Array Of Imaging Atmospheric Cherenkov Telescopes Research Articles

Multi-view Deep Learning for Imaging Atmospheric Cherenkov Telescopes

This research note concerns the application of deep-learning-based multi-view-imaging techniques to data from the High Energy Stereoscopic System Imaging Atmospheric Cherenkov Telescope array. We find that the earlier the fusion of layer information from different views takes place in the neural network, the better our model performs with this data. Our analysis shows that the point in the network where the information from the different views is combined is far more important for the model performance than the method used to combine the information.

The ASTRI Mini-Array: A New Pathfinder for Imaging Cherenkov Telescope Arrays

The ASTRI Mini-Array is an Istituto Nazionale di Astrofisica (INAF) project to build and operate an array of nine Imaging Atmospheric Cherenkov Telescopes (IACTs) at the Teide Astronomical Observatory of the Instituto de Astrofisica de Canarias in Tenerife (Spain) based on a host agreement with INAF and, as such, it will be the largest IACT array until the Cherenkov Telescope Array Observatory starts operations. Implementing the ASTRI Mini-Array poses several challenges from technical, logistic, and management points of view. Starting from the description of the innovative technologies adopted to build the telescopes, we will discuss the solutions adopted to overcome these challenges, making the ASTRI Mini-Array a great instrument to perform deep observations of the galactic and extra-galactic sky at very high energies.

A study on performance boost of a 17 m class Cherenkov telescope with a SiPM-based camera

The current generation of Imaging Atmospheric Cherenkov Telescopes (IACTs), comprised of major installations such as the MAGIC, H.E.S.S. and VERITAS telescopes, is sometimes called the 3rd generation of such instruments. These telescopes use multipixel cameras composed of hundreds up to thousands photomultiplier tubes (PMTs). The supremacy of PMTs is currently being challenged by photon sensors, rapidly spreading in popularity: the silicon photomultipliers (SiPMs), that are becoming a valid alternative thanks to their high PDE, low operating voltage and flexibility in installation. In this report, we investigate the performance of an existing 3rd generation IACT array (taking as a working example MAGIC), in which PMTs would be replaced with SiPMs, applying generalized simulations, not tuned for a specific hardware solution. We find an energy-dependent improvement in sensitivity, reaching a factor of three at the current trigger threshold energy. Interestingly, we also find that the stronger sensitivity of SiPMs in the red part of the spectrum, a source of background for IACTs, does not affect this performance.

Impact of satellite trails on H.E.S.S. astronomical observations

Context. The number of satellites launched into Earth’s orbit has almost tripled in the last three years (to over 4000) due to the increasing commercialisation of space. Multiple satellite constellations, consisting of over 400 000 individual satellites, have either been partially launched or are proposed for launch in the near future. Many of these satellites are highly reflective, resulting in a high optical brightness that affects ground-based astronomical observations. Despite this caveat, the potential effect of these satellites on gamma-ray-observing Imaging Atmospheric Cherenkov Telescopes (IACTs) has largely been assumed to be negligible due to their nanosecond-scale integration times. However, this assumption has not been verified to date. Aims. As IACTs are sensitive to optical wavelength light, we aim to identify satellite trails in data taken by the High Energy Stereoscopic System (H.E.S.S.) IACT array. In particular, this study is aimed at quantifying the potential effects on data quality and extensive air shower event classification and reconstruction. Methods. Using night sky background measurements from H.E.S.S., we determined which observation times and pointing directions are affected most by these satellite trails. We then evaluated their impact on the standard Hillas parameter variables used for event analysis. Results. Due to the brightest trails, false trigger events can occur, however, for most modern analyses, the effect on astronomical results will be minimal. We observe a mild increase in the rate of trail detections over time (approximately doubling in three years), which is partially correlated with the number of satellite launches. Overall, the fraction of H.E.S.S. data affected (~0.2% of dark time observations) is currently minimal. We note that these trails could still have a non-negligible effect on future Cherenkov Telescope Array observations if advanced analysis techniques designed to lower the energy threshold of the instrument are applied.

