HEGRA Research Articles

Estimation of the excess noise factor for TACTIC photomultiplier tubes using Gaussian mixture model based machine learning algorithm

The Excess Noise Factor (ENF) of a Photomultiplier Tube (PMT) is basically the additional noise introduced because of the statistical nature of electron multiplication inside PMT. In the field of Very High Energy (VHE) gamma-ray astronomy, PMT is used as a Cherenkov photon detector. Hence, the noise of PMT plays an important role in estimating the energy of VHE gamma rays. Conventionally, estimation of ENF of PMT requires a very low-intensity, tunable light source experiment in a lab where no external light background interferes. In this study, we propose the application of a novel Machine Learning (ML) technique, the Gaussian Mixture Model (GMM) to estimate the ENF of the telescope camera for an in-situ observation.

TAIGA - An advanced hybrid detector complex for astroparticle physics and high energy gamma-ray astronomy

The physical motivations, present status, main results in study of cosmic rays and in the field of gamma-ray astronomy as well future plans of the TAIGA-1 (Tunka Advanced Instrument for cosmic ray physics and Gamma Astronomy) project are presented. The TAIGA observatory addresses ground-based gamma-ray astronomy and astroparticle physics at energies from a few TeV to several PeV, as well as cosmic ray physics from 100 TeV to several EeV. The pilot TAIGA-1 complex is located in the Tunka valley, ~50 km west from the southern tip of the lake Baikal.

Galactic cosmic rays: latest results from the DAMPE mission

The space-based DAMPE (DArk Matter Particle Explorer) particle detector has been taking data for more than 6 years since its successful launch in December 2015. Its main scientific goals include the indirect search of Dark Matter signatures in the cosmic lepton spectra, the study of Galactic Cosmic Rays up to energies of hundreds of TeV and high-energy gamma ray astronomy. This talk will focus on Galactic Cosmic Rays and the measurement of their spectra, fundamental to investigate the mechanisms of acceleration at their sources and propagation through the interstellar medium. The most recent results on Proton and Helium, which revealed new spectral features, will be highlighted. Ongoing analyses regarding the cosmic ray light component, medium and heavy mass nuclei will be discussed alongside studies on the so-called secondary cosmic rays.

Latest results from the DAMPE space mission

The DArk Matter Particle Explorer (DAMPE) is a particle detector hosted on board a satellite orbiting around the Earth since December 2015. The space mission has been promoted by the Chinese Academy of Science and results from an international effort also including Italian and Swiss institutions. The scientific goals include: indirect detection of Dark Matter signatures in cosmic lepton spectra, study of Cosmic Ray energy spectra up to energies of hundreds of TeV and high-energy gamma ray astronomy. A general overview of the mission will be presented and its main results about the all-electron, proton, helium, light-component (p+He) and heavier nuclei energy spectra, as well as studies on gamma-ray sources, will be discussed.

Latest results from the DAMPE space mission

The space-based DAMPE (DArk Matter Particle Explorer) particle detector has been taking data for more than 7 years since its successful launch in December 2015. Its main scientific goals include the indirect search for dark matter signatures in the cosmic lepton and gamma-ray spectra, the study of galactic cosmic rays up to energies of hundreds of TeV and studies on highenergy gamma ray astronomy. The measurement of galactic cosmic ray spectra are reported here, those being fundamental tools to investigate the mechanisms of acceleration at their sources and propagation through the interstellar medium. Results on proton and helium, which revealed new spectral features, are described. Ongoing analyses on the cosmic ray light, medium and heavy mass nuclei are outlined, together with studies of the so-called secondary cosmic rays. Latest results on gamma-ray astronomy and dark matter search will be also summarized.

The Crab Pulsar and Nebula as Seen in Gamma-Rays

Slightly more than 30 years ago, Whipple detection of the Crab Nebula was the start of Very High Energy gamma-ray astronomy. Since then, gamma-ray observations of this source have continued to provide new surprises and challenges to theories, with the detection of fast variability, pulsed emission up to unexpectedly high energy, and the very recent detection of photons with energy exceeding 1 PeV. In this article, we review the impact of gamma-ray observations on our understanding of this extraordinary accelerator.

