The operation of a small-size Cherenkov gamma-ray telescope TAIGA-IACT with camera on SiPMs OnSemi MicroFJ-60035 has been modelled by multiparticle Monte Carlo (MC) methods. The model implies that telescope camera is equipped with two specific types of filters of 290-590 nm (visible+NUV) and 220-320 nm (MUV+UVB)-bands, each covering half of the camera pixels in some uniform order. This allows one to measure the fraction of UV-radiation in total amount of Cherenkov radiation of an extensive air shower (EAS), that can be used for efficient gamma-hadron separation. The corresponding quality factor takes values up to 5.07 in the 10-100 TeV range depending on the distance to EAS axis and camera orientation.

Results are presented from Monte Carlo calculations of the effective areas and count rates of the TAIGA–IACT Cherenkov telescope with an upgraded SiPM OnSemi MicroFJ-60035 camera equipped with SL 290–590 and SL 280–390 filters. It is shown that the threshold energy (by triggering cosmic gamma-quanta) of the TAIGA–IACT telescope with a SiPM camera and an SL 290–590 filter is ≈0.4 TeV, slightly lower than that of the current TAIGA–IACT configuration with a PMT camera (0.5 TeV). The threshold energy of the TAIGA–IACT telescope with a SiPM camera and an SL 280–390 filter is ≈0.7 TeV, which is quite acceptable for Cherenkov telescopes with a mirror area of 10 m2. These results, plus the expected stability of the SiPM design with excessive illumination and the ability to use UV filters (which allow observations at twilight and on moonlit nights without raising the trigger threshold signal appreciably), indicate that the TAIGA–IACT telescope with a SiPM camera is a promising instrument for observing cosmic gamma radiation in the TeV range.

A quantitative simulation of a system of optical concentrators based on Winston’s hexagonal cones, intended for the registration camera of the TAIGA-IACT Cherenkov gamma-ray telescope, has been performed. The data on the transmission of the cones are obtained; the distributions of the photon flux intensity in the plane of the detector are given. On the basis of the results obtained, an optimal configuration of optical concentrators is proposed, taking into account the design features of the mount, mirror and TAIGA-IACT camera, as well as the features of its new detector units.

We present modeling of effective area and count rates of a TAIGA-IACT Cherenkov gamma-ray telescope unit with an upgraded camera based on semiconductor photo detectors (SiPM) OnSemi MicroFJ-60035 and optical filters SL 290-590 and SL 280-390. In comparison with the current configuration of TAIGA-IACT where classic vacuum photomultipliers are employed, the threshold detection energy of cosmic gamma-quanta by a TAIGA-IACT unit equipped with a SIiPM-based camera and a wide-band optical filter SL 290590 would be reduced down to about 0.4 TeV, and with a narrower filter SL 280-390 down to about 0.7 TeV. Application of semiconductor photo detectors, which are stable against excess illumination, and optical filters of the near-UV band allows one to substantially increase the duty cycle of a Cherenkov gamma-ray telescopes due to the possibility to carry out observations during moonlit nights and at twilight even without a need to substantially increase the trigger threshold. Hence, one may conclude that a TAIGA-IACT unit with an upgraded camera with SiPM detectors will be an efficient instrument for studies of TeV-range emission from space gamma-ray objects.

A new experimental detector cluster for the TAIGA-IACT telescope has been developed. The cluster contains 28 pixels based on MicroFJ-60035 silicon photomultipliers, whose signal is digitized with an analog memory chip (switched capacitor array) DRS4 at a frequency of up to 5 GHz. The paper describes the device and the operation principles of the detector cluster, reveals the peculiarities encountered in the development process. Dark chamber tests of the cluster with a point source of short pulses of ultraviolet light have allowed us to obtain dependencies of the cluster conversion coefficient and the maximum value of the recorded signal on the overvoltage of the silicon detectors.

The Unruh effect for a massive scalar field in (3+1) D space-time is considered in the case of accelerated motion with a certain starting instant. A stationary energy-angular distribution of the corresponding Unruh radiation to be observed by a two-opposite-horn antenna is obtained. It is shown that this distribution is significantly anisotropic, and the Unruh radiation cannot be considered as thermal (equilibrium) radiation. It is also shown that Fulling modes are not completely relevant wave functions for the description of particles in semi-eternal Rindler space-time. More appropriate candidates are suggested for this aim.

The temperature dependences of the dark count and efficiency of single-photon detection for two silicon avalanche photodetectors of ultraviolet radiation (OnSemi/SensL MicroFJ-60035 and Hamamatsu VUV4 S13371-6050CQ-02) operating at a wavelength of 277 nm are studied experimentally in the context of development of a new recording camera for the TAIGA-IACT gamma-telescope. It is shown that the main characteristics of these detectors correspond to the manufacturer certification. The signal readout systems have been developed and tested. It is concluded that such detectors are applicable in recording cameras of Cherenkov gamma telescopes.