The effect of night sky background on source reconstruction in atmospheric Cherenkov telescope arrays and implications for image cleaning

Abstract Gamma-ray astronomy through imaging atmospheric Cherenkov telescope (IACTs) has enabled the study of numerous astrophysical sources in the GeV-TeV domain. The presence of night sky background (NSB) in IACT images affects the reconstruction of the gamma-ray being observed and is usually dealt through image cleaning. In this work, we propose that the image cleaning approach might be adapted to the reconstruction method and NSB levels. In particular, we study the effect of NSB on source reconstruction for gamma-rays observed through IACT arrays through Monte Carlo simulations. We find that the presence of NSB can have a smoothing effect on the shower images enabling better source reconstruction under certain circumstances. We also find that there is an optimum cleaning threshold depending on the energy of the initial gamma-ray. This threshold shows very little dependence on the NSB level or core position. Based on these results, we propose and test a hybrid image cleaning method for source reconstruction. We find that this hybrid image cleaning method results in better source reconstruction when observations are made in high NSB conditions. Whereas at low NSB levels, the application of a single image cleaning threshold yields better results. We finally conclude that this implies that different image cleaning methods might be needed to best carry out source reconstructions at different NSB levels.

The ASTRI Mini-Array

The ASTRI Mini-Array is an INAF project to build and operate a facility to study astronomical sources emitting very high energy in the TeV spectral band. It consists of a group of nine innovative aplanatic dual mirror Imaging Atmospheric Cherenkov Telescopes (IACT) of 4 m diameter. The telescopes are being installed at the Teide Astronomical Observatory of the Instituto de Astrofisica de Canarias in Tenerife (Canary Islands, Spain) based on a host agreement with INAF. Thanks to its expected overall performance, better than those of current IACT arrays, for energies above about 5 TeV and up to 100 TeV and beyond, the ASTRI Mini-Array will represent an important instrument to perform deep observations of the Galactic and extra-Galactic sky at these energies with high angular resolution (a few arcmins). It will be complementary to the wide-field particle shower arrays (based on water Cherenkov and scintillator detectors) like HAWC and LHAASO already operated in the northern hemisphere. The ASTRI Mini-Array is currently under construction and in this paper, we will present its status, discussing its design and expected performance.

Current status and operation of the H.E.S.S. array of imaging atmospheric Cherenkov telescopes

The High Energy Stereoscopic System (H.E.S.S.) is an array of five imaging atmospheric Cherenkov telescopes (IACTs) to study gamma-ray emission from astrophysical objects in the Southern hemisphere. It is the only hybrid array of IACTs, composed of telescopes with different collection area and footprint, individually optimised for a specific energy range. Collectively, the array is most sensitive to gamma rays in the range of 100GeV to 100TeV. The array has been in operation since 2002 and has been upgraded with new telescopes and cameras multiple times. Recent hardware upgrades and changes in the operational procedures increased the amount of observing time, which is of key importance for time-domain science. H.E.S.S. operations saw record data taking in 2020 and 2021 and we describe the current operations with specific emphasis on system performance, operational processes and workflows, quality control, and (near) real-time extraction of science results. In light of this, we will briefly discuss the early detection of gamma-ray emission from the recurrent nova RS Oph and alert distribution to the astrophysics community.

Detection of extended γ-ray emission around the Geminga pulsar with H.E.S.S.

Geminga is an enigmatic radio-quiet γ-ray pulsar located at a mere 250 pc distance from Earth. Extended very-high-energy γ-ray emission around the pulsar was discovered by Milagro and later confirmed by HAWC, which are both water Cherenkov detector-based experiments. However, evidence for the Geminga pulsar wind nebula in gamma rays has long evaded detection by imaging atmospheric Cherenkov telescopes (IACTs) despite targeted observations. The detection of γ-ray emission on angular scales ≳2º poses a considerable challenge for the background estimation in IACT data analysis. With recent developments in understanding the complementary background estimation techniques of water Cherenkov and atmospheric Cherenkov instruments, the H.E.S.S. IACT array can now confirm the detection of highly extended γ-ray emission around the Geminga pulsar with a radius of at least 3º in the energy range 0.5–40 TeV. We find no indications for statistically significant asymmetries or energy-dependent morphology. A flux normalisation of (2.8 ± 0.7) × 10−12 cm−2 s−1 TeV−1 at 1 TeV is obtained within a 1º radius region around the pulsar. To investigate the particle transport within the halo of energetic leptons around the pulsar, we fitted an electron diffusion model to the data. The normalisation of the diffusion coefficient obtained of D0 = 7.6−1.2+1.5 × 1027 cm2 s−1, at an electron energy of 100 TeV, is compatible with values previously reported for the pulsar halo around Geminga, which is considerably below the Galactic average.

Background rejection using image residuals from large telescopes in imaging atmospheric Cherenkov telescope arrays

Identification of Cherenkov light generated by muons has been suggested as a promising way to dramatically improve the background rejection power of Imaging Atmospheric Cherenkov Telescope (IACT) arrays at high energies. However, muon identification remains a challenging task, for which efficient algorithms are still being developed. We present an approach in which, rather than identifying Cherenkov light from muons, we simply consider the presence of Cherenkov light other than the main shower image in IACTs with large mirror area. We show that in the case of the H.E.S.S. array of five telescopes this approach results in background rejection improvements at all energies above 1 TeV. In particular, the rejection power can be improved by a factor sim ,3–4 at energies above 20 TeV while keeping sim ,90% of the original gamma-ray efficiency.

