4. Prospects
Fig. 7. Sensitivity of HAWC to GRB emission, as a function of the emission duration. The lines labeled ‘‘HAWC’’ show the sensitivity of the standard observation mode, while those labeled ‘‘scaler’’ refer to the lower-energy scaler observations. The flux is normalized to the emission at 10 GeV, as observed by e.g. the Fermi LAT, and is assumed to extend with a spectral index of 1.6 as observed for GRB 090510. The actual flux and duration of GRB 090510 are indicated with an inverted triangle.
When complete, HAWC will be the world’s most sensitive wide field of view observatory in the VHE gamma ray band. Its field of view and sensitivity to high energy gamma rays will allow it to make important measurements of the high energy spectra of Galactic sources, of extended Galactic sources and diffuse gamma ray emission in the Galactic plane, and of emission from transient sources such as GRBs.
Fig. 8. Simulated data from HAWC observation of GRB 090510, had it occurred overhead with HAWC fully operational. The hard spectral component observed by the Fermi LAT is assumed to extend with the observed index up to energies where it is cut off by EBL attenuation.
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T. DeYoung / Nuclear Instruments and Methods in Physics Research A 692 (2012) 72–76
Construction of the HAWC array began in February 2011, and will continue for the next three years. In the fall of 2012 we expect to have 30 of a planned 300 tanks operational, providing a sensitivity comparable to Milagro’s. The full array will be complete in the fall of 2014. The original plan for HAWC called for three 8 in. Hamamatsu PMTs, recycled from Milagro, to be placed in each water tank. After the project was approved, additional funding was provided to purchase a fourth PMT for each tank. The fourth PMT will be a 10 in. Hamamatsu super-bialkali PMT, providing approximately twice the photon collection efficiency of the original tubes. This will substantially increase HAWC’s effectiveness for low energy gamma ray showers. All of the simulated performance plots included here are based on the original, three PMT plans, and are thus conservative estimate’s of HAWC’s final capabilities.
References [1] A.A. Abdo, et al., The Milagro Collaboration, The Astrophysical Journal 658 (2007) L33. [2] A.A. Abdo, et al., The Milagro Collaboration, The Astrophysical Journal 664 (2007) L91. [3] A.A. Abdo, et al., Fermi LAT and Fermi GBM Collaboration, Science 323 (2009) 1688. [4] A.A. Abdo, et al., The Fermi/GBM and The Fermi/LAT and The Swift Team Collaborations, The Astrophysical Journal 706 (2009) L138. [5] M. Ackermann, et al., The Fermi LAT and GBM Collaborations, The Astrophysical Journal 716 (2010) 1178. [6] M. Ackermann, et al., The Fermi Collaboration, The Astrophysical Journal 729 (2011) 114. [7] A.U. Abeysekara, et al., The HAWC Collaboration, Astroparticle Physics, forth coming. arXiv:1108.6034 (astro-ph.HE). [8] R.C. Gilmore, P. Madau, J.R. Primack, R.S. Somerville, F. Haardt, Monthly Notices of the Royal Astronomical Society 399 (2009) 1694.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The HAWC observatory Tyce DeYoung Department of Physics, Pennsylvania State University, University Park, PA 16802, USA
For the HAWC Collaboration a r t i c l e i n f o
a b s t r a c t
Available online 21 January 2012
The High Altitude Water Cherenkov (HAWC) observatory is a new very high energy water Cherenkov gamma ray telescope, now under construction at 4100 m altitude at Sierra Negra, Mexico. Due to its increased altitude, larger surface area and improved design, HAWC will be about 15 times more sensitive than its predecessor, Milagro. With its wide field of view and high duty factor, HAWC will be an excellent instrument for the studies of diffuse gamma ray emission, the high energy spectra of Galactic gamma ray sources, and transient emission from extragalactic objects such as GRBs and AGN, as well as surveying a large fraction of the VHE sky. & 2012 Elsevier B.V. All rights reserved.