Full-particle massive modeling of physical processes in the Earth’s atmosphere leading to generation of Cherenkov radiation in γ-ray- and proton-induced extensive air showers (EASs), as well as Monte Carlo modeling of photon transport in small-size Cherenkov telescope and signal registration with a camera based on OnSemi MicroFJ type semiconductor photomultiplier (SiPM) detectors have been performed. Calculations have been carried out for primaries with energies in the 0.3–30 TeV range and a Cherenkov telescope with an ≃10 m2 mirror similar to that employed at the TAIGA observatory. It is shown that, even with strict selection criteria ensuring high-quality EAS images, the threshold detection energy of the SiPM-based camera would not exceed 0.8 TeV, which is about twice as low as the detection threshold (≃1.5 TeV) of a small-size TAIGA-IACT telescope camera based on vacuum photomultipliers.

In the context of the development of a new detecting camera for the Cherenkov gamma-ray telescope TAIGA-IACT, the temperature dependences of the dark count rate and the efficiency of ultraviolet photon detection of two silicon avalanche photodetectors: OnSemi / SensL MicroFJ-60035 and Hamamatsu VUV4 S13371-6050CQ-02 at a wavelength of 277 nm were experimentally investigated. It is shown that the main characteristics of these detectors correspond to those declared by the manufacturers. Front-end readout electronics was developed and tested. Conclusions are drawn about the applicability of such detectors in the detecting cameras of Cherenkov gamma-ray telescopes.

Full-particle modeling of gamma-ray and proton induced extensive air showers (EASs) in the Earth's atmosphere as well as Monte Carlo modeling of photon transport in a small size Cherenkov telescope and signal registration with its camera based on the OnSemi MicroFJ SiPM detectors have been performed. Calculations have been carried out for primaries within the 0.3 - 30 TeV range and a telescope with a 10 m(2) mirror similar to the unit employed at the TAIGA observatory. It has been shown that even with strict selection criteria aimed at high quality EAS images, the threshold detection energy of the SiPM-based camera would not exceed 0.8 TeV - about twice as low as the current threshold of the TAIGA-IACT camera based on vacuum photomultipliers.

A quantitative simulation of a system of optical concentrators based on Winston's hexagonal cones, intended for the registration camera of the TAIGA-IACT Cherenkov gamma-ray telescope, has been performed. The data on the transmission of the cones are obtained; the distributions of the photon flux intensity in the plane of the detector are given. On the basis of the results obtained, an optimal configuration of optical concentrators is proposed, taking into account the design features of the mount, mirror and TAIGA-IACT camera, as well as the features of its new detector units.

We have modeled propagation of source signal and noise for a future camera of TAIGA-IACT Cherenkov gamma-ray telescope, which will be based on silicon photomultipliers sensitive in the 240 -- 600 nm range. It has been shown that employment of such detectors instead of traditional vacuum photomultipliers will allow one to decrease the energy threshold by a factor of about 2.5: from ~ 1.5 TeV down to ~ 0.6 TeV. We have also shown that application of a standard ultraviolet filter ZWB3 will decrease the source signal by a factor of 3, while the noise signal from the night sky will be decreased by a factor of 6. In this way the duty cycle of the telescope can be extended (the telescope will be able to operate during moonlit nights and during twilight) and the energy threshold further decreased down to ~ 0.3 TeV. A narrow 260 -- 300 nm filter can be employed to improve gamma-hadron separation of primary cosmic particles in the ~ 25 -- 50 TeV band.

Source and noise signals in a new camera of the TAIGA-IACT Cherenkov γ-ray telescope based on silicon photomultipliers (SiPM) have been simulated. It is shown that application of modern silicon photomultipliers as detecting elements of TAIGA-IACT (instead of the currently used conventional vacuum photomultipliers) will make it possible to reduce the threshold detection energy of cosmic γ quanta by a factor of about 2.5 (from ≃1.5 to ≃0.6 TeV). It is also shown that employment of a standard ZWB3 UV filter mask in the TAIGA-IACT camera would reduce the average signal level by a factor of about 3 and the noise (background) level from the night sky by a factor of about 6, which would allow one to extend the duty cycle of the telescope (in particular, to carry out observations during moonlit nights and twilights) and to additionally reduce the threshold detection energy down to ≃0.3 TeV. The application of a narrower UV filter of 260–300 nm bandwidth can increase the efficiency of determining the primary particle type (γ-hadron separation) in the energy range from ~25 up to ~50 TeV.