Search for Astrophysical Nanosecond Optical Transients with TAIGA-HiSCORE Array

A wide-angle Cerenkov array TAIGA-HiSCORE (FOV $\sim$0.6 sr), was originally created as a part of TAIGA installation for high-energy gamma-ray astronomy and cosmic ray physics. Array now consist on nearly 100 optical stations on the area of 1 km$^2$. Due to high accuracy and stability ($\sim$1 ns) of time synchronization of the optical stations the accuracy of EAS arrival direction reconstruction is reached 0.1$^\mathrm{o}$. It was proven that the array can also be used to search for nanosecond events of the optical range. The report discusses the method of searching for optical transients using the HiSCORE array and demonstrates its performance on a real example of detecting signals from an artificial Earth satellite. The search for this short flares in the HiSCORE data of the winter season 2018--2019 is carried out. One candidate for double repeater has been detected, but the estimated probability of random simulation of such a transient by background EAS events is not less than 10%, which does not allow us to say that the detected candidate corresponds to a real astrophysical transient. An upper bound on the frequency of optical spikes with flux density of more than $10^{-4} \mathrm{erg/s/cm}^2$ and a duration of more than 5\,ns is established as $\sim 2 \times 10^{-3}$ events/sr/hour.

TAIGA—an advanced hybrid detector complex for astroparticle physics and high energy gamma-ray astronomy in the Tunka valley

The TAIGA observatory addresses ground-based gamma-ray astronomy at energies from a few TeV to several PeV, cosmic ray physics from 100 TeV to several EeV as well as for search for axion-like particles, Lorentz violations and another evidence of New Physics. In 2020 year a one square kilometer TAIGA setup should be put in operation.

A fast muon tagger method for Imaging Atmospheric Cherenkov Telescopes

The Cherenkov Telescope Array (CTA) will be the next major observatory for Very High Energy gamma-ray astronomy. Its optical throughput calibration relies on muon Cherenkov rings. This work is aimed at developing a fast and efficient muon tagger at the camera level for the CTA telescopes. A novel technique to tag muons using the capabilities of silicon photomultiplier Compact High-Energy Camera CHEC-S, one of the design options for the camera of the small size telescopes, has been developed, studying and comparing different algorithms such as circle fitting with the Taubin method, machine learning using a neural network and simple pixel counting. Their performance in terms of efficiency and computation speed was investigated using simulations with varying levels of night sky background light. The application of the best performing method to the large size telescope camera has also been studied, to improve the speed of the muon preselection.

Silicon photomultipliers in Very High Energy gamma-ray astrophysics

In the last ten years silicon photomultipliers (SiPMs) have gained terrain in experiments and applications in which photomultiplier tubes have been the dominant photosensors during decades. Imaging Atmospheric Cherenkov Telescopes (IACTs) for very high energy (VHE, E>50 GeV) gamma-ray astronomy are experiencing the same process. Until now FACT was the only IACT using SiPMs. In the Cherenkov Telescope Array (CTA), the next-generation VHE gamma-ray observatory, at least 70 telescopes equipped with SiPMs are planned to be built. The first prototypes have already been constructed and are now being commissioned. Here we discuss some of the advantages and drawbacks of using SiPMs in VHE gamma-ray astronomy and provide a brief overview of different developments related to the use of SiPMs in IACTs.

The golden age of high-energy gamma-ray astronomy: the Cherenkov Telescope Array in the multimessenger era

High-energy photons are a powerful tool to understand the most violent phenomena in our Universe. However, detecting gamma-ray photons is a daunting task both owing to the overwhelming presence of cosmic rays and to the opacity of our atmosphere. Thus, direct detection of gamma-ray photons must be done in space, while instruments on the ground can see very high-energy photons through the secondary particles they produce in the atmosphere. Mastering space instruments as well as ground-based ones required decades of relentless efforts implying hardware and software developments as well as theoretical studies and extensive simulations. Space instruments, as well as ground-based ones, are now producing a steady flow of important results often in conjunction with observatories working at different wavelengths. Multi-messenger astronomy, which combines the electromagnetic channel with gravitational waves and neutrinos, is the newly born discipline to which high-energy gamma-ray detectors provide an essential contribution. Building instruments to continuously cover the gamma-ray sky is, thus, a priority for the astronomical community which is looking forward for the Cherenkov Telescope Array, a new generation ground based gamma-ray observatory designed to monitor, to study and to unveil the mysteries hidden in the very high energy sky.

