Ground-based Gamma-ray Telescopes Research Articles

The Upcoming GAMMA-400 Experiment

The upcoming GAMMA-400 experiment will be implemented aboard the Russian astrophysical space observatory, which will be operating in a highly elliptical orbit over a period of 7 years to provide new data on gamma-ray emissions and cosmic-ray electron + positron fluxes, mainly from the galactic plane, the Galactic Center, and the Sun. The main observation mode will be a continuous point-source mode, with a duration of up to ~100 days. The GAMMA-400 gamma-ray telescope will study high-energy gamma-ray emissions of up to several TeV and cosmic-ray electrons + positrons up to 20 TeV. The GAMMA-400 telescope will have a high angular resolution, high energy and time resolutions, and a very good separation efficiency for separating gamma rays from the cosmic-ray background and the electrons + positrons from protons. A distinctive feature of the GAMMA-400 gamma-ray telescope is its wonderful angular resolution for energies of >30 GeV (0.01° for Eγ = 100 GeV), which exceeds the resolutions of space-based and ground-based gamma-ray telescopes by a factor of 5–10. GAMMA-400 studies can reveal gamma-ray emissions from dark matter particles’ annihilation or decay, identify many unassociated, discrete sources, explore the extended sources’ structures, and improve the cosmic-ray electron + positron spectra data for energies of >30 GeV.

VTSCat: The VERITAS Catalog of Gamma-Ray Observations

The ground-based gamma-ray observatory Very Energetic Radiation Imaging Telescope Array System (VERITAS, https://veritas.sao.arizona.edu/) is sensitive to photons of astrophysical origin with energies in the range between ≈85 GeV and ≈30 TeV. The instrument consists of four 12 m diameter imaging Cherenkov telescopes operating at the Fred Lawrence Whipple Observatory in southern Arizona. VERITAS started four-telescope operations in 2007 and collects about 1100 hr of good-weather data per year. The VERITAS collaboration has published over 100 journal articles since 2008 reporting on gamma-ray observations of a large variety of objects: Galactic sources like supernova remnants, pulsar wind nebulae, and binary systems; extragalactic sources like star-forming galaxies, dwarf-spheroidal galaxies, and highly variable active galactic nuclei. This note presents VTSCat: the catalog of high-level data products from all VERITAS publications.

Constraining branon dark matter from observations of the Segue 1 dwarf spheroidal galaxy with the MAGIC telescopes

We present the first search for signatures of brane-world extra-dimensional dark matter (DM) in the very-high-energy gamma-ray band by scrutinizing observations of the dwarf spheroidal galaxy Segue 1 with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescope system. Branons are new degrees of freedom that appear within flexible brane-world models: they are weakly interacting massive particles and natural DM candidates. The ground-based gamma-ray telescopes MAGIC could indirectly detect branon DM in the multi-TeV mass range by observing secondary products of DM annihilation into Standard Model particles. In the absence of a signal, we place constraints on the branon DM parameter space by using a binned likelihood analysis of almost 160-hours deep exposure on the Segue 1 dwarf spheroidal galaxy by the MAGIC telescopes. Our most stringent limit to the thermally-averaged annihilation cross-section (at 95% confidence level) corresponds to 〈σv〉 ≃ 1.4 × 10-23 cm3 s-1 at a branon mass of ∼ 0.7 TeV.

Gamma- and Cosmic-Ray observations with the GAMMA-400 Gamma-Ray telescope

The future space-based GAMMA-400 gamma-ray telescope will operate onboard the Russian astrophysical observatory in a highly elliptic orbit during 7 years to observe Galactic plane, Galactic Center, Fermi Bubbles, Crab, Vela, Cygnus X, Geminga, Sun, and other regions and measure gamma- and cosmic-ray fluxes. Observations will be performed in the point-source mode continuously for a long time (∼100 days). GAMMA-400 will measure gamma rays in the energy range from ∼ 20 MeV to several TeV and cosmic-ray electrons + positrons up to several tens TeV. GAMMA-400 instrument will have very good angle and energy resolutions, high separation efficiency of gamma rays from cosmic-ray background, as well as electrons + positrons from protons. The main feature of GAMMA-400 is the unprecedented angular resolution for energies > 30 GeV better than the space-based and ground-based gamma-ray telescopes by a factor of 5–10. GAMMA-400 observations will permit to resolve gamma rays from annihilation or decay of dark matter particles, identify many discrete sources, clarify the structure of extended sources, specify the data on cosmic-ray electron + positron spectra.

