A new Cherenkov telescope of the SPHERE type is under development. Its main goal is to promote the solution of the problem of the primary cosmic ray mass composition at ultra high energies (1–100 PeV) using a newly developed technique of the primary mass assignment to EAS event on event-by-event basis. The telescope will carry out measurements of both the Cherenkov light reflected from the snow surface as well as the direct one. Sensitivity of the direct Cherenkov images’ shapes to the primary mass is demonstrated.

A new SPHERE-3 telescope is being developed for the study of the cosmic ray spectrum and mass composition in the 5–1000 PeV energy range. Registration of extensive air showers using reflected Cherenkov light method applied in the SPHERE detector series requires a good trigger system for accurate separation of events from the background produced by starlight and airglow photons reflected from the snow. Here, we present the results of convolutional networks application for the classification of images obtained from Monte Carlo simulation of the detector. The simulated detector response includes photon tracing through the optical system, silicon photomultiplier operation, and the electronics response and digitization process. The results are compared to the SPHERE-2 trigger system performance.

The new SPHERE-3 detector is under development. The main purpose of this experiment is to study the mass composition of primary cosmic rays in the energy range of 1–1000 PeV. The difference between this detector and the previous ones in this series is the registration of not only the Cherenkov light reflected from the snow, but also direct light entering the detector. Several options are being considered for recording direct Cherenkov light. The article presents the estimated parameters of the optical scheme of the detector and the first estimations of its sensitivity to the energy and mass of the primary particle. The first approximations of methods used for energy and mass of the primary particle assessment in each event are given, and their accuracy is considered. At the moment, methods for reflected Cherenkov light and direct Cherenkov light processing are being considered independently of each other.

It has been previously demonstrated by Panov et al. (2021) that the TAIGA-HiSCORE Cherenkov array, originally built for cosmic ray physics and ultrahigh-energy gamma-ray astronomy studies using the extensive air shower method, can be used in conventional optical astronomy for wide-field searches for rare nanosecond optical transients of astrophysical origin. The field of view of the facility is on the scale of 1 ster, and it is capable of detecting very rare transients in the visible light range with fluxes greater than approximately 3000 quanta/m2/10 ns (10 ns is the apparatus integration time) and pulse durations of 10 ns. Among the potential sources of distant nanosecond optical transients are the evaporation of primary black holes, magnetic reconnection in the accretion disks of black holes, and signals from distant lasers of extraterrestrial civilizations. The paper describes the methods and results of the search for optical transients using the TAIGA-HiSCORE Cherenkov array from 2018 to 2022 (four winter seasons of data collection). No reliable astrophysical candidates for optical transients were found. We set an upper bound on the flux of the searched events as ∼ 1pt 1 ×10^ - 3 events/ster/h.

Based on the experience of successful implementation of the SPHERE-2 experiment, further development of the method for studying PCR by recording the reflected EAS Cherenkov light is planned. In this work, we show the possibility of simultaneous detection of direct and reflected EAS Cherenkov light. The prospect of creating a new SPHERE-3 detector is discussed, and the first simulation results are presented.

Further development of the way of studying primary cosmic rays by detecting the reflected extensive air shower Cherenkov light is planned, based on the successful implementation of the SPHERE-2 aerostat experiment. The possibility of simultaneously detecting direct and reflected Cherenkov light from extensive air showers is demonstrated. Prospects for creating a new SPHERE-3 detector are discussed and the first results from modeling are presented.

The paper is devoted to the modeling and analysis of data detected by the TAIGA-IACT installation in the stereo mode. Five Imaging Atmospheric Cherenkov Telescopes (IACT) with a viewing angle of 9.6° are expected to be included in the installation. Today there are three telescopes spaced far apart (from 320 to 500 m) in the installation. The effective area of the installation is as large as 0.6 km2; therefore, it is possible to conduct statistically significant measurements of weak γ-ray sources in the energy range above 10 TeV over a reasonable observation time (300–400 h). The Monte Carlo procedure for simulating the hadrons and γ-rays detected by the telescopes is described as is the procedure for reconstructing the parameters of extensive air showers, such as the arrival direction of an event, the axis position, the depth of the maximum of shower development (Xmax), and the primary-particle energy. In order to solve the problem of γ-hadron separation, the criteria for selecting γ-rays detected in the stereo mode have been optimized and the effective area of the installation has been calculated.

The study of cosmic rays in the energy range from 1 to 1000 PeV is crucial for understanding their origins and propagation paths. As part of this research, a new SPHERE-3 installation is being developed, featuring enhanced light sensitivity and optical resolution, based on the experience gained with the balloon-borne SPHERE-2 installation. This report describes a computational complex designed for simulating the formation of Cherenkov light on the detector grid of the SPHERE-3 telescope.

In November 2020, the IceCube Neutrino Observatory registered a neutrino event with an energy of 150 TeV directed at the Cygnus Cocoon gamma-ray source. In the Carpet-2 experiment, as part of the Baksan Neutrino Observatory (BNO), a sharp increase in the flow of events with an energy above 300 TeV was recorded from the same direction within the angular accuracy of the events. This flux is 4 orders of magnitude higher than the expected intensity of gamma quanta of this energy region according to data in the region of less than 100 TeV. It was expected that such a powerful flare could be registered by the TAIGA-HiSCORE installation of the TAIGA astrophysical complex. We analyzed the events of the EAS recorded by the installation of TAIGA-HiSCORE for 18 h in October–November 2020 from the Cygnus Cocoon source. This article provides the upper limit of the expected excess flow.