Keyword: Gamma ray astronomy
1. Introduction The last decade has seen considerable advances in the field of very high energy (VHE, Eg \ 100 GeV) gamma ray astronomy, as imaging air Cherenkov telescopes such as HESS, MAGIC, and VERITAS have provided ever more detailed views of sources such as supernova remnants, pulsar wind nebulae, and Active Galactic Nuclei (AGN). New satellite instruments such as the Fermi LAT have contributed to our view of these objects at lower energies, with a wide field of view that enhances our understanding of the variability of these objects. Wide field of view instruments are also useful for measuring diffuse fluxes and for observing transient emitters such as gamma ray bursts (GRBs). In the VHE band, observatories such as Milagro, Tibet AS-g, and ARGO have provided useful observations but with a lower sensitivity than the current generation of air Cherenkov telescopes. The High Altitude Water Cherenkov (HAWC) observatory, now under construction at Sierra Negra, Mexico, will be the world’s most sensitive wide field of view VHE gamma ray detector. The detector will consist of 300 water-filled tanks, each instrumented with four photomultiplier tubes (PMTs) sensitive to the Cherenkov radiation emitted by relativistic particles in the tanks. The tanks are 7.2 m in diameter and 4.3 m tall, and cover a total area of around 20,000 m2, of which approximately 60% is active Cherenkov medium. The planned layout of HAWC is shown in Fig. 1. When VHE gamma rays strike the atmosphere above the detector, some particles from the resulting extensive air shower will survive to ground level and be detected in these tanks. The
E-mail addresses: [email protected], [email protected] 0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2012.01.026
incident gamma ray direction can be reconstructed from the timing of the air shower front sweeping across the detector, and the background of cosmic ray air showers can be reduced by rejecting showers with evidence of muons, which are rarely produced in gamma ray showers. At 4100 m elevation, the Sierra Negra site allows the detection of significantly lower energy air showers than HAWC’s predecessor, Milagro, which substantially enhances HAWC’s sensitivity to gamma ray sources.
2. Detector performance The Sierra Negra site lies on a saddle between the peaks of Sierra Negra and Pico de Orizaba in the Parque Nacional Pico de Orizaba, near the city of Puebla, Mexico. The site, which also hosts the Large Millimeter Telescope, provides a large flat area at 4100 m above sea level, approximately 1500 m higher than the site of the Milagro telescope. The site is located at 181590 4100 N, 971180 2800 W, providing a view of most of the Northern Hemisphere sky and somewhat more of the southern sky than was visible to Milagro. The effective area of HAWC as a function of energy is shown in Fig. 2, in comparison to Milagro. The effect of the higher altitude of the HAWC site is clearly visible in the order of magnitude increase in the effective area at trigger level for gamma rays with energies below several TeV. This improvement is further reinforced by the effect of a typical cut to reject background events produced by hadronic cosmic ray air showers, as indicated by the dashed lines. The combination will substantially extend the sensitivity of HAWC to extragalactic sources such as GRBs and AGN, which are observable primarily at low energies due to the attenuation of VHE gamma rays through interactions with the
T. DeYoung / Nuclear Instruments and Methods in Physics Research A 692 (2012) 72–76
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Fig. 1. Planned layout of the 300 HAWC water tanks, along with the VAMOS engineering array of seven tanks at left. The center of the HAWC array will contain the electronics shack, and the gaps between the rows of tanks permit physical access as well as providing space for cable runs and water drainage. Fig. 3. Angular resolution of HAWC as compared to Milagro. The solid line indicates 1s resolution: the angular error within which 68% of gamma rays are reconstructed. The bin size which optimizes sensitivity to point sources is slightly smaller, and is shown with a dashed line. For comparison, the angular resolution of Milagro is shown with the dot-dashed line.
Fig. 2. Effective area of HAWC for the detection of gamma rays, as compared to Milagro. The effective area is averaged over the solid angle within 301 of zenith. Solid lines indicate the effective area at trigger level, while dashed lines include the effect of a typical cut against hadronic showers. HAWC provides a substantial increase at energies below 10 TeV due to its higher altitude.
extragalactic background light (EBL). The array of outriggers surrounding Milagro was sufficient to detect showers at the highest energies; HAWC’s instrumented area is comparable to that of the Milagro outrigger array, so the effective area for gamma rays above 10 TeV is comparable. HAWC’s advantage over Milagro at those energies is in its superior angular resolution and energy resolution, as shown below. The higher altitude and greater active Cherenkov area of HAWC mean that substantially higher numbers of particles will be detected, as compared to Milagro. This allows considerably higher accuracy in the reconstruction of the shower front, leading to much better angular resolution than was possible with Milagro. The improvement is shown in Fig. 3. At energies above 10 TeV, HAWC’s angular resolution will be 0.1–0.21, with a resolution better than 1.01 for energies above roughly 500 GeV. By contrast, Milagro was limited to 0.51 even for the highest energy air showers. The higher efficiency for detecting particles also leads to better energy resolution than was possible with Milagro, as shown in Fig. 4. The energy resolution is limited by two effects: the accuracy with which the amount of energy reaching the detector can be reconstructed, and the stochastic history of the shower development in the atmosphere. The latter element is dominated
Fig. 4. Energy resolution of HAWC, compared to Milagro. The dashed line indicates the precision of reconstruction of the total energy of the particles reaching ground level, which is the experimental observable. The solid line indicates the resolution on the primary gamma ray energy; the difference between solid and dashed lines is due to the intrinsically stochastic nature of air shower development in the atmosphere, which is an irreducible uncertainty for a ground array such as HAWC.
