Cherenkov Telescope Research Articles

High-Density Near-Ultraviolet Silicon Photomultipliers: Characterization of photosensors for Cherenkov light detection

In recent years, Silicon Photomultipliers (SiPMs) have proven to be highly suitable devices for applications where high sensitivity to low-intensity light and fast responses are required. Among their many advantages are their low operational voltage when compared with classical photomultiplier tubes, mechanical robustness, and increased photon detection efficiency (PDE).Here we present a full characterization of a SiPM device technology developed in Italy by Fondazione Bruno Kessler, which is suitable for Cherenkov light detection in the Near-Ultraviolet (NUV) band. This device is a High-Density (HD) NUV SiPM, based on a microcell of 40μm×40μm and with an area of 6 × 6mm2, providing low levels of dark noise and high PDE peaking in the NUV band. This particular device has been selected to equip a part of the focal plane of the Schwarzschild–Couder Telescope (SCT) prototype proposed for the Cherenkov Telescope Array (CTA) Observatory.

EUSO-SPB2: A balloon experiment for UHECR and VHE neutrino observation

The Extreme Universe Space Observatory on a Super Pressure Balloon 2 (EUSO-SPB2) experiment will make new measurements from suborbital space as a precursor for future space missions that will address the challenge of the extremely low fluxes of ultra-high energy cosmic rays (UHECR) and very high energy (VHE) neutrinos. The EUSO-SPB2 detector is comprised of two 1m diameter aperture telescopes. The Fluorescence Telescope (FT) will point in nadir and will record fluorescence light from cosmic ray EAS with energies above 1 EeV in its field of view of 36 by 12 degrees. The Cherenkov Telescope (CT) features a silicon photomultiplier focal surface with a field of view of 12 by 6 degrees. The CT will switch between two observation modes: one which points the CT above the limb to measure the Cherenkov emission of cosmic ray EAS with energies above 1 PeV and one which points the CT below the limb to record the Cherenkov emission produced by PeV scale EAS initiated by neutrino-sourced tau decay. As it is the first time such an instrument has been flown, one of the priorities of the CT will be the study of the optical background for observing neutrinos in this way. EUSO-SPB2 is undergoing the final integration steps for launch on a NASA super pressure balloon payload in the spring of 2023 from Wanaka NZ. The CT was field-tested in March 2022 and the FT in August - September 2022. The data collected during the EUSO-SPB2 mission will be essential to advancing the development of a space-based multi-messenger observatory such as the Probe of Extreme Multi-Messenger Astrophysics (POEMMA). In this contribution, we discuss the EUSO-SPB2 science goals, the instruments, the expected performance and the current status.

A 16-channel programmable SiPM bias supply system for Cherenkov telescope imaging camera

In recent years, the Silicon Photomultipliers (SiPMs) have emerged as a promising photodetector in various applications such as high energy physics, nuclear physics and medical instruments. One such application is in the pixelated camera of Imaging Atmospheric Cherenkov Telescopes (IACTs). IACTs are used to detect very high-energy celestial gamma rays using Atmospheric Cherenkov Technique. The SiPM gain is proportional to the overvoltage which is the difference between applied bias voltage and breakdown voltage. As the SiPM breakdown voltage increases with temperature, the overvoltage and hence the gain decreases proportionally (2-3%/°C) at a constant applied bias voltage. Also, the actual bias voltage across the SiPM changes with load current due to voltage drop across a series resistor in SiPM bias circuit, thus causing a change in overvoltage and gain. To maintain a constant gain of the SiPM, the applied bias voltage need to be adjusted to compensate for the changes in temperature and load. We are developing a 256-pixel imaging camera with SiPM as its photo-sensor. The camera will be placed at the focal point of telescope and will be operated during night in outdoor environment at Hanle, Ladakh, India. The night temperature at Hanle typically varies by ∼ 10°C overnight and ∼ 40°C (-20°C to +20°C) over the year. Thus, gain of SiPMs, exposed to the environment, may vary considerably during observations. A prototype 8-channel bias supply board with real time temperature & load compensation is developed to operate the camera SiPMs at fixed gain throughout the observation night.The voltage range of the bias supply for each channel is from 10 V to 80 V with 5 mV resolution and current upto 4 mA. The output voltage and current can be monitored with a resolution of ∼ 5 mV and ∼ 0.3 μA respectively. The single board computer, Raspberry Pi, is connected to the bias supply board over a 7-wire Customized Serial Peripheral Interface (CSPI) for control and monitoring. A multi-channel SiPM bias supply system is realized by daisy-chaining 8-channel boards through CSPI to cater bias voltage to all the pixels of the camera. The system is operated remotely using Ethernet interface of Raspberry-Pi. This paper discusses the design, development and performance of the prototype 16-channel closed-loop, programmable bias supply system with built-in compensation for changes in temperature and load.