Evolution of Data Formats in Very-High-Energy Gamma-Ray Astronomy

Most major scientific results produced by ground-based gamma-ray telescopes in the last 30 years have been obtained by expert members of the collaborations operating these instruments. This is due to the proprietary data and software policies adopted by these collaborations. However, the advent of the next generation of telescopes and their operation as observatories open to the astronomical community, along with a generally increasing demand for open science, confront gamma-ray astronomers with the challenge of sharing their data and analysis tools. As a consequence, in the last few years, the development of open-source science tools has progressed in parallel with the endeavour to define a standardised data format for astronomical gamma-ray data. The latter constitutes the main topic of this review. Common data specifications provide equally important benefits to the current and future generation of gamma-ray instruments: they allow the data from different instruments, including legacy data from decommissioned telescopes, to be easily combined and analysed within the same software framework. In addition, standardised data accessible to the public, and analysable with open-source software, grant fully-reproducible results. In this article, we provide an overview of the evolution of the data format for gamma-ray astronomical data, focusing on its progression from private and diverse specifications to prototypical open and standardised ones. The latter have already been successfully employed in a number of publications paving the way to the analysis of data from the next generation of gamma-ray instruments, and to an open and reproducible way of conducting gamma-ray astronomy.

Search for spectral features in extragalactic background light with gamma-ray telescopes

Context. Cumulative optical and infrared emission from galaxies accumulated over cosmological time scales, the extragalactic background light (EBL), could be probed by complementary techniques of direct observations and source counting in the visible and infrared as well as via its imprint on the signal of distant active galactic nuclei in γ-rays. Aims. We compare the visible and infrared measurements with the γ-ray constraints and study if the discrepancies of the measurements with different methods could be due to the presence of features in the EBL spectrum that are localised in the micron wavelength range. Methods. We combined data on time-averaged spectra of selected blazars that were obtained by Fermi and ground-based gamma-ray telescopes. We also modelled the effect of absorption on EBL while allowing for the existence of a previously unaccounted spectral feature. Results. We show that a previously reported “excess” in EBL flux in the ∼1 micron wavelength range is consistent with γ-ray measurements, that is, if the excess has the form of a narrow feature of the width δλ < λ and an overall flux of up to 15 nW m−2 sr−1 above the “minimal” EBL, which is estimated from the visible and infrared source counts. Such ’bump-like’ spectral features could originate, for example, from decaying dark-matter particles, or either axions or peculiar astrophysical processes in the course of star-formation history. We discuss the possibilities for the search of spectral features in the EBL with the Cherenkov Telescope Array.

Extragalactic background light models and GeV-TeV observation of blazars

ABSTRACT In this work, we use two different methods to determine the opacity of the TeV gamma-rays caused by the extragalactic background light (EBL) via e—e+ production due to photon-photon interaction. The first method, Model-Dependent Approach, uses various EBL models for estimating the opacity as a function of the redshift and energy of the TeV photons. The second method, Model-Independent Approach, relies on using the simultaneous observations of blazars in the MeV-GeV energy range from the Fermi-LAT and in the TeV band from the ground-based gammaray telescopes. We make the underline assumption that the extrapolation of the LAT spectrum of blazars to TeV energies is either a good estimate or an upper limit for the intrinsic TeV spectrum of a source. We apply this method on the simultaneous observations of a few blazars at different redshifts to demonstrate a comparative study of six prominent EBL models. Opacities of the TeV photons predicted by the model-independent approach are systematically larger than the ones estimated from the modeldependent method. Therefore, the gamma-ray observations of blazars can be used to set a strict upper limit on the opacity of the Universe to the TeV photons at a given redshift.