by the depth to which the primary gamma ray penetrated into the atmosphere before first interacting, which cannot be determined by an air shower array such as HAWC. As shown in the figure, the error in the determination of the ground-level energy is substantially smaller than the total energy resolution, indicating that HAWC approaches the intrinsic limit of energy resolution for an extensive air shower array at this altitude. As is the case for the angular resolution, the higher altitude of the HAWC site leads to substantially better energy resolution than that of Milagro. Finally, the higher altitude and larger detector footprint of HAWC lead to substantial improvements in the accuracy of hadronic background rejection, as compared to Milagro. Air showers produced by hadronic cosmic rays are far more numerous than those
74
T. DeYoung / Nuclear Instruments and Methods in Physics Research A 692 (2012) 72–76
Fig. 6. Sensitivity of HAWC vs. energy for one year (dashed) and five years (solid) observation time. For atmospheric Cherenkov telescopes, 50 h of observation is assumed, while for Fermi and the air shower detectors the observation duration corresponds to the integrated time the source is present in the field of view. The thin dashed lines indicate the spectrum of the Crab nebula, scaled to 100%, 10%, and 1% of its actual flux. Fig. 5. Fraction of hadronic air showers surviving an energy-dependent cut which preserves 50% of the gamma rays at that estimated energy. Above 10 TeV, the hadronic background can be reduced by a factor of nearly 300, although the efficiency for detecting muons falls off with decreasing shower energy. At all energies, background rejection is considerably more effective than with Milagro, due to the much larger instrumented area of HAWC.
produced by gamma rays, so background rejection is extremely important for determining the sensitivity of a VHE gamma ray detector. The primary discriminant used by HAWC is the presence of muons in hadronic air showers. Muons striking a tank significantly removed from the central axis of the air shower will produce a high amplitude signal in one or more PMTs, much higher in amplitude than the signals produced by the electromagnetic particles (electrons, positrons, and photons) which fall off in number with distance from the shower axis. Searching for large signals at a distance of tens of meters from the reconstructed shower core thus allows us to reject a substantial fraction of the hadronic background, as shown in Fig. 5, especially for the higher energy showers where muon production is common. The same technique was used with Milagro, but due to the relatively small size of the central Milagro pond, muons were typically only seen if they were near the shower axis (and thus relatively difficult to detect).
3. Sensitivity to astrophysical sources The combination of the improved effective area, angular resolution, and background rejection available with HAWC will lead to a sensitivity nearly 15 times greater than that of Milagro. The sensitivity of HAWC in different energy ranges is compared to that of Milagro and to the Fermi LAT and various air Cherenkov telescopes in Fig. 6. Because HAWC observes sources whenever they are overhead, the sensitivity of HAWC to the high energy end of the spectrum is considerably better than is possible with air Cherenkov telescopes, given realistic observing budgets. With five years of observation, HAWC will survey approximately half the sky to a sensitivity of around 50 mCrab. In addition to its sensitivity to high energy emission from point sources, HAWC’s wide field of view makes it ideally suited to detect extended or diffuse emission from Galactic regions which may be too large for convenient observations with air Cherenkov telescopes. Milagro observations of the Cygnus region of the Galaxy [1,2] indicated a complex region, containing several point sources as well as apparently extended emission, perhaps from the interactions of cosmic rays on dust in the Galactic plane.
