Analog front end board electronics proposal for the HAWC Observatory

The High Altitude Water Cherenkov (HAWC) observatory is a wide field-of-view detector sensitive to gamma rays of 100 GeV to a few hundred TeV. Located in central Mexico at 19 degrees North latitude and 4100 m above sea level, HAWC will observe gamma rays and cosmic rays with an array of water Cherenkov detectors. The full HAWC array is scheduled to be operational in Spring 2015. In this paper, we study the HAWC sensitivity to the gamma-ray signatures of high-mass (multi- TeV) dark matter annihilation. The HAWC observatory will be sensitive to diverse searches for dark matter annihilation, including annihilation from extended dark matter sources, the diffuse gamma-ray emission from dark matter annihilation, and gamma-ray emission from non-luminous dark matter subhalos. Here we consider the HAWC sensitivity to a subset of these sources, including dwarf galaxies, the M31 galaxy, the Virgo cluster, and the Galactic center. We simulate the HAWC response to gamma rays from these sources in several well-motivated dark matter annihilation channels. If no gamma-ray excess is observed, we show the limits HAWC can place on the dark matter cross-section from these sources. In particular, in the case of dark matter annihilation into gauge bosons, HAWC will be able to detect a narrow range of dark matter masses to cross-sections below thermal. HAWC should also be sensitive to non-thermal cross-sections for masses up to nearly 1000 TeV. The constraints placed by HAWC on the dark matter cross-section from known sources should be competitive with current limits in the mass range where HAWC has similar sensitivity. HAWC can additionally explore higher dark matter masses than are currently constrained.

The High-Altitude Water Cherenkov (HAWC) Observatory was completed and began full operation on March 20, 2015. The detector consists of an array of 300 water tanks, each containing 200 ktons of purified water and instrumented with 4 PMTs. Located at an elevation of 4100m a.s.l. near the Sierra Negra volcano in central Mexico, HAWC has a threshold for gamma-ray detection well below 1 TeV and a sensitivity to TeV-scale gamma-ray sources an order of magnitude better than previous air-shower arrays. The detector operates 24 hours/day and observes the overhead sky ( 2 sr), making it an ideal survey instrument. We describe the configuration of HAWC with an emphasis on how the design was optimized, describe the data acquired, reconstructed and analyzed. Finally, we will demonstrate the sensitivity of the detector using the observation of the Crab. This paper serves as a detailed technical description of the foundations of the numerous analyses presented at this meeting by members of the HAWC collaboration.

The High-Altitude Water Cherenkov (HAWC) Observatory is designed to observe astrophysical sources of cosmic and gamma rays with energies from several hundred GeV up to several hundred TeV. HAWC is a survey instrument that maps a significant fraction of the gamma-ray sky due to its wide field of view. HAWC has extensively studied galactic sources of gamma rays, measuring their energy spectra and morphology. HAWC observes and measures variable and transient sources due to its continuous operation. HAWC has also measured the energy spectrum and anisotropy of the arrival directions of cosmic rays. HAWC participates in multimessenger studies with other observatories, including the IceCube neutrino and LIGO/Virgo gravitational wave observatories. HAWC has also performed indirect dark matter searches and studies of beyond-standard model particle physics measurements.

Sensitivity of the high altitude water Cherenkov detector to sources of multi-TeV gamma rays

The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory continuously detects TeV photons and particles within its large field of view, accumulating every day a deeper exposure of two-thirds of the sky. We analyzed 1523 days of HAWC live data acquired over four and a half years, in a follow-up analysis of 138 nearby (z < 0.3) active galactic nuclei from the Third Catalog of Hard Fermi-LAT sources culminating within 40° of the zenith at Sierra Negra, the HAWC site. This search for persistent TeV emission used a maximum-likelihood analysis assuming intrinsic power-law spectra attenuated by pair production of gamma-ray photons with the extragalactic background light. HAWC clearly detects persistent emission from Mkn 421 and Mkn 501, the two brightest blazars in the TeV sky, at 65σ and 17σ level, respectively. Marginal evidence, just above the 3σ level, was found for three other known very high-energy emitters: the radio galaxy M87 and the BL Lac objects VER J0521+211 and 1ES 1215+303, the latter two at z ∼ 0.1. We find a 4.2σ evidence for collective emission from the set of 30 previously reported very high-energy sources, with Mkn 421 and Mkn 501 excluded. Upper limits are presented for the sample under the power-law assumption and in the predefined (0.5–2.0), (2.0–8.0), and (8.0–32.0) TeV energy intervals.

