Vol 83, No 8 (2019)

The 1-cubic km deep Baikal-GVD underwater Cherenkov detector, a new-generation neutrino telescope, is now being deployed in Lake Baikal. The telescope’s status is described and the first physical results from its operation are presented.

The search for neutrino signals from gravitational core collapses in the Milky Way is one of the main lines of research at the Baksan Underground Scintillation Telescope (BUST). Neutrons generated by cosmic-ray muons produce a considerable background in such experiments. The contribution from the elastic scattering of fast neutrons off protons to the overall single-event background rate measured in the inner detector planes is estimated via neutron activation analysis. The resulting value of the fast-neutron flux agrees with one based on the results from FLUKA modeling for the depth of the BUST detector. The same technique can be used to estimate the neutron background in similar experiments focused on rare events.

The atmospheric muon and neutrino fluxes at energies from 10 GeV to 10 PeV are calculated with the Kimel–Mokhov (KM), SIBYLL 2.1, EPOS LHC, and QGSJET-II hadronic models for known parameterizations of the cosmic ray spectra by Zatsepin--Sokolskaya and Hillas–Gaisser. The calculations with the KM, EPOS LHC, and SIBYLL 2.1 models are agree well with the measurements of the atmospheric muon spectra in IceCube and IceTop experiments. The measurement of high-energy muon spectra indicates the presence of the prompt muon component at energies above 500 TeV, which is well reproduced in calculations with the quark-gluon string model. The calculations of the atmospheric muon neutrino spectrum performed for the KM, EPOS LHC, and SIBYLL 2.1 models give good fit with the measurement data of IceCube, ANTARES, and Super-Kamiokande experiments.

An experiment for measuring the flux of cosmic diffuse gamma rays with energies higher than 100 TeV (the Carpet-3 project) is under preparation at the Baksan Neutrino Observatory. The experiment suggests to increase areas of both the muon detector and the surface shower detectors. The experiment’s sensitivity to the showers initiated by primary gamma rays is estimated in this paper for different configurations of the array. In addition, preliminary estimates of the upper limit on diffuse gamma rays with energy above 700 TeV are presented. They are derived from experimental data of the Carpet-2 array (previous version of the experiment) for the net exposure of 9.2 years of observation.

The diffuse gamma ray emission produced by ultrahigh-energy cosmic rays (E > 4 × 10¹⁹ eV) in intergalactic space is discussed. Particles with monoenergetic or hard injection spectra, formed probably upon the acceleration of particles in supermassive black holes, are considered. It is found that intensity of the diffuse gamma ray emission depends on the shape of the injection spectrum. It is therefore concluded that data on intergalactic gamma ray emission can be used to investigate supermassive black holes.

The TAIGA (Tunka Advanced Instrument for cosmic ray physics and Gamma-ray Astronomy) experiment aims at observing gamma-rays in the energy range from 1 TeV to several 100 TeV. The operation of the observatory is based on a new hybrid approach that combines imaging air Cherenkov telescopes (IACTs) and wide-angle Cherenkov detectors (TAIGA-HiSCORE) for measuring times of extensive air shower (EAS) light front arrival. Monte Carlo simulations are compared to real data to determine the performance of the detector setup. Dedicated software and algorithms are described, model parameters are given, and an overview of the current status of model-based performance studies is presented.

Work is currently under way in the Tunka Valley, 50 km from Lake Baikal, to create the TAIGA gamma observatory for studying gamma radiation and cosmic ray fluxes in the 10¹³–10¹⁸ eV range of energies. To detect gamma rays with energies above tens of TeV, a hybrid method of detecting showers is implemented. It is based on data obtained by the TAIGA Imaging Atmospheric Cherenkov Telescope (IACT) and the wide-angle TAIGA-HiSCORE array. The preliminary results from processing the telescope’s data for the low-energy region (>2–3 TeV) are presented. Joint events with energy more than 50 TeV are analyzed and compared to Monte Carlo calculations.

Amplitudes of Forbush decreases registered in the PAMELA experiment from 2006 to 2014 were analyzed, depending on such characteristics of interplanetary space as the magnitude of the magnetic field modulus, the solar wind velocity, and the plasma temperature. The PAMELA telescope consists of a time-of-flight system, a magnetic spectrometer, anticoincidence systems, an electromagnetic calorimeter, and a neutron detector. It was launched into Earth orbit aboard the Resurs DK1 satellite in June 2006 and continued to work for 10 years.

The main aims of the NUCLEON satellite experiment are direct measurement of the energy spectra of cosmic ray protons and nuclei in the 2–500 TeV range of energies by two different methods (an ionization calorimeter and the new Kinematic Lightweight Energy Meter (KLEM) technique). The energy spectra of protons and helium nuclei are presented, and their characteristics are discussed.

Studying the central part of the Galaxy is of great interest for observational astronomy and astrophysics. Based on the results from observations in the X-ray energy range, remnants of supernova explosions, star clusters, X-ray binaries, and nonthermal filaments have been detected in a small region of the Galactic center. However, the most interesting object of study is the supermassive black hole Sagittarius A*, along with its immediate environment, including the recently discovered extended emission (several parsecs) and nearby molecular clouds. This work presents the preliminary results from a study of the properties of the stellar population in the immediate environment of Sagittarius A*. Using data from the NuSTAR orbital X-ray telescope and the stellar mass population model of the central part of the Galaxy, we estimate the X-ray luminosity per unit stellar mass of the Sagittarius A* extended emission, which is found to be a factor several times higher than the similar value throughout the Galaxy.