HAWC will be able to improve on Milagro’s observations of this region, perhaps resolving additional point sources or measuring the energy spectrum of the diffuse emission. HAWC’s wide field of view also makes it an excellent instrument for searching for VHE emission from transient sources, such as GRBs. Recent observations by the Fermi LAT [3–6] have confirmed the existence of a second spectral component in addition to the band component for several bright GRBs. This second component is an apparent power-law emission spectrum which develops more slowly than the lower energy emission and which extends to the highest energies accessible to the LAT (although in one instance a high energy cutoff may have been observed). The highest energy photon observed by the LAT was at 33.4 GeV [4], corresponding to an emitted energy of 94 GeV after correcting for the source redshift. It is unclear whether this emission extends to the VHE band, as might be expected if this second component reflects the acceleration of hadrons to extremely high energies. For relatively nearby GRBs, HAWC will be well suited to measuring the higher end of this spectral component [7]. Fig. 7 shows the sensitivity of HAWC to GRB emission as a function of the emission duration. It is assumed that the GRB time and location are known from observations at lower energies, and that the power law component observed in the Fermi LAT data extends until it is cut off by EBL absorbtion. Because HAWC’s effective area rises sharply with energy, the detectability of a GRB depends strongly on the index of the power law component; Fig. 7 shows the sensitivity for a relatively hard index of 1.6, as observed in GRB 090510, for various redshifts ranging from 0 to 2. For short GRBs with a redshift below approximately 1.0, the nF n sensitivity of HAWC is comparable that of the Fermi LAT, so HAWC should either detect GRBs or provide meaningful constraints on their emission at energies above those detected by the LAT. Fig. 7 also shows the sensitivity of HAWC observations in scaler mode, where individual gamma ray air showers are not detected but a rise in the flux of soft gamma rays associated with a GRB is observed as a correlated increase in the hit rates of all PMTs. In this mode, HAWC is sensitive to gamma rays at energies above roughly 700 MeV. For relatively hard spectra like the E1:6 spectrum assumed in Fig. 7, scaler mode observations offer relatively little advantage over the normal observation mode. For softer spectra, however, scaler observations are more sensitive than regular observations. Additionally, the different energy ranges to which the regular and scaler observations are sensitive
T. DeYoung / Nuclear Instruments and Methods in Physics Research A 692 (2012) 72–76
75
permit a rough measurement of the energy spectrum at energies of 1–100 GeV. Alternatively, if the spectral index is known from external observations, e.g. by the Fermi LAT, comparison of the observations in scaler and normal mode would allow us to infer the presence of a spectral cutoff. If the hard spectral component is not cut off by intrinsic effects in the GRB, but extends up to the energies where it is cut off by EBL attenuation, HAWC will record a wealth of data about the high energy behavior of the GRB. Fig. 8 shows a simulation of HAWC observations of GRB 090510, assuming that it occurred overhead and that the power law spectrum observed by the Fermi LAT was cut off by EBL attenuation. The Gilmore model of the EBL spectrum [8] was used, and both its effect and the hadronic background contribution are included in Fig. 8.
4. Prospects
Fig. 7. Sensitivity of HAWC to GRB emission, as a function of the emission duration. The lines labeled ‘‘HAWC’’ show the sensitivity of the standard observation mode, while those labeled ‘‘scaler’’ refer to the lower-energy scaler observations. The flux is normalized to the emission at 10 GeV, as observed by e.g. the Fermi LAT, and is assumed to extend with a spectral index of 1.6 as observed for GRB 090510. The actual flux and duration of GRB 090510 are indicated with an inverted triangle.
When complete, HAWC will be the world’s most sensitive wide field of view observatory in the VHE gamma ray band. Its field of view and sensitivity to high energy gamma rays will allow it to make important measurements of the high energy spectra of Galactic sources, of extended Galactic sources and diffuse gamma ray emission in the Galactic plane, and of emission from transient sources such as GRBs.
Fig. 8. Simulated data from HAWC observation of GRB 090510, had it occurred overhead with HAWC fully operational. The hard spectral component observed by the Fermi LAT is assumed to extend with the observed index up to energies where it is cut off by EBL attenuation.
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T. DeYoung / Nuclear Instruments and Methods in Physics Research A 692 (2012) 72–76
Construction of the HAWC array began in February 2011, and will continue for the next three years. In the fall of 2012 we expect to have 30 of a planned 300 tanks operational, providing a sensitivity comparable to Milagro’s. The full array will be complete in the fall of 2014. The original plan for HAWC called for three 8 in. Hamamatsu PMTs, recycled from Milagro, to be placed in each water tank. After the project was approved, additional funding was provided to purchase a fourth PMT for each tank. The fourth PMT will be a 10 in. Hamamatsu super-bialkali PMT, providing approximately twice the photon collection efficiency of the original tubes. This will substantially increase HAWC’s effectiveness for low energy gamma ray showers. All of the simulated performance plots included here are based on the original, three PMT plans, and are thus conservative estimate’s of HAWC’s final capabilities.
References [1] A.A. Abdo, et al., The Milagro Collaboration, The Astrophysical Journal 658 (2007) L33. [2] A.A. Abdo, et al., The Milagro Collaboration, The Astrophysical Journal 664 (2007) L91. [3] A.A. Abdo, et al., Fermi LAT and Fermi GBM Collaboration, Science 323 (2009) 1688. [4] A.A. Abdo, et al., The Fermi/GBM and The Fermi/LAT and The Swift Team Collaborations, The Astrophysical Journal 706 (2009) L138. [5] M. Ackermann, et al., The Fermi LAT and GBM Collaborations, The Astrophysical Journal 716 (2010) 1178. [6] M. Ackermann, et al., The Fermi Collaboration, The Astrophysical Journal 729 (2011) 114. [7] A.U. Abeysekara, et al., The HAWC Collaboration, Astroparticle Physics, forth coming. arXiv:1108.6034 (astro-ph.HE). [8] R.C. Gilmore, P. Madau, J.R. Primack, R.S. Somerville, F. Haardt, Monthly Notices of the Royal Astronomical Society 399 (2009) 1694.