Ground-based Gamma-ray Detectors Research Articles

Detection of Very High-energy Gamma-Ray Emission from the Radio Galaxy M87 with LHAASO

The nearby radio galaxy M87 is a very high-energy (VHE) gamma-ray emitter established by observations with ground-based gamma-ray detectors. Here we report the long-term monitoring of M87 from 2021 to 2024 with the Large High Altitude Air Shower Observatory (LHAASO). M87 has been detected by LHAASO with a statistical significance ∼ 9σ. The observed energy spectrum extends to 20 TeV, with a possible hardening at ∼20 TeV and then a clear softening at higher energies. Assuming that the intrinsic spectrum is described by a single power law up to 20 TeV, a tight upper bound on the extragalactic background light intensity is obtained. A strong VHE flare lasting 8 days, with a rise time of τrrise=1.05±0.49 days and decay time of τddecay=2.17±0.58 days, was found in early 2022. A possible GeV flare is seen also in Fermi Large Area Telescope data during the VHE flare period. The variability time as short as 1 day seen in the LHAASO data suggests a compact emission region with a size of ∼3 × 1015 δ cm (δ being the Doppler factor of the emitting region), corresponding to a few Schwarzschild radii of the central supermassive black hole in M87. The continuous monitoring of the source reveals a duty cycle of ∼1% for VHE flares with a flux above 10−11 erg cm−2 s−1.

Gamma/hadron separation with the HAWC observatory

The High Altitude Water Cherenkov (HAWC) gamma-ray observatory observes atmospheric showers produced by incident gamma rays and cosmic rays with energy from 300 GeV to more than 100 TeV. A crucial phase in analyzing gamma-ray sources using ground-based gamma-ray detectors like HAWC is to identify the showers produced by gamma rays or hadrons. The HAWC observatory records roughly 25,000 events per second, with hadrons representing the vast majority (>99.9%) of these events. The standard gamma/hadron separation technique in HAWC uses a simple rectangular cut involving only two parameters. This work describes the implementation of more sophisticated gamma/hadron separation techniques, via machine learning methods (boosted decision trees and neural networks), and summarizes the resulting improvements in gamma/hadron separation obtained in HAWC.

Searching for gamma-ray counterparts to gravitational waves from merging binary neutron stars with the Cherenkov Telescope Array

The merger of binary neutron star (BNS) systems are predicted to be progenitors of short gamma-ray bursts (GRBs); the definitive probe of this association came with the recent detection of gravitational waves (GWs) from a BNS merger by Advanced LIGO and Advanced Virgo (GW170817), in coincidence with the short GRB 170817A observed by Fermi-GBM and INTEGRAL. Short GRBs are also expected to emit very-high energy (VHE, > 10S0 GeV) photons and VHE electromagnetic (EM) upper limits have been set with observations performed by ground-based gamma-ray detectors and during the intense EM follow-up campaign associated with GW170817/GRB 170817A. In the next years, the searches for VHE EM counterparts will become more effective thanks to the Cherenkov Telescope Array (CTA): this instrument will be fundamental for the EM follow-up of transient GW events at VHE, owing to its unprecedented sensitivity, rapid response (few tens of seconds) and capability to monitor large sky areas via survey-mode operation. We present a comprehensive study on the prospects for joint GW and VHE EM observations of merging BNSs with Advanced LIGO, Advanced Virgo and CTA, based on detailed simulations of the multi-messenger emission and detection. We propose a new observational strategy optimized on the prior assumptions about the EM emission. The method can be further generalized to include other electromagnetic emission models. According to this study CTA will cover most of the region of the GW skymap for the intermediate and most energetic on-axis GRBs associated to the GW event. We estimate the expected joint GW and VHE EM detection rates and we found this rate goes from 0.08 up to 0.5 events per year for the most energetic EM sources.

