TeV ASTROPHYSICS WITH THE MILAGRO AND HAWC OBSERVATORIES

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

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.

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.

Above 10\(^{15}\) eV, the CR flux drops below a few tens of particles per m\(^2\)-year. It is no longer possible to detect the incident particles above the atmosphere before they interact. Direct experiments (characterized by a small geometrical factor \(A\cdot \varDelta \varOmega \) [cm\(^2\) sr]) must be replaced by ground-based instruments that cover up to several thousands of km\(^2\), the extensive air shower (EAS) arrays. They use a completely different approach to CR measurements, started by pioneering experiments soon after World War II by Auger, Kohlhorster and Rossi. The EAS arrays are in most cases large area and long duration experiments studying, as accurately as possible, the nature, flux, mass, direction of primary CRs up to the highest energies. Air showers are initiated by primary CRs, through the interaction with a nucleus in the atmosphere. In addition to the hadronic component the decays of short-lived hadrons lead to a shower of particles: photons, electrons, and positrons constitute the electromagnetic (EM) component; muons and neutrinos constitute the penetrating component. All these particles travel at the speed of light in the atmosphere approximately along the direction of the primary CR. High energy primary \(\gamma \)-rays induce an almost pure EM cascade. In Sect. 4.3 we present a simple model for the EM cascade initiated by a \(\gamma \)-ray, which can be mathematically treated using differential transport equations of \(e^\pm \) and \(\gamma \) in the atmosphere. Some simple features, as derived from approximate solutions of the cascade equations, are presented. The cascades initiated by primary CR protons or nuclei have additional features. They are also characterized by large event-to-event fluctuations. Their description is today achieved using full Monte Carlo simulations, which follow the details of the development of the EM and muonic components. It is interesting, however, to obtain (Sect. 4.4) a first-order estimate of the quantities, which can be measured by experiments both for the muonic and the EM components. In the first sections of the chapter, we enumerate the main showers features (denoted as SF1, SF2,..., SF12)) which characterize EM and muonic component of the cascade initiated by \(\gamma \)-rays, protons, and heavier nuclei. These characteristics are confirmed by detailed Monte Carlo simulations of air showers in the atmosphere (Sect. 4.5), which are used by the experiments to interpret their observations. EAS arrays are installed on ground and are sometimes referred to as indirect detection experiments. Modern shower arrays employ complementary techniques (Sect. 4.6) such as scintillators, air Cherenkov detectors, etc. to measure simultaneously as many shower parameters as possible, in order to reduce the model dependence in the energy and mass number \(A\) determination. The features of the EM and muonic cascades will be used in Sects. 4.8, 4.9 and 4.10 to illustrate how indirect experiments can derive the CR flux and properties in the energy region around the knee of CRs.

We present the first catalogs of the highest-energy (above 56 TeV and 100 TeV) gamma-ray sources seen by the High Altitude Water Cherenkov (HAWC) Observatory. The wide field-of-view of HAWC naturally lends itself to unbiased all-sky surveys and newly developed event-by-event gamma-ray energy reconstruction algorithms have allowed unprecedented energy resolution. The sources presented here are the highest-energy sources ever detected. All are coincident with known lower-energy gamma-ray sources within our Galaxy. These objects may have implications for the sources of Galactic cosmic rays; since Galactic CRs have been observed up to PeV energies, sources accelerating particles to these energies must exist. These sources, called PeVatrons'', would have corresponding hard gamma-ray spectra that extend to high energies without any spectral break or cutoff. We will present measurements of the spectra of these highest-energy gamma-ray sources and discuss if any of them can be identified as PeVatron candidates.

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

The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory is an extensive air shower array located in Puebla, Mexico. The closest radio galaxy within the HAWC field of view, M87, has been detected in very high energies. In this work we report upper limits on the TeV $\gamma$-ray flux of the radio galaxy M87. At a distance of 16 Mpc, M87 is a supergiant elliptical galaxy located in the Virgo Cluster that has been observed from radio wavelengths to TeV $\gamma$-rays. Although a single-zone synchrotron self-Compton model has been successfully used to explain the spectral energy distribution of this source up to a few GeV, the $\gamma$-ray spectrum at TeV has been interpreted within different theoretical models. We discuss the implications of these upper limits on the hadronic interactions, as well as the number of neutrino events expected in the IceCube neutrino telescope.