A scintillation experiment is a part of the TAIGA astrophysical complex located in the Tunka Valley, 50 km from Lake Baikal. It consists of the Tunka-Grande and TAIGA-Muon arrays. Its scientific program is devoted to the study of cosmic rays (CRs) and search for astrophysical gamma rays by detecting charged particles (electrons and muons) of extensive air showers (EASs). We present the current status of the scintillation experiment, methods of EAS and CR parameters’ reconstruction, the main results obtained by the Tunka-Grande array and our scientific program for the future.

The TAIGA astrophysical complex includes now 3 IACTs at the distance 300-500 m between each other and 1km2 area wide-angle timing array TAIGA-HiSCORE. At energies above 40 TeV, a hybrid approach to the detection of gamma-rays becomes possible - the detection of EAS by both IACTs and the TAIGA-HiSCORE installation. The main advantage of the joint operation of the IACTs and timing is their good gamma/hadron separation, even by only few telescopes on the large area, by image parameters information and EAS another parameter (core position, direction and energy) that can be better reconstructed by the timing array. In this paper the following topics of a hybrid method are discussed: data processing and analysis, a comparison experimental results with Monte-Carlo simulations, selection of the first events with the energy more than 100 TeV from Crab Nebula in 250 hours of observation. The data were taken during the period of installation deployment, with one IACT in operation and half of the area of TAIGA-HiSCORE installation.

The study of cosmic rays mass composition is an important problem in high-energy physics. The main goal of the SPHERE-2 experiment was to study the energy spectrum of the primary cosmic rays in the 10–300 PeV energy range. Also the experimental data allow approaching their mass composition. The separation of events into nuclei groups makes it possible to estimate the average masses over the sample. Using machine learning methods, we developed a separation method for the primary nuclei groups that formed extensive air showers based on the simulated events for the SPHERE-2 telescope. Various models of the high energy nucleus-nucleus interaction were used, but their predictions differ significantly. In the SPHERE-2 experiment data analysis, this problem was solved, first, by the use of the data on Cherenkov light, which has weak dependence on the model of hadronic interaction; second, the neural network was trained simultaneously on two interaction models (QGSJET-01 and QGSJETII-04), which differ greatly from each other. Therefore, the independence of experimental data processing from the choice of the nuclear interaction model was ensured. The regression task is solved by machine learning methods. The separation of events into three groups of nuclei—protons (p), nitrogen (N), and iron (Fe)—by using a neural network is more precise than that by using traditional methods.

This paper is devoted to the analysis of simulation results of TAIGA-IACT in stereo mode of observations and describes the technique of gamma-ray detection with energies higher than several TeV. TAIGA-IACT is part of the TAIGA astrophysics complex. Installation currently consists of three operating telescopes located 300-500 m apart. Two additional telescopes will begin operation in the next two years. We describe a new gamma-hadron separation technique for point-like source observations.

In the era of multi-messenger astronomy it is extremely important to obtain data from many instruments to get the most complete information about transient and short-lasting phenomena. The TAIGA observatory has also started observations in multi-messenger regime. The TAIGA observatory is a hybrid detector complex for high-energy gamma-ray astronomy and cosmic ray physics that combines the detection of extensive air showers with different detector systems: timing array TAIGA-HiSCORE, scintillation array TAIGA-Muon and imaging atmospheric Cherenkov telescopes of the TAIGA-IACT installation. The General Coordinates Network (GCN), that distributes online alerts from different instruments around the world, makes it possible to observe GRBs with TAIGA-IACTs to search for very high energy gamma-quanta. This work presents the current status of the TAIGA-IACT telescope control and alert system for GRB observations as well as its performance.

Paper contains the first results on the development of a SPHERE-3 telescope for the primary cosmic ray studies in 1-1000 PeV energy range using reflected and direct Cherenkov light generated by extensive air showers. It also sheds some light on the development of our new approach to the design of the new telescope.

Experiments on detection of charged particles from EAS in the Tunka Valley are the part of the TAIGA astrophysical complex and consists of two arrays: the operating Tunka-Grande facility and TAIGA-Muon array under construction. In report we present description of arrays, methods of EAS parameters reconstruction, scientific programs and the main results of Tunka-Grande array based on 6 seasons of operation: CR energy spectrum in the energy range 10 PeV - 1000 PeV and limit on the flux of the diffuse gamma rays in the same energy range. In addition, we provide prospects for studying primary cosmic radiation in the energy range 100 TeV - 1000 PeV.

The more correct recalculation from the measured Cherenkov light fluxes at distances of 200 (Q200) and 100 (Q100) m from the Extensive Air Shower (EAS) core to the energy of the primary particle has been developed using the results of M-C simulation by the CORSIKA code, assuming a light primary composition of cosmic rays. Using the new conversion expressions, a differential energy spectrum was obtained according to the data of the Tunka-133 array for 7 years of operation and the TAIGA-HiSCORE array for 2 years of operation.