The progresses and prospects of high-energy gamma-ray observations

The high energy gamma-ray emission is the high end of the cosmic electromagnetic radiation. It is associated with the huge energy release, relativistic particle acceleration and the corresponding non-thermal emission processes within the cosmic objects. High energy gamma-ray astronomy is an important window to explore physical processes under extreme conditions in the universe, and is also a power tool to study the origin of cosmic rays, a century puzzle. Thanks to the successful operation of a new generation of gamma-ray detectors, both space-based and ground-based gamma-ray detectors have achieved fruitful results. At energy above 100 MeV, satellite detector Fermi has increased the number of gamma-ray sources from about 300 to more than 3000 since 2008. At energy above 100 GeV, the ground-based Cherenkov telescopes, HESS, MAGIC, VERITAS, and EAS arrays, Tibet AS<italic>γ</italic>, Milagro, ARGO-YBJ, have also increased the number of gammaray sources from about 10 to more than 160 since 2003. At the same time, the next generation of ground-based detectors, CTA and LHAASO, are also steadily pushed forward, which will boost the sensitivity of current detectors at least up to 10 times. This article will review the current status of each gamma-ray experiment and their observation progresses on gamma-ray sources including pulsar and its wind nebula, supernova remnant, gamma-ray binary system, active galactic nuclei, GRB and so on. The potential prospects of future projects CTA and LHAASO are also discussed.

Observation of TeV gamma ray extended sources with ARGO-YBJ

More than 80% of TeV galactic gamma ray sources are spatially extended and many of them are still unidentified. The extended emission could be the result of cosmic ray interactions with the ambient medium which provides the target to produce TeV gamma rays. The sensitivity of ground based gamma ray detectors decreases for extended sources; shower detectors, due to their large field of view, are less affected with respect to Cherenkov telescopes. The ARGO-YBJ experiment (Yangbajing Cosmic Ray Laboratory, Tibet, China, 4300m of altitude) is an air shower detector devoted to gamma ray astronomy at energies above a few hundred GeV, with an integrated sensitivity ranging from 0.24 to ~1 Crab units, depending on the source declination. In this paper the observation of galactic extended sources with ARGO-YBJ during 5 years is reviewed.

Pionic photons and neutrinos from cosmic ray accelerators

Identifying the accelerators that produce the Galactic and extragalactic cosmic rays has been a priority mission of several generations of high energy gamma ray and neutrino telescopes; success has been elusive so far. Detecting the gamma-ray and neutrino fluxes associated with cosmic rays reaches a new watershed with the completion of IceCube, the first neutrino detector with sensitivity to the anticipated fluxes, and the construction of CTA, a ground-based gamma ray detector that will map and study candidate sources with unprecedented precision. In this paper, we revisit the prospects for revealing the sources of the cosmic rays by a multiwavelength approach; after reviewing the methods, we discuss supernova remnants, gamma ray bursts, active galaxies and GZK neutrinos in some detail.

Pionic Photons and Neutrinos from Cosmic Ray Accelerators

Abstract Identifying the accelerators that produce the Galactic and extragalactic cosmic rays has been a priority mission of several generations of high energy gamma ray and neutrino telescopes; success has been elusive so far. Detecting the gamma-ray and neutrino fluxes associated with cosmic rays reaches a new watershed with the completion of IceCube, the first neutrino detector with sensitivity to the anticipated fluxes, and the construction of CTA, a ground-based gamma ray detector that will map and study candidate sources with unprecedented precision. In this paper, we revisit the prospects for revealing the sources of the cosmic rays by a multiwavelength approach; after reviewing the methods, we discuss supernova remnants, gamma ray bursts, active galaxies and GZK neutrinos in some detail.

A Gamma-Ray Burst Remnant in Our Galaxy: HESS J1303-631

We present an investigation of the multiwavelength data on HESS J1303-631, an unidentified TeV source serendipitously discovered in the Galactic plane by the HESS collaboration. Our results strongly suggest the identification of this particular source as the remnant of a gamma-ray burst (GRB) that happened some few tens of thousands years ago in our Galaxy at a distance on the order of ≳10 kpc from us. We show through detailed calculations of particle diffusion, interaction, and radiation processes of relativistic particles in the interstellar medium that it is possible for a GRB remnant (GRBR) to be a strong TeV emitter with no observable synchrotron emission. We predict spectral and spatial signatures that would unambiguously distinguish GRBRs from ordinary supernova remnants, including (1) large energy budgets inferred from their TeV emission, but at the same time (2) suppressed fluxes in the radio through GeV wave bands; (3) extended center-filled emission with an energy-dependent spatial profile; and (4) possible elongation in the direction of the past pair of GRB jets. While GRBRs can best be detected by ground-based gamma-ray detectors, the future GLAST mission will play a crucial role in confirming the predicted low level of GeV emission.