The particle lateral distribution function (LDF) of air showers at a given altitude is sensitive to the mass composition and primary energy of cosmic rays. Studies of the LDF are difficult to perform due to experimental effects such as sampling bias, as well as shower-to-shower fluctuations in particle density. The High Altitude Water Cherenkov (HAWC) observatory, a dense air shower array located in central Mexico at 4100 m a.s.l., is well-suited to perform detailed event-by-event studies of the LDF of multi-TeV cosmic-ray showers. The detector is instrumented with 1,200 photomultipliers (PMTs) in close-packed water Cherenkov tanks containing a total of 60 ML of water. We present a study of the LDF of cosmic-ray air showers recorded by HAWC in 2016 with energies between 3 TeV and 300 TeV and zenith angle < 16.7º. The data are used to determine the optimal parameterization of the LDF at the HAWC site. From here the lateral shower age is obtained and its sensitivity to the cosmic ray mass composition is analyzed.

HAWC: A next-generation all-sky gamma-ray telescope

The Andromeda Galaxy (M31) is a nearby (∼780 kpc) galaxy similar to our own Milky Way. Observational evidence suggests that it resides in a large halo of dark matter (DM), making it a good target for DM searches. We present a search for gamma rays from M31 using 1017 days of data from the High Altitude Water Cherenkov (HAWC) Observatory. With its wide field of view and constant monitoring, HAWC is well-suited to search for DM in extended targets like M31. No DM annihilation or decay signal was detected for DM masses from 1 to 100 TeV in the bb̄, tt̄, τ+τ−, μ+μ−, and W+W− channels. Therefore we present limits on those processes. Our limits nicely complement the existing body of DM limits from other targets and instruments. Specifically the DM decay limits from our benchmark model are the most constraining for DM masses from 25 TeV to 100 TeV in the bb̄ and tt̄ channels. In addition to DM-specific limits, we also calculate general gamma-ray flux limits for M31 in 5 energy bins from 1 TeV to 100 TeV.

Compact binary systems can provide us with unique information on astrophysical particle acceleration and cosmic ray production. However, only five binary systems have ever been observed in TeV $\gamma$ rays. The High Altitude Water Cherenkov (HAWC) Observatory has high uptime (duty cycle $>95\%$) and a wide field of view (2 sr), making it well-suited for observing transient sources such as binaries. Using two years of data from HAWC, we have searched for TeV emission from three known TeV binary systems in the field of view and twenty-eight TeV binary candidates. We have searched the HAWC data for evidence of orbital modulation or flares from these objects, and report estimates of their $\gamma$-ray flux.

The High-Altitude Water-Cherenkov (HAWC) experiment is a TeV $\gamma$-ray observatory located 4100 m above sea level on the Sierra Negra mountain in Puebla, Mexico. The detector consists of 300 water-filled tanks, each instrumented with 4 photomultiplier tubes that utilize the water-Cherenkov technique to detect atmospheric air showers produced by cosmic $\gamma$ rays. Construction of HAWC was completed in March of 2015. The experiment's wide instantaneous field of view (2 sr) and high duty cycle (> 95%) make it a powerful survey instrument sensitive to pulsars, supernova remnants, and other $\gamma$-ray sources. The mechanisms of particle acceleration at these sources can be studied by analyzing their high-energy spectra. To this end, we have developed an event-by-event energy-reconstruction algorithm using an artificial neural network to estimate energies of primary $\gamma$ rays at HAWC. We will present the details of this technique and its performance as well as the current progress toward using it to measure energy spectra of $\gamma$-ray sources.

The High Altitude Water Cherenkov (HAWC) gamma-ray observatory is carrying out a detailed survey of the northern sky at TeV energies. 2HWC J2006+341 is a newly discovered source reported by the point source search in the 2HWC HAWC Observatory Gamma Ray Catalog. Using $\sim$1038 days of HAWC data we carried out a detailed analysis of the region revealing extended emission. No emission has been detected by other TeV instruments at the location of 2HWC J2006+341. We also analyzed publicly available data from the Ferm}-LAT that show for the first time the existence of an extended GeV source with a hard spectrum in the region of 2HWC J2006+341. Combined modeling of the data from HAWC and the Fermi-LAT allowed us to explore different scenarios for the origin of 2HWC J2006+341.

