From 2012 to 2023, the PRISMA-32 array operated at the Experimental Complex NEVOD (MEPhI, Moscow). The purpose of the array was to study extensive air showers detecting their neutron and electron-photon components using unshielded neutron detectors. To expand the capabilities of this facility, including the study of cosmic and geophysical phenomena with a neutron flux, its upgrade was carried out. In this upgrade, a measurement channel to study the variations of the neutron background and the processes affecting them was added. To achieve this, the photomultipliers, the integrating amplifiers, the digitalizing electronics and the high-voltage power supply system were replaced. The paper describes the structure of the upgraded array, which was named PRISMA-36, and presents the results of studying the characteristics of the main elements of its "variation" channel. A method for identifying signals caused by neutron capture and the determined criteria for their selection are discussed. An example of a Forbush decrease, caused by a X1.1-class flare and recorded with the variation channel of the PRISMA-36 array, is given.

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.

A trigger system of the Experimental Complex NEVOD (EC NEVOD) unique scientific facility is described. Detectors and setups that differ in area, physical principles of detection, and background counting rate are included in the EC NEVOD to detect various components of cosmic rays. Each detector is equipped with its own original data-acquisition and trigger system and is able to operate independently. In addition, the detectors are all combined by a single system of data triggering and synchronization. Features of the individual detector systems, their main characteristics and methods of their initialization and integration 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.

In this paper, we describe the quasi-spherical optical module QSM-6M to detect Cherenkov radiation in water. The module is based on six photomultiplier tubes (PMTs) with flat photocathodes Hamamatsu R877. We discuss the results of the photomultiplier testing, as well as the choice of the high-voltage divider providing the PMT dynamic range from 1 to 105 photoelectrons. The techniques for studying QSM-6M characteristics, as well as the results of the underwater testing of the module for an 18-month period are presented. We also present the results of the analysis of the QSM-6M response to single-muon and multiparticle events detected by the installations of the Experimental Complex NEVOD.

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.

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.

The data of cosmic ray NEVOD-DECOR experiment on the investigation of inclined muon bundles for a long time period (May 2012 - March 2021) are presented. The analysis showed that the observed intensity of muon bundles at primary cosmic ray energies of about 1 EeV and higher can be compatible with the expectation in frame of widely used hadron interaction models only under the assumption of an extremely heavy mass composition. This conclusion is consistent with data of several experiments on investigations of muon content in air showers, but contradicts the available fluorescence data on X_{max} Xmax which favor a light mass composition at these energies. In order to clarify the nature of the "muon puzzle", investigations of the muon bundle energy deposit in the detector material were carried out. For the first time, experimental estimates of the average energy of muons in the bundles of inclined air showers initiated by primary particles with energies from 10 to 1000 PeV have been obtained.

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.

The status of the TAIGA experiment (Tunka Advanced Instrument for cosmic-ray physics and Gamma-ray Astronomy) located in the Tunka Valley is presented. The paper presents mainly the tasks, developed approaches for their solution, and first results on high-energy gamma-ray astronomy (10 TeV and higher) obtained from a two- to three-year exposure. The current tasks of gamma-ray astronomy and plans for development of the installation are discussed.

The concept of the TAIGA experiment is to combine wide-angle timing and imaging Cherenkov telescopes as well as electron and muon detectors. The TAIGA facility aims at gamma-ray astrophysics at energies from a few TeV to several PeV and cosmic-ray physics from 100 TeV to several EeV but also pursues searches for astrophysical nanosecond transients, axion-like particles, Lorentz invariance violation and other unexpected manifestations of New Physics. TAIGA-1, a hybrid detector complex with an area of 1 km2, operating since 2021 in the Tunka valley, 50 km to the West from the southernmost tip of lake Baikal, and the plans for its upgrade are presented.

The Tunka Advanced Instrument for Gamma-ray and cosmic ray Astrophysics (TAIGA) is a hybrid experiment for the measurement of Extensive Air Showers (EAS) with good spectral resolution in the TeV to PeV energy range. In this domain, the long-sought Pevatrons can be detected. Currently the hybrid TAIGA detector combines two wide angle shower front Cherenkov light sampling timing arrays (HiSCORE and Tunka-133), two 4m class, 10 degrees aperture Imaging Air Cherenkov Telescopes (IACTs) and 240 m(2) surface and underground charged particle detector stations. Our goal is to introduce a new hybrid reconstruction technique, combining the good angular and shower core resolution of HiSCORE with the gamma-hadron separation power of imaging telescopes. This approach allows to maximize the effective area and simultaneously to reach a good gamma-hadron separation at low energies (few TeV). At higher energies, muon detectors are planned to enhance gamma-hadron separation. During the commissioning phase of the first and second IACT, several sources were observed. First detections of known sources with the first telescope show the functionality of the TAIGA IACTs. Here, the status of the TAIGA experiment will be presented, along with first results from the current configuration.

The Tunka-Grande experiment is a scintillation array with about 0.5 km ^2 2 sensitive area at Tunka Valley, Siberia, for measuring charged particles and muons in extensive air showers (EASs). Tunka-Grande is optimized for cosmic ray studies in the energy range 10 PeV to about 1 EeV, where exploring the composition is of fundamental importance for understanding the transition from galactic to extragalactic origin of cosmic rays. This paper attempts to provide a synopsis of the current results of the experiment. In particular, the reconstruction of the all-particle energy spectrum in the range of 10 PeV to 1 EeV based on experimental data from four observation seasons is presented.

An analysis is performed of the spectrum of gamma rays from the Crab Nebula in the 4–100 TeV range of energies, obtained using data from two Atmospheric Cherenkov Telescopes that are part of the TAIGA complex. A way of selecting and restoring the energy of gamma rays is described that includes a procedure for restoring the energy spectrum.

The Tunka-Grande scintillation array is described. Scientific results obtained over the first five years of its operation are presented. Prospects for studying cosmic rays in the 1016–1018 eV range of energies are discussed.