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.

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 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 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.

A new scenario of the formation of observed ultrahigh-energy cosmic-ray electron/positron and nuclei spectra is discussed within a nonclassical (superdiffusion) model of particle propagation in highly inhomogeneous interstellar and intergalactic media. It is shown that the proposed scenario provides a consistent description of data from highly precise satellite measurements, ground-based hybrid EAS arrays, and Cherenkov telescopes using the spectra of leptons and nuclei, and the mass composition of cosmic rays in the super- and ultrahigh ranges of energy.

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.

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.

The differential energy spectrum of cosmic rays in the energy range of 3 × 1014–3 × 1018 eV and the corrected dependence of the mean depth of the maximum 〈Xmax〉 of an extensive air shower (EAS) inside the wide energy range of 1015–3 × 1017 eV have been obtained from the data of the Tunka-133 array for 7 years of operation (2009–2017) and the TAIGA-HiSCORE array for the 2019–2020 season of operation. At the extremely high energy, our results agree with the results of the Pierre Auger Observatory based on direct measurements of the maximum depth by the observation of fluorescent light from EAS. The recalculation from the 〈Xmax〉 to the parameter $$\left\langle {\ln A} \right\rangle $$ , which characterizes the average composition of the primary cosmic rays, is presented.

High-energy cosmic-ray research via the detection of Cherenkov radiation from extensive air showers was begun in the Tunka valley (50 km to the west from the southern extremity of Lake Baikal) in the early 1990s. A series of large arrays combined into the TAIGA (Tunka Advanced Instrument for cosmic-ray physics and Gamma Astronomy) astrophysical facility and designed to study gamma rays and charged cosmic rays have been created in the elapsed time. Descriptions of the facility arrays and the main results obtained while investigating high-energy cosmic rays are presented. Plans for a further development of the astrophysical facility are discussed.

A study is made of the astroclimatic conditions for performing nighttime astrophysical observations on the territory of the Greater Altai region. Nighttime data from both the VIIRS radiometer of the Suomi NPP satellite platform and the AIRS hyperspectrometer of the Aqua satellite are used. Topographic and astroclimatic criteria show that the Chuya steppe region (Altai Republic, Russia) and the plateau of Lake Khubsugul (Aimak Khuvsgel, Mongolia) are best suited for the deployment a full-scale gamma astronomy experiment. Infrastructure considerations make the territory in the western part of the Chuya steppe preferable.

The results of a complex approach to the study of sensitivity of spatial distributions of electron and muon components of extensive air showers (EAS), measured by scintillation detectors, to variations in the temperature profile of the atmosphere are presented. To describe the lateral dependence of the spatial distribution function (SDF) of electrons in electron–photon cascades (EPC) and the SDF of muons of EAS, the method of the adjoint equations and also the variational theory of sensitivity, developed by the authors, were used. Spatial distributions of electron and muon components of EAS, as well as the corresponding differential temperature coefficients, were simulated by Monte Carlo methods. To assess the effect of variations in the temperature profile of the atmosphere on the spatial distribution of particles satellite monitoring of main parameters of the system ‘‘atmosphere–underlying surface’’ was carried out at locations of Yakutsk complex EAS array and TAIGA observatory. As a result, coefficients of differential sensitivity of spatial distributions of electrons and muons to variations in the temperature profile of the atmosphere were obtained for the first time. Corrective function that relates the energy release in scintillation detectors of various thicknesses with the electrons’ density above the detector at various distances from the shower axis were established. Based on the obtained data, a method for correcting the EAS detectors’ readings in view of the temperature effect has been developed. It is shown that changes in SDFs of the EAS electromagnetic component due to variations in the atmospheric temperature profile in one annual cycle of operation can exceed 10 $$\%$$ .

A corrected energy dependence of the depth of the maximum in the wide range of energies 10 15 to 10 18 eV is obtained using data collected at the Tunka-133 facility over 7 years of operation (2009–2017) and the TAIGA-HiSCORE facility in the 2019–2020 season. At the highest energies, our results match those of the Pierre Auger observatory. The results are converted to parameter ❬ln A ❭, which characterizes the mean EAS composition.

The TAIGA hybrid gamma-ray observatory is currently being developed in the Tunka Valley, 50 km from Lake Baikal, to study gamma radiation and charged cosmic ray fluxes in the 1013–1018 eV range. The first results are presented for detecting gamma rays from the Crab Nebula in 44 h of observation, and from the blazar Markarian 421 in 62 h of observation with a significance of around 5–6 σ by one of the TAIGA IACT telescopes.

The prototype of a small wide-angle telescope was installed within the astrophysical complex TAIGA in the Tunka Valley in September 2019. Data were collected on the prototype’s performance during the last eight observation sessions. Operating regimes of the telescope’s systems, the trigger system, and the system for synchronizing with the TAIGA complex were tested. The first results from analyzing data on the prototype telescope’s performance are presented.