Detections of very-high-energy (VHE; > 100 GeV) photons from Gamma-ray bursts (GRBs) can provide fundamental information on the involved radiative processes, physical composition of the ejecta and acceleration processes. The High Altitude Water Cherenkov (HAWC) gamma-ray observatory is the best gamma-ray instrument to study transient phenomena over a long period of time from 100 GeV to 100 TeV. Its large field of view and duty cycle (> 95\%) allow it to search blindly for sources of GRBs and to follow up on external alerts from satellite instruments, such as Fermi and Swift, searching for their VHE counterpart. We present results from the on-going GRB monitoring program and VHE upper limits of the latest interesting low-redshift and/or powerful bursts in HAWC's field of view: GRB 170206A, GRB 170817A, GRB 180720B and GRB 190114C.

Many gamma-ray bursts (GRBs) have been observed from radio wavelengths, and a few at very high energies (VHEs, >100 GeV). The High Altitude Water Cherenkov (HAWC) gamma-ray observatory is well suited to study transient phenomena at VHEs owing to its large field of view and duty cycle. These features allow for searches of VHE emission and can probe different model assumptions of duration and spectra. In this paper, we use data collected by HAWC between 2014 December and 2020 May to search for emission in the energy range from 80 to 800 GeV coming from a sample of 47 short GRBs that triggered the Fermi, Swift, and Konus satellites during this period. This analysis is optimized to search for delayed and extended VHE emission within the first 20 s of each burst. We find no evidence of VHE emission, either simultaneous or delayed, with respect to the prompt emission. Upper limits (90% confidence level) derived on the GRB fluence are used to constrain the synchrotron self-Compton forward-shock model. Constraints for the interstellar density as low as 10−2 cm−3 are obtained when assuming z = 0.3 for bursts with the highest keV fluences such as GRB 170206A and GRB 181222841. Such a low density makes observing VHE emission mainly from the fast-cooling regime challenging.

The High Altitude Water Cherenkov (HAWC) observatory recently published the discovery of SS 433 as a TeV source, reporting the observation of multi-TeV gamma-ray emission from the jet interaction regions e1 and w1, suggesting in-situ particle acceleration. This showed the first direct evidence of acceleration in jets at energies greater than a few TeV. SS 433 was the first microquasar to be discovered and is still considered special in that the accretion is supercritical and the luminosity of the system is very high ($\sim10^{40}$ erg s$^{-2}$). The lobes of the supernova remnant W 50 in which the jets terminate, about 40 parsecs from the central binary, are expected to accelerate charged particles, and indeed radio and X-ray emission consistent with electron synchrotron emission in a magnetic field have been observed. SS 433 has also been a strong candidate for hadronic acceleration due to spectroscopic evidence of ionized nuclei in the inner jets. However, multiwavelength fits including the HAWC measurements favor the leptonic production of the observed gamma rays. Here, we present new follow-up measurements of the jet interaction regions of SS 433 using the most recent data from HAWC.

Gamma-ray binaries are luminous in gamma rays, composed of a compact object orbiting a massive companion star. The interaction between these two objects can drive relativistic outflows, either jets or winds, in which particles can be accelerated to energies reaching hundreds of teraelectronvolts (TeV). However, it is still debated where and under which physical conditions particles are accelerated in these objects and ultimately whether protons can be accelerated up to PeV energies. Among the well-known gamma-ray binaries, LS 5039 is a high-mass X-ray binary with an orbital period of 3.9 days that has been observed up to TeV energies by the High Energy Stereoscopic System. We present new observations of LS 5039 obtained with the High Altitude Water Cherenkov (HAWC) observatory. Our data reveal that the gamma-ray spectrum of LS 5039 extends up to 200 TeV with no apparent spectral cutoff. Furthermore, we confirm, with a confidence level of 4.7σ, that the emission between 2 and 118 TeV is modulated by the orbital motion of the system, and find a 2.2σ hint of variability above 100 TeV. This indicates that these photons are likely produced within or near the binary orbit, where they can undergo absorption by the stellar photons. In a leptonic scenario, the highest energy photons detected by HAWC can be emitted by ∼200 TeV electrons inverse Compton scattering stellar photons, which would require an extremely efficient acceleration mechanism operating within LS 5039. Alternatively, a hadronic scenario could explain the data through proton–proton or proton–gamma collisions of protons accelerated to petaelectronvolt energies.