GammaLearn - first steps to apply Deep Learning to the Cherenkov Telescope Array data

The Cherenkov Telescope Array (CTA) is the next generation of ground-based gamma-ray telescopes for gamma-ray astronomy. Two arrays will be deployed composed of 19 telescopes in the Northern hemisphere and 99 telescopes in the Southern hemisphere. Due to its very high sensitivity, CTA will record a colossal amount of data that represent a computing challenge to the reconstruction software. Moreover, the vast majority of triggered events come from protons that represent a background for gamma-ray astronomy. Deep learning developments in the last few years have shown tremendous improvements in the analysis of data in many domains. Thanks to the huge amount of simulated data and later of real data, produced by CTA, these algorithms look well-suited and very promising. Moreover, the trained neural networks show very good computing performances during execution. Here we present a first study of deep learning architectures applied to CTA simulated data to perform the reconstruction of the particles energy and incoming direction and the development of a specific framework, GammaLearn, to accomplish this task.

GAMMA-400 Project

Extraterrestrial gamma-ray astronomy is now a source of a new knowledge in the fields of astrophysics, cosmic-ray physics, and the nature of dark matter. The next absolutely necessary step in the development of extraterrestrial high-energy gamma-ray astronomy is the improvement of the physical and technical characteristics of gamma-ray telescopes, especially their angular and energy resolutions. Such a new generation telescope will be GAMMA-400, currently under development. Together with an X-ray telescope, it will perform precise and detailed observations in the energy range of ~20 MeV to ~10 000 GeV and 3–30 keV the Galactic plane, especially, toward the Galactic Center, Fermi Bubbles, Crab, Cygnus, etc. The GAMMA-400 will operate in the highly elliptic orbit continuously for a long time with the unprecedented angular (~0.01◦ at Eγ = 100 GeV) and energy (~1% at Eγ = 100 GeV) resolutions, exceeding the Fermi-LAT as well as ground-based gamma-ray telescopes by a factor of 5–10. GAMMA-400 will permit resolving gamma rays from annihilation or decay of dark matter particles, identifyingmany discrete sources (many of which are variable), clarifying the structure of extended sources, specifying the data on the diffuse emission, as well as measuring electron + positron fluxes and specifying electron + positron spectrum in the energy range from 1 GeV to 10 000 GeV.

AGN at very high energies: Cosmic accelerators and probes of space-time

During the last decade, very high energy astrophysics emerged as a new branch of astronomy with major discoveries achieved by the present ground-based gamma-ray Cherenkov telescopes. The sample of cosmic sources firmly detected at very high energy (VHE) now exceeds two hundred objects, including active galactic nuclei (AGN), pulsar wind nebulae, and several other types of sources of which a significant number are unidentified ones. The scientific return from recent VHE data is particularly interesting for AGN science, shedding new light on particle acceleration and emission processes around supermassive black holes, and probing the intergalactic space by the analysis of VHE photons propagating from bright remote sources to the Earth. The perspectives of this research field are promising with new generation VHE instruments such as CTA, a project of open observatory at extreme energies at the horizon 2023, allowing a deep analysis of the sky in the highest part of the electromagnetic spectrum, from 20 GeV to 300 TeV.