A novel cluster of sensitive detectors based on silicon photomultipliers (SiPM) is being developed for the Cherenkov gamma-ray telescope TAIGA-IACT (Tunka valley, Republic of Buryatia, Russia). The cluster will be able to detect Cherenkov radiation from extensive air showers in two wide bands: 250-300 nm (UV) and 250-700 nm (visible and UV). Each pixel consists of a Winston cone, 4 SiPMs with the total sensitive area of 144 mm(2), and readout electronics based on fast analogue memory. During operation in the UV band, a UV-bandpass filter is used to suppress cluster sensitivity in the visible range. In order to evaluate the detection efficiency of the selected SiPMs, a specific software simulator of SiPM output signal has been developed. This simulator takes into account such inherent parameters of SiPMs as total number of microcells, their recharge time, the dark count rate, the effective detection area, the quantum efficiency, the crosstalk between microcells, as well as conditions of SiPM operation, namely, the background noise and the Ohmic load in the readout (front-end) electronics. With this simulator it is possible to determine the expected trigger threshold under given conditions and parameters of selected detectors. Based on preliminary simulations, OnSemi MicroFJ-60035 SiPM chips have been chosen for the novel cluster of TAIGA-IACT. These SiPMs have sensible efficiency in the ultraviolet range (5-20% in the 250-300 nm band) and are distinguished by the presence of a fast output, which allows one to capture a low amplitude signal above a relatively high background noise.

Sensitive observations of energetic space phenomena in the 0.3 – 10 MeV range are needed to investigate a number of hot issues of modern astrophysics and cosmology. With the advanced technology of thick double sided silicon strip detectors (Si-DSSDs) it is possible to construct a space-borne gamma-ray telescope with sensitivity 30-100 times better than that of CGRO/COMPTEL. Synergies of MeV range observations from space with GeV-TeV range spectra to be obtained with future fast and sensitive imaging atmospheric Cherenkov arrays (CTA, TAIGA-IACT, ALEGRO) would allow one to reveal complex mechanisms of energy conversion in transient gamma-ray objects, in particular, sources of gamma-ray bursts. In this short note we briefly outline most demanding objectives of 0.3 – 10 MeV range astronomy and present a schematic view of HERMES gamma-ray spectrometer being developed at the Ioffe Institute.

Представлены результаты моделирования черенковского излучения широких атмосферных ливней (ШАЛ), порожденных космическими гамма-квантами и протонами космических лучей. Для наблюдений на высоте 2 и 5 km над уровнем моря рассчитана спектральная плотность черенковского излучения ШАЛ в диапазоне 240−700 nm при энергиях первичных частиц 3GeV−10 TeV. Отношение количества фотонов черенковского излучения ШАЛ в оптическом и ультрафиолетовом (УФ) диапазоне существенно зависит от типа первичной частицы, поэтому измерение и анализ этого отношения может применяться для выделения гаммасобытий из фона, создаваемого частицами космических лучей. Использование детектирующих элементов на основе специализированных кремниевых фотоумножителей, обладающих высокой чувствительностью в УФ диапазоне, позволяет существенно увеличить продолжительность рабочего цикла черенковского гаммателескопа за счет наблюдений в лунные ночи. При использовании в телескопах черенковской гаммаобсерватории ALEGRO разработанных в ФТИ им. А.Ф. Иоффе кремниевых лавинных умножителей (SiPM), порог регистрации гамма-событий только на основе измерений в УФ диапазоне составит около 40GeV на высоте 5 km и около 80GeV на высоте 2 km.

The Unruh effect is considered for the case of a massless scalar field in an (1+1)D space-time. It is shown that under some natural assumptions like finitness of the integration volume or finitness of the interaction propagation speed the effect should be anisotropic.

Cherenkov emission of extensive air showers (EAS) from cosmic gamma rays and cosmic-ray protons has been modelled. Spectral density of such emission in the 240–700 nm band has been determined for 3 GeV–10 TeV primaries of both sorts. It has been shown that the ratio of EAS Cherenkov emission fluxes in the optical and ultraviolet (UV) bands substantially depends on the sort of the primary, so the ratio can be employed to select gamma-ray events from the background produced by cosmic rays. Detection of EAS Cherenkov emission with specialized silicon photomultipliers sensitive in the UV band would allow one to substantially extend the duty cycle of a Cherenkov gamma-ray telescope, as it would be possible to observe the showers during moonlit nights. The UV-sensitive silicon photomultipliers developed at the Ioffe Institute would allow to detect gamma-ray-induced EAS at the Cherenkov gamma-ray observatory ALEGRO over a threshold of about 40 GeV at 5 km above sea level and about 80 GeV at 2 km above sea level.

A brief overview of the history of atmospheric Cherenkov gamma-ray telescopes is given. Topical problems of modern astrophysics and fundamental physics to be solved with these instruments are listed. The ALEGRO project of a low-threshold gamma-ray observatory is characterized in detail. The aim of this project is to examine cosmic gamma-ray sources (especially the rapidly variable gamma-ray sources, gamma-ray transients) with high statistics of detected photons in the energy range of 5–50 GeV.