Simulations of the M@TETelescope

AbstractThe M@TE (Monitoring at TeV Energies) project consists of the installation of a small Imaging Air Cherenkov Telescope (IACT) at the Observatorio Astronómico Nacional de San Pedro Mártir, Mexico, for long‐term monitoring of the TeV emission from blazars. M@TE has a similar design to the First G‐APD Cherenkov Telescope (FACT), featuring higher photon detection efficiency SiPM sensors and mechanical and electronics upgrades. In this work, simulations are used to assess M@TE's performance. FACT's simulation software was adapted to take into account M@TE's new features. This work presents the simulations as well as results on M@TE's angular resolution and energy reconstruction.

A new multi‐messenger description of starburst galaxies emission: perspectives for neutrino and gamma‐ray observations

AbstractStar‐forming and starburst galaxies (SBGs), which are well‐known cosmic‐ray (CR) reservoirs, are expected to emit gamma rays and neutrinos predominantly via hadronic collisions. In this work we analyze the 10‐year Fermi‐Low Energy Technique (LAT) spectral energy distributions of 13 nearby galaxies by means of a physical model that accounts for high‐energy proton transport in starburst nuclei and includes the contribution of primary and secondary electrons. In particular, we test the hypothesis that the observed gamma‐ray fluxes are mostly due to star‐forming activity, which is in agreement with the available star formation rates coming from infrared (IR) and ultraviolet (UV) observations. Through this observation‐based approach, we determine the most‐likely neutrino counterparts from star‐forming and SBGs and quantitatively assess the ability of current and upcoming neutrino telescopes to detect them as point‐like sources. We also generate mock gamma‐ray data to simulate the Cherenkov Telescope Array (CTA) performance in detecting these sources. Moreover, we propose a test to discriminate between the two different CR transport models for the starburst nuclei by looking at the different gamma‐ray expectations. We point out that current data already gives a slight preference to CR models, which are dominated by advection.

Expectation on observations of Fermi-LAT gamma-ray sources using the HADAR experiment

High altitude detection of astronomical radiation (HADAR) is an innovative array of atmospheric Cherenkov telescopes that employs pure water as its medium. By utilizing large-aperture hemispherical lenses, HADAR can capture atmospheric Cherenkov light, enabling the detection of gamma rays and cosmic rays in the energy range of 10 GeV to 10 TeV. Compared to traditional Imaging Atmospheric Cherenkov telescopes, HADAR offers distinct advantages such as a low energy threshold, high sensitivity, and a wide field of view. The telescope mainly consists of a hemispherical lens with a diameter of 5 m acting as a Cherenkov light collector, a cylindrical metal tank with a 4 m radius and 7 m height, and an imaging system at the bottom of the tank. The sky region covered by HADAR is much larger than the current generation of Imaging Atmospheric Cherenkov Telescopes. The field of view of HADAR can reach up to 60 degrees. Its continuous scanning capability allows for comprehensive observations of gamma-ray sources throughout the entire celestial sphere, making it an ideal instrument for studying transient and variable sources. In this study, the observational capabilities of HADAR are thoroughly investigated using the latest 4FGL-DR3 and 4LAC-DR3 gamma-ray source catalogs from Fermi-LAT. For extragalactic sources, the energy spectra in the high energy range have been extrapolated to the very high energy range, taking into account the absorption effect caused by extragalactic background light. By comparing the extrapolated results with existing VHE experimental data, the feasibility of this extrapolation method has been demonstrated. Through simulated analyses of the significance of these sources, it is anticipated that HADAR will detect a total of 93 gamma-ray sources with a significance exceeding 5 standard deviations during one year of operation. These sources comprise 45 galactic sources, 39 extragalactic sources, 3 sources of unknown type, and 6 unassociated sources.