Fundamental physics with blazar spectra: a critical appraisal

ABSTRACT Very-high-energy (VHE) BL Lacertae (BL Lac) spectra extending above $10 \, \rm TeV$ provide a unique opportunity for testing physics beyond the standard model of elementary particle and alternative blazar emission models. We consider the hadron beam scenario, the conversion of photons to axion-like particles (ALPs) and the Lorentz invariance violation (LIV) by analysing their consequences and induced modifications to BL Lac spectra. In particular, we consider how different processes can provide similar spectral features (e.g. hard tails) and we discuss the ways they can be disentangled. We use data from High-Energy Gamma-Ray Astronomy (HEGRA) of a high state of Markarian 501 and the High-Energy Stereoscopic System (H.E.S.S.) spectrum of the extreme BL Lac (EHBL) 1ES 0229+200. In addition, we consider two hypothetical EHBLs similar to 1ES 0229+200 located at redshifts z = 0.3 and z = 0.5. We observe that both the hadron beam and the photon–ALP oscillations predict a hard tail extending to energies larger than those possible in the standard scenario. Photon–ALP interaction predicts a peak in the spectra of distant BL Lacs at about $20\rm {-}30 \, \rm TeV$, while LIV produces a strong peak in all BL Lac spectra around $\sim 100 \, \rm TeV$. The peculiar feature of the photon–ALP conversion model is the production of oscillations in the spectral energy distribution, so that its detection/absence can be exploited to distinguish between the considered models. The above-mentioned features of the three models might be detected by the upcoming Cherenkov Telescope Array. Thus, future observations of BL Lac spectra could eventually shed light on new physics and alternative blazar emission models, driving fundamental research towards a specific direction.

Cherenkov Telescope Array and High-Energy Gamma Ray Astronomy

Cherenkov Telescope Array and High-Energy Gamma Ray Astronomy

Multiple Coulomb scattering method to reconstruct low-energy gamma–ray direction in the GAMMA-400 space-based gamma–ray telescope

The GAMMA-400 currently developing space-based gamma-ray telescope is designed to measure the gamma-ray fluxes in the energy range from ∼20 MeV to several TeV in the highly elliptic orbit (without shadowing the telescope by the Earth) continuously for a long time. The physical characteristics of the GAMMA-400 gamma-ray telescope, especially the angular and energy resolutions (at 100-GeV gamma rays they are ∼0.01° and ∼1%, respectively), allow us to consider this space-based experiment as the next step in the development of extraterrestrial high-energy gamma-ray astronomy. In this paper, a method to improve the reconstruction accuracy of incident angle for low-energy gamma rays in the GAMMA-400 space-based gamma-ray telescope is presented. The special analysis of topology of pair-conversion events in thin layers of converter was performed. Applying the energy dependence of multiple Coulomb scattering for pair components, it is possible to estimate the energies for each particle, and to use these energies as weight in the angle reconstruction procedure. To identify the unique track in each projection the imaginary curvature method is applied. It allows us to obtain significantly better angular resolution in comparison with other methods applied in current space-based experiments. When using this method for 50-MeV gamma rays the GAMMA-400 gamma-ray telescope angular resolution is about 4°.

The Light-Trap: A novel concept for a large SiPM-based pixel for Very High Energy gamma-ray astronomy and beyond

Among the main disadvantages of using silicon photomultipliers (SiPMs) in large experiments are their limited physical area (increasing the cost and the complexity of the readout of a camera) and their sensitivity to unwanted wavelengths. This explains why photomultiplier tubes (PMTs) are still selected for the largest cameras of present and future Very High Energy (VHE) gamma-ray telescopes. These telescopes require photosensors that are sensitive to the fast and dim optical/near-UV Cherenkov radiation emitted due to the interaction of gamma rays with the atmosphere. Here we introduce a low-cost pixel consisting of a SiPM attached to a PMMA disk doped with a wavelength-shifting material, which collects light over a much larger area than standard SiPMs, increases sensitivity to near-UV light and improves background rejection. We also show the measurements performed in the laboratory with a proof-of-concept Light-Trap pixel that is equipped with a 3 × 3 mm2 SiPM collecting light only in the 300–400 nm band, covering an area ∼20 times larger than that of the same SiPM itself. We also present results from simulations performed with Geant4 to evaluate its performance. In addition to VHE astronomy, this pixel could have other applications in fields where detection area and cost are critical.