Capture and decay of electroweak WIMPonium

The spectrum of Weakly-Interacting-Massive-Particle (WIMP) dark matter generically possesses bound states when the WIMP mass becomes sufficiently large relative to the mass of the electroweak gauge bosons. The presence of these bound states enhances the annihilation rate via resonances in the Sommerfeld enhancement, but they can also be produced directly with the emission of a low-energy photon. In this work we compute the rate for SU(2) triplet dark matter (the wino) to bind into WIMPonium—which is possible via single-photon emission for wino masses above 5 TeV for relative velocity v < O(10−2) —and study the subsequent decays of these bound states. We present results with applications beyond the wino case, e.g. for dark matter inhabiting a nonabelian dark sector; these include analytic capture and transition rates for general dark sectors in the limit of vanishing force carrier mass, efficient numerical routines for calculating positive and negative-energy eigenstates of a Hamiltonian containing interactions with both massive and massless force carriers, and a study of the scaling of bound state formation in the short-range Hulth'{e}n potential. In the specific case of the wino, we find that the rate for bound state formation is suppressed relative to direct annihilation, and so provides only a small correction to the overall annihilation rate. The soft photons radiated by the capture process and by bound state transitions could permit measurement of the dark matter's quantum numbers; for wino-like dark matter, such photons are rare, but might be observable by a future ground-based gamma-ray telescope combining large effective area and a low energy threshold.

ROBAST: Development of a ROOT-based ray-tracing library for cosmic-ray telescopes and its applications in the Cherenkov Telescope Array

We have developed a non-sequential ray-tracing simulation library, ROOT-basedsimulatorforraytracing (ROBAST), which is aimed to be widely used in optical simulations of cosmic-ray (CR) and gamma-ray telescopes. The library is written in C++, and fully utilizes the geometry library of the ROOT framework. Despite the importance of optics simulations in CR experiments, no open-source software for ray-tracing simulations that can be widely used in the community has existed. To reduce the dispensable effort needed to develop multiple ray-tracing simulators by different research groups, we have successfully used ROBAST for many years to perform optics simulations for the Cherenkov Telescope Array (CTA). Among the six proposed telescope designs for CTA, ROBAST is currently used for three telescopes: a Schwarzschild–Couder (SC) medium-sized telescope, one of SC small-sized telescopes, and a large-sized telescope (LST). ROBAST is also used for the simulation and development of hexagonal light concentrators proposed for the LST focal plane. Making full use of the ROOT geometry library with additional ROBAST classes, we are able to build the complex optics geometries typically used in CR experiments and ground-based gamma-ray telescopes. We introduce ROBAST and its features developed for CR experiments, and show several successful applications for CTA.

Very high-energyγ-ray emission from high-redshift blazars

We study the possible detection of and properties of very high-energy (VHE) gamma-ray emission (in the energy band above 100 GeV) from high redshift sources. We report on the detection of VHE gamma-ray flux from blazars with redshifts z>0.5. We use the data of Fermi telescope in the energy band above 100 GeV and identify significant sources via cross-correlation of arrival directions of individual VHE gamma-rays with the positions of known Fermi sources. There are thirteen high-redshift sources detected in the VHE band by Fermi/LAT telescope. The present statistics of the Fermi signal from these sources is too low for a sensible study of the effects of suppression of the VHE flux by pair production through interactions with Extragalactic Background Light photons. We find that the detection of these sources with ground-based gamma-ray telescopes would be challenging. However, several sources including BL Lacs PKS 0426-380 at z=1.11, KUV 00311-1938 at z=0.61, B3 1307+433 at z=0.69, PG 1246+586 at z=0.84, Ton 116 at z=1.065 as well as a flat-spectrum radio quasar 4C +55.17 at z=0.89 should be detectable by HESS-II, MAGIC-II and CTA. A high-statistics study of a much larger number of VHE gamma-ray sources at cosmological distances would be possible with the proposed high-altitude Cherenkov telescope 5@5.

The camera of the fifth H.E.S.S. telescope. Part I: System description

In July 2012, as the four ground-based gamma-ray telescopes of the H.E.S.S. (High Energy Stereoscopic System) array reached their tenth year of operation in Khomas Highlands, Namibia, a fifth telescope took its first data as part of the system. This new Cherenkov detector, comprising a 614.5m2 reflector with a highly pixelized camera in its focal plane, improves the sensitivity of the current array by a factor two and extends its energy domain down to a few tens of GeV.The present part I of the paper gives a detailed description of the fifth H.E.S.S. telescope׳s camera, presenting the details of both the hardware and the software, emphasizing the main improvements as compared to previous H.E.S.S. camera technology.