TeV emission from Gamma Ray Bursts, checking the hadronic model

Gamma-Ray Bursts (GRBs) are the most luminous explosions in the Universe. Their luminous prompt emission makes them detectable from cosmological distances. Most GRBs have been detected below a few MeV, however at least a hundred GRBs have been detected at high (0.1 GeV) energies and observed up to tens of GeV with the Fermi Large Area Telescope (LAT). Some GRBs have been observed at (0.1–1) TeV by ground-based imaging atmospheric Cherenkov telescopes. To date, the high energy emission mechanism is not understood. In this paper we review the possible leptonic and hadronic mechanisms capable of producing the ∼ TeV emission detected in GRBs. In paricular we concentrate on the hadronic origin of this radiation component and discuss in detail the numerical simulation elaborated to reproduce the observed sub-TeV observations of GRB190114C.

Detailed Analysis of the TeV γ-Ray Sources 3HWC J1928+178, 3HWC J1930+188, and the New Source HAWC J1932+192

The latest High Altitude Water Cherenkov (HAWC) point-like source catalog up to 56 TeV reported the detection of two sources in the region of the Galactic plane at galactic longitude 52° < ℓ < 55°, 3HWC J1930+188 and 3HWC J1928+178. The first one is associated with a known TeV source, the supernova remnant SNR G054.1+00.3. It was discovered by one of the currently operating Imaging Atmospheric Cherenkov Telescope (IACT), the Very Energetic Radiation Imaging Telescope Array System (VERITAS), detected by the High Energy Stereoscopic System (H.E.S.S), and identified as a composite SNR. However, the source 3HWC J1928+178, discovered by HAWC and coincident with the pulsar PSR J1928+1746, was not detected by any IACT despite their long exposure on the region, until a recent new analysis of H.E.S.S. data was able to confirm it. Moreover, no X-ray counterpart has been detected from this pulsar. We present a multicomponent fit of this region using the latest HAWC data. This reveals an additional new source, HAWC J1932+192, which is potentially associated with the pulsar PSR J1932+1916, whose γ-ray emission could come from the acceleration of particles in its pulsar wind nebula. In the case of 3HWC J1928+178, several possible explanations are explored, in an attempt to unveil the origins of the very-high-energy γ-ray emission.

Prospective study on observations of gamma-ray emission from active galactic nuclei using the HADAR experiment

The High Altitude Detection of Astronomical Radiation (HADAR) experiment is a refracting terrestrial telescope array based on the atmospheric Cherenkov imaging technique. It is a hybrid array consisting of four water-lens telescopes and a surrounding scintillation detector array for observing Cherenkov light induced by 10 GeV–10 TeV cosmic rays and gamma rays in the atmosphere. The water-lens telescope mainly consists of a hemispherical lens with a diameter of 5 m acting as a Cherenkov light collector, a cylindrical metal tank with a 4 m radius and 7 m height, and an imaging system at the bottom of the tank. The sky region covered by HADAR is much larger than the current generation of Imaging Atmospheric Cherenkov Telescopes, and even the CTA. The field-of-view (FOV) of HADAR can reach up to 60 degrees. The HADAR experiment possesses the advantages of a large field-of-view and low energy threshold, so it can continuously scan wide portions of the sky and easily observe extragalactic gamma-ray sources. The majority of the extragalactic gamma-ray sources detected at very high energy (VHE) energies are active galactic nuclei (AGNs). In this study, we present the potential of using the HADAR experiment for detecting AGN. Based on the AGN catalog sources of the Fermi Large Area Telescope (Fermi-LAT), the observed energy is extrapolated to the VHE range. The VHE gamma rays propagating over cosmological distances can interact with the low-energy of the extragalactic background light (EBL) and produce electron-positron pairs. Therefore, we consider the absorption effects of different EBL models when calculating the expected gamma ray spectra of the AGN sample. We select the sample with redshift measurements and locations inside the FOV of HADAR from 4LAC catalog. In total, there are 375 BL Lacertae objects (BL Lacs) and 289 flat-spectrum radio quasars (FSRQs) satisfying the selection conditions. The integral gamma ray spectra are derived and compared with the sensitivity curve of HADAR, the number of sources with fluxes above the sensitivity of HADAR is counted. Further, we calculate the statistical significance of HADAR for AGN source observation based on the equi-zenith angle sky scanning analysis method. The simulation results reveal that a total of 31 sources of Fermi-LAT AGN can be detected by HADAR with a significance greater than five standard deviations over a one-year survey period, most of which are BL Lacs.