Comparison of the Measurement and Simulation with KM2A Prototype Array

The Large High Altitude Air Shower Observatory (LHAASO) is a new hybrid array for very high energy gamma ray astronomy and cosmic ray physics. The KM2A array, one of the main parts of LHAASO, covers an area of 1.3 km2 to observe gamma rays above 10 TeV up to 1 PeV for many sources. A prototype with 1% the size of the whole KM2A has been in stable operation for more than two years. A Monte Carlo simulation program named G4KM2A was developed; based on this work, the trigger rate, hit multiplicity, angular and core reconstruction are compared with KM2A prototype data. Finally, the moon shadow with -6.5 significance was obtained. The G4KM2A simulation results are consistent with KM2A prototype data and can be used for the whole KM2A array in future.

Performance optimization of the air shower simulation program for the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA), currently under construction, is the next-generation instrument in the field of very high energy gamma-ray astronomy. The first data are expected by the end of 2018, while the scientific operations will start in 2022 for a duration of about 30 years. In order to characterize the instrument response to the Cherenkov light emitted when cosmic ray showers develop in the atmosphere, detailed Monte Carlo simulations will be regularly performed in parallel to CTA operation. The estimated CPU time associated to these simulations is very high, of the order of 200 millions HS06 hours per year. Reducing the CPU time devoted to simulations would allow either to reduce infrastructure cost or to better cover the large phase space. In this paper, we focus on the main computing step (70% of the whole CPU time) implemented in the CORSIKA program, and specifically on the mod-ule responsible for the propagation of Cherenkov photons in the atmosphere. We present our preliminary studies about different options of code optimization, with a particular focus on vectorization facilities (SIMD instructions). Our proposals take care, as automatically as possible, of the hardware portability constraints introduced by the grid computing environment that hosts these simulations. Performance evaluation in terms of running-time and accuracy is provided.

Helium flux and elemental composition of galactic Cosmic Rays with the DAMPE space mission

DAMPE (DArk Matter Particle Explorer) is a space mission project promoted by the Chinese Academy of Sciences (CAS), in collaboration with Universities and Institutes from China, Italy and Switzerland. The detector is collecting data in a stable sun-synchronous orbit lasting 95 minutes at an altitude of about 500 km. It has been launched in December 17th, 2015, from the Jiuquan Satellite Launch Center, in the Gobi Desert. The main goals of the mission are: indirect search for Dark Matter, looking for signatures in the electron and photon spectra with energies up to 10 TeV; analysis of the flux and composition of primary Cosmic Rays with energies up to hundreds of TeV; high energy gamma-ray astronomy. Preliminary results about the Helium flux and Cosmic Ray composition will be presented and discussed.

The Cherenkov Telescope Array production system for data-processing and Monte Carlo simulation

The Cherenkov Telescope Array (CTA) is the next-generation instrument in the field of very high energy gamma-ray astronomy. It will be composed of two arrays of Imaging Atmospheric Cherenkov Telescopes, located at La Palma (Spain) and Paranal (Chile). The construction of CTA has just started with the installation of the first telescope on site at La Palma and the first data expected by the end of 2018. The scientific operations should begin in 2022 for a duration of about 30 years. The overall amount of data produced during these operations is around 27 PB per year. The associated computing power for data processing and Monte Carlo (MC) simulations is of the order of hundreds of millions of CPU HS06 hours per year. In order to cope with these high computing requirements, we have developed a production system prototype based on the DIRAC framework, that we have intensively exploited during the past 6 years to handle massive MC simulations on the grid for the CTA design and prototyping phases. CTA workflows are composed of several inter-dependent steps, which we used to handle separately within our production system. In order to fully automatize the whole workflows execution, we have partially revised the production system by further enhancing the data-driven behavior and by extending the use of meta-data to link together the different steps of a workflow. In this contribution we present the application of the production system to the last years MC campaigns as well as the recent production system evolution, intended to obtain a fully data-driven and automatized workflow execution for efficient processing of real telescope data.

High-energy Gamma-ray Astronomy in the Multimessenger Era

High-energy Gamma-ray Astronomy in the Multimessenger Era