The STACEE ground-based gamma-ray detector

We describe the design and performance of the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) in its complete configuration. STACEE uses the heliostats of a solar energy research facility to collect and focus the Cherenkov photons produced in gamma-ray induced air showers. The light is concentrated onto an array of photomultiplier tubes located near the top of a tower. The large Cherenkov photon collection area of STACEE results in a gamma-ray energy threshold below that of previous ground-based detectors. STACEE is being used to observe pulsars, supernova remnants, active galactic nuclei, and gamma-ray bursts

STEREO ARRAY of 30 m imaging atmospheric Čerenkov telescopes: A next-generation detector for ground-based high energy gamma-ray astronomy

The construction of the H.E.S.S. ( High Energy Stereoscopic System), a superior system of four 12 m imaging atmospheric Čerenkov telescopes, has been completed recently in the Namib desert close to Windhoek, at 1800 m above sea level. This new generation of ground-based gamma-ray detectors has an energy threshold of about 100 GeV in observations at zenith and a sensitivity of about 1% Crab flux for a point-like γ-ray source. Such high sensitivity was achieved due to 0.1° angular resolution, as well as, severe cosmic ray background rejection acquired from multi-fold imaging of individual atmospheric showers. In addition H.E.S.S. has a rather good energy resolution of 15%. H.E.S.S. has been taking routine observations of the γ-ray sources since December 2003. Similar stereoscopic arrays are currently under construction at Kitt Peak, Arizona, and at Woomera, Australia. Two telescopes of somewhat larger size of 17 m diameter are being built by the MAGIC collaboration on the Canary Island of La Palma. The first of these two telescopes has been taking data since the Fall of 2003. The outstanding physics results achieved with H.E.S.S. already in a first year of its exploitation strongly encourage further development of imaging atmospheric Čerenkov technique for high-quality γ-ray observations, which is basically driven by father reduction of the energy threshold of a forthcoming major future γ-ray detector. Here we are dealing with such a detector, which may allow us to achieve an energy threshold as low as 10 GeV given a unique sensitivity of about 2 × 10 −13 erg cm −2 s −1. Basic results on performance and sensitivity for a single stand-alone 30 m imaging atmospheric Čerenkov telescope, as well as for a system of two and five Čerenkov telescopes, derived from appropriate Monte Carlo simulations, are discussed here.

The STACEE-32 ground based gamma-ray detector

We describe the design and performance of the Solar Tower Atmospheric Cherenkov Effect Experiment detector in its initial configuration (STACEE-32). STACEE is a new ground-based gamma-ray detector using the atmospheric Cherenkov technique. In STACEE, the heliostats of a solar energy research array are used to collect and focus the Cherenkov photons produced in gamma-ray induced air showers. The large Cherenkov photon collection area of STACEE results in a gamma-ray energy threshold below that of previous detectors.

The laser calibration system for the STACEE ground-based gamma ray detector

We describe the design and performance of the laser system used for calibration monitoring of components of the STACEE detector. STACEE is a ground based gamma ray detector which uses the heliostats of a solar power facility to collect and focus Cherenkov light onto a system of secondary optics and photomultiplier tubes. To monitor the gain and check the linearity and timing properties of the phototubes and associated electronics, a system based on a dye laser, neutral density filters and optical fibres has been developed. In this paper we describe the system and present some results from initial tests made with it.

Large-area atmospheric Cherenkov detectors for high-energy gamma-ray astronomy

This paper describes the development of new ground-based gammaray detectors to explore the energy region between 20 and 200 GeV. This region in energy is interesting because it is currently unexplored by any experiment. The proposed detectors use the atmospheric Cherenkov technique, in which Cherenkov radiation produced in the gamma-ray air showers is detected using mirrors and light-sensitive devices. The important feature of the proposed experiments is the use of large mirror collection areas, which should allow for a significant improvement (i.e. reduction) in energy threshold over existing experiments. Large mirror areas are available for relatively low cost at central tower solar power plants, and there are two groups developing gamma-ray experiments using solar heliostat arrays. This paper summarizes the progress in the design of experiments using this novel approach.