Synchrotron self-compton emission in the two-component jet model for gamma-ray bursts

Gamma-ray bursts (GRBs) are intense bursts of high-energy photons (prompt emissions) caused by relativistic jets. After the emissions, multi-wavelength afterglows, from radio to very-high-energy (VHE) gamma-ray, last for more than a few days. In the past three years, the VHE gamma-ray photons from four GRBs (GRBs 180720B, 190114C, 190829A and 201216C) were detected by ground-based Imaging Atmospheric Cherenkov Telescopes, such as the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes and the High Energy Stereoscopic System (H.E.S.S.). One of them, GRB 190829A, had some peculiar features of showing achromatic peaks in X-ray and optical bands at 1.4×103 s and being classified as low-luminosity GRBs. Previously, we proposed a two-component jet model, which has ‘narrow jet’ with a small initial opening half-angle θ0=0.015 rad and large bulk Lorentz factor Γ0=350, and ‘wide jet’ with θ0=0.1 rad and Γ0=20. The narrow jet explained the early X-ray and optical emissions and apparently small isotropic gamma-ray energy and peak energy in the off-axis viewing case. Furthermore, the late X-ray and radio (1.3 and 15.5 GHz) afterglows were emitted from the wide jet. Here, we calculate the VHE gamma-ray flux by the synchrotron self-Compton (SSC) emission. The multi-wavelength afterglows of GRB 190829A including the VHE gamma-ray emission are well explained by our two-component jet model. The afterglow emissions from our two-component jet are also consistent with the observational results of GRBs 180720B, 190114C and 201216C, when the jets are viewed on-axis. Furthermore, we discuss the detectability of off-axis orphan afterglows by the Cherenkov Telescope Array (CTA).

Shell‐type supernova remnants and very high energy cosmic rays

AbstractThe investigation of very high energy (VHE) γ‐ray sources by any methods, including mirror Cherenkov telescopes, touches on the problem of the cosmic‐ray origin and, accordingly, the role of the Galaxy in their generation. The SHALON observations have yielded the results on Galactic supernova remnants (SNR) of different ages. Among them are: the shell‐type SNRs Tycho's SNR, Cas A, IC 443, γCygni SNR, G166.0+4.3. For each of SNRs, the observation results are presented with spectral energy distribution by SHALON in comparison with other experiment data and images by SHALON in together with data from X‐ray by Chandra and radio‐data by CGPS. The experimental data presented here have confirmed the prediction of the theory about the hadronic generation mechanism of VHE 800 GeV–100 TeV γ‐rays in Tycho's SNR, Cas A and IC 443, and also the collected experimental data help to make clear the origin of TeV γ‐ray emission in the older SNRs like γCygni SNR and G166.0+4.3.

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.

Very high energy emission mechanism in the extreme blazar PGC 2402248

ABSTRACT Extreme high-frequency peaked BL Lacs (EHBLs) are characterized by a synchrotron peak frequency exceeding 1017 Hz and a second peak that can be in the energy range of few GeVs to several TeVs. The Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC) detected multi-TeV gamma-rays on 2018 April 19 for the first time from the EHBL PGC 2402248, which was simultaneously observed in multiwavelength by several other instruments. The broad-band spectral energy distribution of the source is conventionally modelled using the leptonic and the hadronic models. Due to the success of the photohadronic model in interpreting the enigmatic very high energy (VHE) flaring events from many high-energy blazars, we extend this model to explain the VHE events from PGC 2402248 observed by MAGIC telescopes and compare our results with other models. We conclude that the photohadronic fits are comparable and even fare better than most other models. Furthermore, we show that the spectrum is not hard and is in a low-emission state. The estimated bulk Lorentz factor for this flaring event is found to be ≲34.

Proton synchrotron, an explanation for possible extended VHE gamma-ray activity of TXS 0506+056 in 2017