Current and Future γ-Ray Searches for Dark Matter Annihilation Beyond the Unitarity Limit

For decades, searches for electroweak-scale dark matter (DM) have been performed without a definitive detection. This lack of success may hint that DM searches have focused on the wrong mass range. A proposed candidate beyond the canonical parameter space is ultraheavy DM (UHDM). In this work, we consider indirect UHDM annihilation searches for masses between 30 TeV and 30 PeV—extending well beyond the unitarity limit at ∼100 TeV—and discuss the basic requirements for DM models in this regime. We explore the feasibility of detecting the annihilation signature, and the expected reach for UHDM with current and future very-high-energy (VHE; >100 GeV) γ-ray observatories. Specifically, we focus on three reference instruments: two Imaging Atmospheric Cherenkov Telescope arrays, modeled on VERITAS and CTA-North, and one extended air shower array, motivated by HAWC. With reasonable assumptions on the instrument response functions and background rate, we find a set of UHDM parameters (mass and cross section) for which a γ-ray signature can be detected by the aforementioned observatories. We further compute the expected upper limits for each experiment. With realistic exposure times, the three instruments can probe DM across a wide mass range. At the lower end, it can still have a point-like cross section, while at higher masses the DM could have a geometric cross section, indicative of compositeness.

Evidence for γ-ray emission from the remnant of Kepler’s supernova based on deep H.E.S.S. observations

Observations with imaging atmospheric Cherenkov telescopes (IACTs) have enhanced our knowledge of nearby supernova (SN) remnants with ages younger than 500 yr by establishing Cassiopeia A and the remnant of Tycho’s SN as very-high-energy (VHE) γ-ray sources. The remnant of Kepler’s SN, which is the product of the most recent naked-eye SN in our Galaxy, is comparable in age to the other two, but is significantly more distant. If the γ-ray luminosities of the remnants of Tycho’s and Kepler’s SNe are similar, then the latter is expected to be one of the faintest γ-ray sources within reach of the current generation IACT arrays. Here we report evidence at a statistical level of 4.6σ for a VHE signal from the remnant of Kepler’s SN based on deep observations by the High Energy Stereoscopic System (H.E.S.S.) with an exposure of 152 h. The measured integral flux above an energy of 226 GeV is ∼0.3% of the flux of the Crab Nebula. The spectral energy distribution (SED) reveals a γ-ray emitting component connecting the VHE emission observed with H.E.S.S. to the emission observed at GeV energies with Fermi-LAT. The overall SED is similar to that of the remnant of Tycho’s SN, possibly indicating the same nonthermal emission processes acting in both these young remnants of thermonuclear SNe.

Muons as a tool for background rejection in imaging atmospheric Cherenkov telescope arrays

The presence of muons in air-showers initiated by cosmic ray protons and nuclei is well established as a powerful tool to separate such showers from those initiated by gamma rays. However, so far this approach has been fully exploited only for ground level particle detecting arrays. We explore the feasibility of using Cherenkov light from muons as a background rejection tool for imaging atmospheric Cherenkov telescope arrays at the highest energies. We adopt an analytical model of the Cherenkov light from individual muons to allow rapid simulation of a large number of showers in a hybrid mode. This allows us to explore the very high background rejection power regime at acceptable cost in terms of computing time. We show that for very large (gtrsim 20 m mirror diameter) telescopes, efficient identification of muon light can potentially lead to background rejection levels up to 10^{-5} whilst retaining high efficiency for gamma rays. While many challenges remain in the effective exploitation of the muon Cherenkov light in the data analysis for imaging Cherenkov telescope arrays, our study indicates that for arrays containing at least one large telescope, this is a very worthwhile endeavor.

Predictions of TeV emission for a set of hard BL Lac objects

ABSTRACT We focus our analysis on 55 BL Lac objects with a hard Fermi gamma-ray spectrum, and for which a redshift or a lower limit to it has been determined by a previous study of ours. We extrapolate the spectral fits given by the 4FGL catalogue to the VHE band (>0.1 TeV), which can be explored by imaging atmospheric Cherenkov telescopes. Furthermore, we take into account the absorption due to the extragalactic background light, strongly depending on the redshift. Finally, we compare our results with publicly available sensitivity curves for a selection of imaging atmospheric Cherenkov telescope arrays currently operating or under construction. From our extrapolations and simulations we find a large number of promising candidates for observation with the forthcoming Cherenkov Telescope Array observatory.