The sensitivity of the lateral distribution of the EAS electron component to variations in the atmospheric temperature profile is investigated. An approach developed by the authors on the basis of the variational theory of sensitivity is used to solve the problem. The differential sensitivity coefficients of the lateral distribution of EAS electrons to variations in the temperature profile are obtained for the first time. A method of adjusting a ground-based EAS array’s data for the temperature effect is proposed.

Tunka-Rex is an antenna array located in the Tunka Valley that measures the radio emissions of cosmic ray air showers with energies of up to 100 PeV. In this work, a precise technique for reconstructing a shower maximum from Tunka-Rex data is presented. A model is developed for calculating detector efficiency that considers different parameters: primary particle energy and mass ranges, shower geometry, and detector configuration. The systematic error introduced by the atmosphere in reconstructing a shower’s maximum depth is estimated, and the distribution of the mean shower maximum versus energy is determined.

The spatial distribution of cascade particles in extensive air showers from cosmic rays with energies of E₀ ≥ 10¹⁷ eV is studied at the Yakutsk array using surface scintillation detectors in continuous observations from 1977 to 2017. The experimental data are compared to calculations obtained with different models of hadron interactions. The mass composition of the cosmic rays changes from 〈lnA〉 ∼ 2.5 to the mass composition of protons in the energy range of (1–20) × 10¹⁷ eV.

Results on the longitudinal development of extensive air showers (EAS) of ultrahigh energies are obtained from observations of radio emission at the Yakutsk array for the periods from 1986 to 1989 and from 2009 to 2014. The maximum depth of individual showers is determined and a statistical analysis of X_max is performed to estimate the fluctuations in the development of EAS σ(X_max) in the energy region of 10¹⁷–10¹⁸ eV. It is shown that σ(X_max) is 50–60 g cm² in the energy range of 10¹⁷–10¹⁸ eV, which does not contradict the mixed composition of cosmic rays (protons and helium nuclei). Data on the dependence of X_max on energy also indicate this.

The Tunka-133 Cherenkov complex for recording extensive air showers (EAS) collected data over seven winters from 2009 to 2017. The differential energy spectra of all particles was acquired in the 6 × 10¹⁵–3 × 10¹⁸ eV range of energies over 2175 h. The TAIGA-HiSCORE complex is continually being expanded and upgraded. Data acquired by 30 first-line stations over 35 days during the period 2017–2018 is analyzed in this work. As at the Tunka-133 setup, the primary particle energies above 10¹⁵ eV are measured using the density of the Cherenkov light flux at a distance of 200 m from a shower’s axis. Data on lower energies are collected by determining the energy of the light flux near a shower’s axis. This results in a spectrum of 2 × 10¹⁴–10¹⁷ eV. The combined spectrum for the two systems covers a range of 2 × 10¹⁴–2 × 10¹⁸ eV.

The TUS detector is the first orbital telescope designed to test techniques for measuring ultraviolet fluorescence and Cherenkov radiation of extensive air showers (EASes). The detector was launched aboard the Lomonosov satellite on April 28, 2016. A two-level event selection system (trigger) was developed and implemented in the digital electronics of the photodetector. This trigger was optimized for the search for EASes, but at the same time operated under conditions of variable atmospheric luminescence of both natural (aurora light, diffused moonlight, thunderstorm phenomena) and anthropogenic origin (city lights, flashes at airports). The frequency of the trigger also depends on that of transient atmospheric phenomena of different origin. Examples of events recorded by the TUS detector and selected welve by the EAS trigger are given.

The Cherenkov radiation of extensive air showers (EASes) is studied using a system of optical tracking detectors based on a camera obscura as part of the Yakutsk EAS complex. These detectors allow photons to be counted from a certain height in the atmosphere and thus reconstruction of longitudinal development of a shower. Good coincidence is observed between EAS cascade curve parameter X_max measured in this way and by modeling. Preliminary results on the cosmic ray mass composition are obtained for energies above 10¹⁶ eV.

Results are presented from studying the anisotropy of the flux of very high-energy cosmic rays on the basis of muon bundles. Events were recorded in the period 2012–2018 by the DECOR coordinate-tracking detector, part of the unique research setup of the NEVOD experimental complex. The results from analyzing the anisotropy for two samples of events with muon bundles corresponding to different ranges of primary particle energy (E > 10¹⁵ eV and E > 10¹⁶ eV) are discussed, and estimates are made of dipole anisotropy parameters (amplitude and phase).

Young supernova remnants (SNRs) are effective accelerators of charged particles. Particle acceleration is accompanied by the amplification of turbulent magnetic fields. The structure of SNR synchrotron intensity and polarization maps is sensitive to features of this field, especially in the X-ray energy band. Numerical modeling of synchrotron SNR maps is performed for different models of magnetic turbulence. The capabilities of a new-generation IXPE polarimeter designed to study magnetic turbulence in the vicinity of SNRs are discussed.