TXS $0506+056$, source of the extreme energy neutrino event, IceCube-170922A, has an interesting environment to study the lepto-hadronic emissions. The Fermi-LAT detector reported high energy (HE) $\ensuremath{\gamma}$-ray flare between 100 MeV and 100 GeV starting from 15 September 2017 from this source. Several follow-ups to trace the very high energy (VHE) gamma-ray counterparts around the IceCube-170922A resulted in no success around 22 September 2022. Only after 28 September, the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes observed the first VHE gamma rays from the blazar above 100 GeV. The $\ensuremath{\sim}41\text{ }\text{ }\mathrm{h}$ survey resulted in VHE $\ensuremath{\gamma}$-ray activity until 31 October 2017, nearly 45 days after the HE flare. Here we propose the extended GeV $\ensuremath{\gamma}$ rays can be explained by taking two production channels, electron synchrotron self Compton and proton synchrotron for HE and VHE emissions, respectively. The 45 days of VHE emission from the peak of the HE flare can be explained with ${L}_{p}^{\ensuremath{'}}\ensuremath{\simeq}{10}^{47}\text{ }\text{ }\mathrm{erg}/\mathrm{sec}$ in the jet frame and magnetic field of 2.4 G, consistent with the ${\mathrm{L}}_{\mathrm{Edd}}$ for a blackhole mass $5\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\text{ }{\mathrm{M}}_{\ensuremath{\bigodot}}$. With the same luminosity of accelerated protons, we explained the observed neutrino flux with proton-varying-ambient interaction.

Correcting MAGIC Telescope data taken under non-optimal atmospheric conditions with an elastic LIDAR

The Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes are a system of two Imaging Atmospheric Cherenkov Telescopes (IACTs). IACTs make calorimetric use of the Earth’s atmosphere, which allows them to reach large effective areas, but also makes them strongly dependent on the quality of the atmosphere at the time of the observations. Dust intrusions or clouds obscuring the observed Cherenkov light can then lead to a wrong reconstruction of the gamma-ray data. In order to mitigate this problem, the MPP group built and has been operating a single wavelength elastic LIDAR (LIght Detection And Ranging) system to perform real time ranged-resolved measurements of the aerosol transmission. This information is then used to quantify the quality of the telescope data, as well as to correct the data taken under suboptimal aerosol conditions. In this talk, the correction of atmospherically impaired IACT data will be described and the first systematic evaluation of the correction capabilities of the LIDAR system will be presented. The results describe the impact of the LIDAR corrections for a variety of atmospheric and observational conditions, and therefore contribute to a better understanding of the telescope’s performance and related systematic uncertainties.

The influence of reduced atmospheric transmission on the CTA-North performance

Cherenkov Telescope Array (CTA) is the next generation ground-based observatory for gamma-ray astronomy at very-high energies which will consist of the northern (CTA-N, La Palma, Spain) and southern (CTA-S, Paranal, Chile) arrays. The atmosphere, as an integral part of the Cherenkov telescope detector, has a great impact on the observed data, especially in means of reduced sensitivity. One of the main contributions to the systematic uncertainties arises from the presence of clouds. To minimize these systematic uncertainties a calibration of the detector response is of great importance. Here, the influence of cloud altitude and transmission on the CTA-N performance degradation using detailed Monte Carlo simulations has been investigated for the case that no measures are taken to characterize clouds and correct their effects. The degradation effect of the presence of clouds is primarily observed at low and middle energies but spans across the entire energy range.

Site environment characterization for Southern Wide-field Gamma-ray Observatory

The Southern Wide-field Gamma-ray Observatory (SWGO) is a proposed gamma-ray observatory based on the ground-level particle detection technique, with close to 100% duty cycle and an order of steradian field of view. SWGO will be located in South America at a latitude between 10 and 30 degrees South and an altitude of 4.4 km or higher, covering an energy range from hundreds of GeV to PeV. The SWGO Site Working Group is gathering, among other information, relevant environmental data to characterize the proposed sites using the AEROSITE instrument. Also atmospheric transparency is monitored to allow for potential enhancement of SWGO with a Cherenkov telescope. This contribution describes our activities in the study of the atmospheric conditions of selected candidate sites using instruments located onsite or installed at nearby stations.