Search for dark matter signals towards a selection of recently detected DES dwarf galaxy satellites of the Milky Way with H.E.S.S.

Dwarf spheroidal galaxy satellites of the Milky Way are prime targets for indirect detection of dark matter with gamma rays due to their proximity, high dark matter content and absence of non-thermal emission processes. Recently, the Dark Energy Survey (DES) revealed the existence of new ultra-faint dwarf spheroidal galaxies in the southern-hemisphere sky, therefore ideally located for ground-based observations with the imaging atmospheric Cherenkov telescope array H.E.S.S. We present a search for very-high-energy ($E\gtrsim100$ GeV) gamma-ray emission using H.E.S.S. observations carried out recently towards Reticulum II, Tucana II, Tucana III, Tucana IV and Grus II satellites. No significant very-high-energy gamma-ray excess is found from the observations on any individual object nor in the combined analysis of all the datasets. Using the most recent modeling of the dark matter distribution in the dwarf galaxy halo, we compute for the first time on DES satellites individual and combined constraints from Cherenkov telescope observations on the annihilation cross section of dark matter particles in the form of Weakly Interacting Massive Particles. The combined 95% C.L. observed upper limits reach $\langle \sigma v \rangle \simeq 1 \times 10^{-23}$ cm$^3$s$^{-1}$ in the $W^+W^-$ channel and $4 \times 10^{-26}$ cm$^3$s$^{-1}$ in the $\gamma\gamma$ channels for a dark matter mass of 1.5 TeV. The H.E.S.S. constraints well complement the results from Fermi-LAT, HAWC, MAGIC and VERITAS and are currently the most stringent in the $\gamma\gamma$ channels in the multi-GeV/multi-TeV mass range.

Demonstration of stellar intensity interferometry with the four VERITAS telescopes

High angular resolution observations at optical wavelengths provide valuable insights in stellar astrophysics, directly measuring fundamental stellar parameters, and probing stellar atmospheres, circumstellar disks, elongation of rapidly rotating stars, and pulsations of Cepheid variable stars. The angular size of most stars are of order one milli-arcsecond or less, and to spatially resolve stellar disks and features at this scale requires an optical interferometer using an array of telescopes with baselines on the order of hundreds of meters. We report on the successful implementation of a stellar intensity interferometry system developed for the four VERITAS imaging atmospheric-Cherenkov telescopes. The system was used to measure the angular diameter of the two sub-mas stars $\beta$ Canis Majoris and $\epsilon$ Orionis with a precision better than 5%. The system utilizes an off-line approach where starlight intensity fluctuations recorded at each telescope are correlated post-observation. The technique can be readily scaled onto tens to hundreds of telescopes, providing a capability that has proven technically challenging to current generation optical amplitude interferometry observatories. This work demonstrates the feasibility of performing astrophysical measurements with imaging atmospheric-Cherenkov telescope arrays as intensity interferometers and the promise for integrating an intensity interferometry system within future observatories such as the Cherenkov Telescope Array.

A run-wise simulation and analysis framework for Imaging Atmospheric Cherenkov Telescope arrays

We introduce a new simulation and analysis paradigm for Imaging Atmospheric Cherenkov Telescope (IACT) arrays, simulating the actual observation conditions as well as individual telescope configuration for each observation unit. Compared to existing frameworks, where simulations are usually generated using pre-defined settings, this run-wise simulation approach implies more realistic simulations and hence reduced systematic uncertainties. The computational effort of this dedicated simulation concept is notably independent of the amount of different observation configurations but just scales linearly with observation time. This corresponds to a large advantage for increasingly complex current and future IACT arrays where the size of the phase space makes it computationally unfeasible to generate simulations that reach the requirements regarding systematics using the classical simulation scheme.