Cosmic rays, with an energy density of $\sim1\,\text{eV}\,\text{cm}^{-3}$, play an important role in the evolution of our Galaxy. Very high energy (TeV) gamma rays provide unique information about the acceleration sites of Galactic cosmic rays. The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory is an all-sky surveying instrument sensitive to gamma rays from 100\:GeV to 100\:TeV with a 2\,steradian instantaneous field of view and a duty cycle of $>95\%$. The array is located in Sierra Negra, Mexico at an elevation of 4,100\,m and was inaugurated in March 2015. Thanks to its modular design, science operation began in Summer 2013 with one third of the array. Using this data, a survey of the inner Galaxy region of Galactic longitude $l\in [+15\degree,\,+50\degree]$ and latitude $b\in[-4\degree,\,+4\degree]$ is performed. To address the ambiguities arising from unresolved sources in the data, a maximum likelihood technique is used to identify point source candidates. Ten sources and candidate sources are identified in this analysis. Eight of these are associated with known TeV sources but not all have differential fluxes compatible with previous measurements. Three sources are detected with significances $>5\,\sigma$ after accounting for statistical trials, and are associated with known TeV sources. With data taken with the full array and improved reconstruction algorithms, the significance on the Crab nebula increases from $3.1\,\sigma/\sqrt{\text{day}}$ to $5.5\,\sigma/\sqrt{\text{day}}$, which allows more sensitive sky surveys and more precise spectral and morphological analyses on individual sources.

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.

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.

We propose a new energy estimator to determine the energies of gamma-ray induced air showers based on the lateral distribution of extensive air showers in the energy range between 10 TeV and 1000 TeV. We carry out a detailed Monte Carlo simulation assuming the Tibet air shower array located at an altitude of 4,300 m above sea level. We define S50, which denotes the particle density at 50 m from the air shower axis, as a new energy estimator. Using S50, the energy resolution is estimated to be approximately 16 % at 100 TeV in the range of the zenith angle 𝜃 < 20∘. We find S50 giving a better energy resolution than 27 % for the air shower size (N e) and 30 % for the sum of detected particles ( $\sum \rho $ ), which have been used so far, at 100 TeV. We also compare the reconstructed age distributions of gamma-ray induced air showers and hadronic cosmic-ray induced air showers. The age parameter may help to discriminate between primary gamma rays and hadronic cosmic rays.

A measurement of the phase distribution of the extensive air showers from the direction of the Crab Nebula

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.

Unlike Imaging Atmospheric Cherenkov Telescopes (IACTs), Extensive Air Shower (EAS) arrays can observe cosmic rays with wide fields of view of about 2 sr and with duty cycles of almost 24 hours every day regardless of weather conditions or the moonlight. Some EAS arrays are located at high altitudes to lower their observation thresholds of cosmic rays and successfully observe gamma rays in the range of several TeV. However, sensitivity of EAS arrays is not as good as IACT mainly because the angular resolution of EAS arrays is poor. In this paper, we report the progress of performance study of an EAS array of scintillation detectors with surface water Cherenkov detectors (WCDs) for improving its angular resolution.

A diffuse gamma-ray emission at $\sim$100 TeV can be expected as a result of the interactions of ultra-high-energy cosmic rays (UHECRs) with the cosmic microwave background (CMB) during their propagation. This radiation carries the information on the distribution of energetic sources and hence the cosmological evolution of the universe. The GRAPES-3 is an extensive air shower (EAS) array, located at Ooty in southern India. It consists of 400 plastic scintillators (each 1 m$^2$) and a large area (560 m$^2$) muon telescope. The muon telescope has the ability to differentiate the gamma-rays from charged cosmic rays through their muon content. We report on the study of isotropic diffuse gamma-ray flux from GRAPES-3 over 10-300 TeV.