Preface

AtmoHEAD (ATmospheric Monitoring for High Energy Astroparticle Detectors) is a periodic 3-days international workshop on the influence of the atmosphere on high-energy cosmic-ray and gamma-ray astroparticle detectors. The 2022 edition is focused on atmospheric monitoring and on the detection and study of events related to atmospheric electricity. The event follows the tradition of previous AtmoHEAD conferences - AtmoHEAD 2013 in Saclay, 2014 in Padova, 2016 in Olomouc and 2018 in Anacapri.The atmosphere is an integral component of many high-energy astroparticle detectors. These include Imaging Atmospheric Cherenkov Telescopes (IACTs) and ground or space based cosmic-ray extensive air shower detectors. All these observatories use the atmosphere as a giant calorimeter where cosmic rays and gamma rays deposit their energy and initiate extensive air showers; it is also the medium through which the resulting Cherenkov and Fluorescence light propagates, and through which it is attenuated and scattered before reaching the detectors. Clearly, a precise and quantitative monitoring of the atmospheric aerosol and molecular conditions are mandatory for an accurate reconstruction of the detected events. Moreover, in the last years there is a growing interest in the detection of electrical phenomena (ELVES, TGFs, ...) developed in atmosphere with high-energy astroparticle detectors, which represent a unique opportunity to study the characteristics of these events with large statistics and precision.New and common monitoring and calibration instruments including lidars, distant laser facilities, cloud monitors, all sky cameras, sun/moon photometers, satellite observations, lightning sensors, as well as simulations, are among the most important tools of these observatories.These online workshop proceedings are grouped in two main sections, Atmospheric Characterization and Atmospheric Electricity and focus on the state-of-the-art and future perspectives of techniques and instruments.Some more detail on the topics treated in the workshop :• the influence of the atmosphere on the measurements of present and future UHECR and gamma-ray experiments• terrestrial gamma-ray flashes, transient luminous events and general topics in atmospheric electricity• analysis techniques and instruments• public global data sets, models and simulationsWe take advantage of this preface to thank all the participants for the very fruitful discussions that came up during the workshop in a friendly and collaborative atmosphere, among participants of different but complementary fields. This was the main goal of AtmoHEAD 2022.AtmoHEAD 2022 was organized by the Department of Physics “E. Pancini ” of the University of Napoli Federico II and by the Napoli Division of the National Institute of Nuclear Physics (INFN).

Assessing aerosol induced errors in Monte Carlo based air-shower reconstruction for atmospheric Cherenkov detectors

Aerosol levels influence the wavelength dependent transmission properties of the atmosphere. Variations in aerosol levels therefore affect the amount of Cherenkov light from air-showers that can reach an atmospheric Cherenkov detector. As the amount of detected Cherenkov light is directly related to a primary shower particle’s energy, deviations between actual and assumed atmospheric transmission properties yield errors in reconstructed particle energies as well as energy axes of instrument response functions. In this work, a scheme is presented to assess this influence and potentially reduce related errors in the air-shower reconstruction. The proposed scheme relies on estimations or measurements of the aerosol optical depth and atmospheric density profile which are then used in radiative transfer simulations to generate atmospheric transmission profiles. As the scheme furthermore uses detector specific quantum efficiencies and generalised shower evolution models, it does not rely on detailed Monte Carlo simulations for the different atmospheric conditions but only on the transmission profile which the initial shower reconstruction algorithm is based on. The approach is derived and presented on the example of the H.E.S.S. experiment which employs imaging atmospheric Cherenkov telescopes in the Khomas Highland of Namibia to detect gamma rays in the GeV to TeV energy range.

Observation of the Cumbre Vieja volcano plume above the Observatorio del Roque de los Muchachos with the Barcelona Raman LIDAR

The Cherenkov Telescope Array Observatory (CTAO), currently under construction, is the next-generation very-high-energy gamma-ray observatory, providing the coverage for photons in the energy range 20GeV to 300TeV. CTAO will increase detection sensitivity in the 100 GeV to 10TeV range by a factor of 5 — 10 with respect to present experiments. CTAO retrieves the properties of very-high-energy gamma-rays by measuring Cherenkov light emitted by atmospheric showers of secondary particles that incident gamma rays produce in upper layers of the atmosphere. The key for reaching the required energy measurement accuracy is a precise knowledge of the atmospheric transmittance for Cherenkov light, which can be obtained using a dedicated Raman LIDAR. The device should operate at 355nm (near the maximum of Cherenkov light spectrum) and have the capability of taking data at specific azimuth and zenith angles up to distances of 30 km, so that atmospheric transmission along all possible air-shower directions can be determined. The Barcelona Raman LIDAR (BRL) is the official CTAO Pathfinder prototype, developed for atmospheric characterization of the Northern CTAO Site at the Observatorio del Roque de los Muchachos (ORM) on the Canary island of La Palma. BRL was deployed at ORM for extensive on-field tests between February 2021 and May 2022. We report on the commissioning results, including the remote operation capabilities of the system and its contribution to the understanding of atmospheric phenomena during its deployment period. In particular, we report on the properties of the volcanic plume from the eruption of the Cumbre Vieja volcano on 22 September 2021.