Mapping large-scale diffuse γ-ray emission in the 10−100 TeV band with Cherenkov telescopes

Context. Measurement of diffuse γ-ray emission from the Milky Way with Imaging Atmospheric Cherenkov Telescopes (IACT) is difficult because of the high level of charged cosmic ray background and the small field of view. Aims. We show that such a measurement is nevertheless possible in the energy band 10−100 TeV. Methods. The minimal charged particle background for IACTs is achieved by selecting the events to be used for the analyses of the cosmic ray electrons. Tight cuts on the event quality in these event selections allow us to obtain a sufficiently low background level to allow measurement of the diffuse Galactic γ-ray flux above 10 TeV. We calculated the sensitivities of different types of IACT arrays for the Galactic diffuse emission measurements and compared them with the diffuse γ-ray flux from different parts of the sky measured by the Fermi Large Area Telescope below 3 TeV and with the astrophysical neutrino signal measured by IceCube telescope. Results. We show that deep exposure of existing IACT systems is sufficient for detection of the diffuse flux from all the Galactic Plane up to Galactic latitude |b| ∼ 5°. The Medium Size Telescope array of the CTA will be able to detect the diffuse flux up 30° Galactic latitude. Its sensitivity will be sufficient for detection of the γ-ray counterpart of the Galactic component of the IceCube astrophysical neutrino signal above 10 TeV. We also propose that a dedicated IACT system composed of small but wide-field-of-view telescopes could be used to map the 10−100 TeV diffuse γ-ray emission from across the whole sky. Conclusions. Detection and detailed study of diffuse Galactic γ-ray emission in the previously unexplored 10−100 TeV energy range is possible with the IACT technique. This is important for identification of the Galactic component of the astrophysical neutrino signal and for understanding the propagation of cosmic rays in the interstellar medium.

Very high-energy constraints on the infrared extragalactic background light

Context. Measurements of the extragalactic background light (EBL) are a fundamental source of information on the collective emission of cosmic sources. Aims. At infrared wavelengths, however, these measurements are precluded by the overwhelming dominance from interplanetary dust emission and the Galactic infrared foreground. Only at λ > 300 μm, where the foregrounds are minimal, has the infrared EBL (IR EBL) been inferred from analysis of the COBE maps. The present paper aims to assess the possibility of evaluating the IR EBL from a few micrometers up to the peak of the emission at > 100 μm using an indirect method that avoids the foreground problem. Methods. To this purpose we exploit the effect of pair-production from gamma-gamma interaction by considering the highest-energy photons emitted by extragalactic sources and their interaction with the IR EBL photons. We simulate observations of a variety of low-redshift emitters with the forthcoming Imaging Atmospheric Cherenkov Telescope (IACT) arrays (CTA in particular) and water Cherenkov observatories (LHAASO, HAWC, SWGO) to assess their suitability to constrain the EBL at such long wavelengths. Results. We find that even under the most extremely favorable conditions of huge emission flares, extremely high-energy emitting blazars are not very useful for our purpose because they are much too distant (> 100 Mpc the nearest ones, MKN 501 and MKN 421). Observations of more local AGNs displaying very high-energy emission, like low-redshift radio galaxies (M 87, IC 310, Centaurus A), are better suited and will potentially allow us to constrain the EBL up to λ ≃ 100 μm.

Potential for measuring the longitudinal and lateral profile of muons in TeV air showers with IACTs

Muons are copiously produced within hadronic extensive air showers (EAS) occurring in the Earth’s atmosphere, and are used by particle air shower detectors as a means of identifying the primary cosmic ray which initiated the EAS. Imaging Atmospheric Cherenkov Telescopes (IACTs), designed for the detection of γ-ray initiated EAS for the purposes of Very High Energy (VHE) γ-ray astronomy, are subject to a considerable background signal due to hadronic EAS. Although hadronic EAS are typically rejected for γ-ray analysis purposes, single muons produced within such showers generate clearly identifiable signals in IACTs and muon images are routinely retained and used for calibration purposes. For IACT arrays operating with a stereoscopic trigger, when a muon triggers one telescope, other telescopes in IACT arrays usually detect the associated hadronic EAS. We demonstrate for the first time the potential of IACT arrays for competitive measurements of the muon content of air showers, their lateral distribution and longitudinal profile of production slant heights in the TeV energy range. Such information can provide useful input to hadronic interaction models.

Atmospheric monitoring and inter-calibration of the telescope optical throughput efficiencies using the trigger rates of the Cherenkov Telescope Array

We discuss a calibration method for imaging atmospheric Cherenkov telescope arrays, based on the detection of cosmic rays. The focus lies on the monitoring of transmission of Cherenkov light in the atmosphere and on the relative calibration of telescope optical throughput efficiencies. We present an approach that addresses both issues by surveying and comparing trigger rates of telescopes in a stereoscopic configuration. A Monte Carlo feasibility study was conducted to explore dependencies of stereo trigger rates on the array layout and observing conditions of the Cherenkov Telescope Array (CTA). Analytical expressions for most of these dependencies have been found and implemented in an extension of the method of the Cherenkov Transparency Coefficient (CTC). In the investigated examples, the resolution of the method for the atmospheric and array calibration has been shown to be 4% and 4–7%, respectively.