VERITAS OBSERVATIONS OF RELATIVISTIC OUTFLOWS IN LOW- AND INTERMEDIATE-FREQUENCY-PEAKED BL LAC OBJECTS

The majority of blazars detected at very high energies (VHE; E &gt; 100 GeV) are high-frequency-peaked BL Lac objects (HBLs). Low- and intermediate-frequency-peaked BL Lacs (LBLs/IBLs with synchrotron-peak frequencies in the infrared and optical regime) are generally more powerful, more luminous, and have a richer jet environment than HBLs. However, only a handful of these IBL and LBLs have been detected by ground-based gamma-ray telescopes, typically during high-flux states. The VERITAS array has been monitoring five known VHE LBLs/IBLs since 2009: 3C 66A, W Comae, PKS 1424+240, S5 0716+714 and BL Lacertae, with typical exposures of 5-10 hours per year. The results of these long-term observations are presented, including a bright, subhour-scale VHE flare of BL Lacertae in June 2011, the first low-state detections of 3C 66A and W Comae, and the detection and characterization of the IBL B2 1215+30.

Gamma rays from cosmic rays in supernova remnants

Context: Cosmic rays are thought to be accelerated at supernova remnant (SNR) shocks, but conclusive evidence is lacking. Aims: New data from ground-based gamma-ray telescopes and the Large Area Telescope on the Fermi Gamma-ray Space Telescope are used to test this hypothesis. A simple model for gamma-ray production efficiency is compared with measured gamma-ray luminosities of SNRs, and the GeV to TeV fluxes ratios of SNRs are examined for correlations with SNR ages. Methods: The supernova explosion is modeled as an expanding spherical shell of material that sweeps up matter from the surrounding interstellar medium (ISM). The accumulated kinetic energy of the shell, which provides the energy available for nonthermal particle acceleration, changes when matter is swept up from the ISM and the SNR shell decelerates. A fraction of this energy is assumed to be converted into the energy of cosmic-ray electrons or protons. Three different particle radiation processes---nuclear pion-production interactions, nonthermal electron bremsstrahlung, and Compton scattering---are considered. Results: The efficiencies for gamma-ray production by these three processes are compared with gamma-ray luminosities of SNRs. Our results suggest that SNRs become less gamma-ray luminous at >~ 10^4 yr, and are consistent with the hypothesis that supernova remnants accelerate cosmic rays with an efficiency of ~10% for the dissipation of kinetic energy into nonthermal cosmic rays. Weak evidence for an increasing GeV to TeV flux ratio with SNR age is found.

Design constraints on Cherenkov telescopes with Davies–Cotton reflectors

This paper discusses the construction of high-performance ground-based gamma-ray Cherenkov telescopes with a Davies–Cotton reflector. For the design of such telescopes, usually physics constrains the field-of-view, while the photo-sensor size is defined by limited options. Including the effect of light-concentrators, it is demonstrated that these constraints are enough to mutually constrain all other design parameters. The dependability of the various design parameters naturally arises once a relationship between the value of the point-spread functions at the edge of the field-of-view and the pixel field-of-view is introduced. To be able to include this constraint into a system of equations, an analytical description for the point-spread function of a tessellated Davies–Cotton reflector is derived from Taylor developments and ray-tracing simulations. Including higher order terms renders the result precise on the percent level.Design curves are provided within the typical phase space of Cherenkov telescopes. The impact of all design parameters on the overall design is discussed. Allowing an immediate comparison of several options with identical physics performance allows the determination of the most cost efficient solution. Emphasis is given on the possible application of solid light concentrators with their typically about two times better concentration compared with hollow cones which allows the use of small photo sensors such as Geiger-mode avalanche photo diodes. This is discussed in more details in the context of possible design options for the Cherenkov Telescope Array. In particular, a solution for a 60mm2 photo sensor with hollow cone is compared to a 36mm2 with solid cone.

Probing the 130 GeV gamma-ray line with ground-based gamma-ray telescopes

The evidence of a monochromatic γ-ray line-like structure at ∼130 GeV, recently obtained from Fermi Large Area Telescope (Fermi-LAT) data, poses serious challenges for standard astrophysical processes. It is generally believed that the narrow profile of the spectral line is very likely to be originated from dark matter annihilation or decay. Further careful observations with either satellite detectors or ground-based detectors are necessary. Here we suggest that ground-based γ-ray telescopes, such as High Energy Stereo System-II (HESS II) (under construction) and the forthcoming high performance experiment Cerenkov Telescope Array (CTA), will be very effective in testing the monochromatic nature of the tentative 130 GeV γ-ray line. With approximately 10-h exposure, HESS II (CTA) may reach a 9σ (100σ) detection of the line, if it is indeed a γ-ray line.

Active Galactic Nuclei under the scrutiny of CTA

Active Galactic Nuclei (hereafter AGN) produce powerful outflows which offer excellent conditions for efficient particle acceleration in internal and external shocks, turbulence, and magnetic reconnection events. The jets as well as particle accelerating regions close to the supermassive black holes (hereafter SMBH) at the intersection of plasma inflows and outflows, can produce readily detectable very high energy gamma-ray emission. As of now, more than 45 AGN including 41 blazars and 4 radiogalaxies have been detected by the present ground-based gamma-ray telescopes, which represents more than one third of the cosmic sources detected so far in the VHE gamma-ray regime. The future Cherenkov Telescope Array (CTA) should boost the sample of AGN detected in the VHE range by about one order of magnitude, shedding new light on AGN population studies, and AGN classification and unification schemes. CTA will be a unique tool to scrutinize the extreme high-energy tail of accelerated particles in SMBH environments, to revisit the central engines and their associated relativistic jets, and to study the particle acceleration and emission mechanisms, particularly exploring the missing link between accretion physics, SMBH magnetospheres and jet formation. Monitoring of distant AGN will be an extremely rewarding observing program which will inform us about the inner workings and evolution of AGN. Furthermore these AGN are bright beacons of gamma-rays which will allow us to constrain the extragalactic infrared and optical backgrounds as well as the intergalactic magnetic field, and will enable tests of quantum gravity and other “exotic” phenomena.

VERITAS OBSERVATIONS OF THE TeV BINARY LS I +61° 303 DURING 2008-2010

We present the results of observations of the TeV binary LS I +61 303 with the VERITAS telescope array between 2008 and 2010, at energies above 300 GeV. In the past, both ground-based gamma-ray telescopes VERITAS and MAGIC have reported detections of TeV emission near the apastron phases of the binary orbit. The observations presented here show no strong evidence for TeV emission during these orbital phases; however, during observations taken in late 2010, significant emission was detected from the source close to the phase of superior conjunction (much closer to periastron passage) at a 5.6 standard deviation (5.6 sigma) post-trials significance. In total, between October 2008 and December 2010 a total exposure of 64.5 hours was accumulated with VERITAS on LS I +61 303, resulting in an excess at the 3.3 sigma significance level for constant emission over the entire integrated dataset. The flux upper limits derived for emission during the previously reliably active TeV phases (i.e. close to apastron) are less than 5% of the Crab Nebula flux in the same energy range. This result stands in apparent contrast to previous observations by both MAGIC and VERITAS which detected the source during these phases at >10% of the Crab Nebula flux. During the two year span of observations, a large amount of X-ray data were also accrued on LS I +61 303 by the Swift X-ray Telescope (XRT) and the Rossi X-ray Timing Explorer Timing (RXTE) Proportional Counter Array (PCA). We find no evidence for a correlation between emission in the X-ray and TeV regimes during 20 directly overlapping observations. We also comment on data obtained contemporaneously by the Fermi Large Area Telescope (LAT).