We have carried out multiwavelength spectroscopic and photometric studies of the object SRGe J194401.8+284452 (2RXS J194401.4+284456, XMMSL2 J194402.0+284451, 2SXPS J194401.7+284450), the brightest X-ray source in the position uncertainty ellipse of the unidentified gamma-ray source 4FGL J1943.9+2841, with the goal of determining its nature and the possible association with the gamma-ray source. The object is shown to be a cataclysmic variable with an orbital period of about 1.5 h and clear evidence for the presence of an accretion disk around the white dwarf. It can be classified by its properties as an intermediate polar whose association with the gamma-ray source is unlikely. SRGe J194401.8+284452 exhibits abrupt transitions between its high and low luminosity states simultaneously in the optical and X-ray bands, which remain relatively stable on time scales of several months/years. This may be related to the change in the accretion rate by an order of magnitude. We have obtained constraints on the mass (0.3–0.9 M_⊙ ) and temperature ( 14 750± 1250 K) of the accreting white dwarf in the low state, the mass of the donor star ( ≤0.08± 0.01 M_⊙ ), and the orbital inclination of the binary system ( 40^∘-75^∘ ). In the low state we have detected an 8-min brightness variability in the optical band that is most likely related to the white-dwarf spin and not to the nonradial pulsations. In the high state we have revealed stochastic brightness variations on time scales of 1–15 min with amplitudes of 0.2– 0.6^m . SRGe J194401.8+284452 replenishes the small group of intermediate polars with the shortest orbital periods lying below the gap in the period distribution of these systems and exhibiting transitions between the states with high and low accretion rates. The brightness of the source at a level of 17-20^m in the 2000–8000 Å range and (5-50)× 10^-13 erg s ^-1 cm ^-2 in the range 0.3–10 keV makes it an interesting object for a detailed study of the physics of such systems.

Quasi-periodic pulsations (QPPs) were observed in the light curves of solar and stellar flares. The number of stellar QPP events is quite small (213 in 2021), which is why they are interesting. QPPs can be used to investigate the properties of flare regions and they are still a problem for flare models. QPPs were identified in stellar flares across a wide range of wavelengths, from radio to X-rays. In the optical band they are searched mainly in the Kepler and TESS data. The periods of QPPs detected from these sources are longer (tens of minutes) than from the high cadence data. Observations with the 6-m telescope BTA and the MANIA complex, as well as the space observatory GALEX, reveal QPP periods of only tens of seconds. On the other hand, solar flares are typically weaker (energies $\ll 10^{32}$ erg) than stellar flares, that possibly makes them more difficult to detect in white light. Our data on 157 flares of dM-type stars (EV Lac, Wolf 359, Wolf 424, V577 Mon, and UV Ceti), obtained from 70 hours of observations with the BTA telescope in $U$-band with microsecond time resolution, represents a significant breakthrough in the search for QPPs in stars. In this paper, we present the characteristics of 13 QPPs found in 44 flares. The QPPs have periods ranging from 6 to 107 seconds, and were identified using both Fourier transform and empirical mode decomposition methods. We classify the observed QPPs based on the evolution of their oscillation envelopes and fractional flux amplitudes.

We present a brief description of a joint Russian-Korean project abbreviated as EXPLANATION (EXoPLANet And Transient events InvestigatiON). The project is aimed at a massive photometric, speckle-interferometric, spectral, and radio bolometric search for non-stationary events in the Universe, as well as the study of exoplanets. The core of the project consists of several 0.07–2.5-m optical telescopes, 6-m telescope BTA and a six-hundred-meter radio telescope RATAN-600 of the Special Astrophysical Observatory of RAS (Russia), Moscow State University observatory (Russia), Kourovka Observatory (Russia), Crimean Astrophysical Observatory (Crimea), Korea Astronomy and Space Science Institute (Republic of Korea). We discuss the philosophy of the project and its instrumentation, as well as the first obtained results. In this paper we report the results related to the detection of several types of transient events and the study of exoplanets.

Abstract—The design and functions of the main component of the automated complex for the study of astrophysical objects with high time resolution at the 6-m telescope of SAO RAS—a permanently available photopolarimeter installed at the N1 focus of the BTA for alert observations of optical transient sources are described. The device operates in several modes (up to six in perspective)—spectral, polarimetric, photometric, the choice of which is determined after real-time analysis of the image of the object localization area, recorded in the subview with a field of view of $$2\mathop .\limits^\prime 5 \times 3'$$ . The radiation from the vicinity of the detected source is transferred to a diaphragm of variable size from $$10'' \times 10''$$ to $$60'' \times 10''$$ and, depending on its brightness, passed through one of the $$UBVR$$ filters or dispersed by the Abbe prism, is recorded by the EMCCD with a time resolution of 0.1 s. In this case, it is possible to introduce a double Wollaston prism into the input beam, which ensures the measurement of the object’s linear polarization. To compensate for the rotation of the instrument’s field of view during observations on the balcony of the N1 focus, a turntable is used, the position of which is set by the telescope’s control system. Control of the observation process, selection and change of modes is carried out using a graphical interface. The automatic implementation of these operations is being tested.

In this paper we present some results of high temporal resolution multichannel optical observations of a transitional mil-lisecond pulsar J1023+0038 with the panoramic photometer-polarimeter mounted on the 6-meter BTA telescope of SpecialAstrophysical Observatory. Besides the double-peaked optical pulsations with the neutron star rotation period, similar tothose reported by other authors, during one of the nights we observed dramatic state change of the system. For about 230seconds instead of typical double-peaked structure single peak with strongly increased amplitude was seen. Simultaneousobservations in two channels also allowed us to detect energy redistribution in the spectrum of the system in this event. Wealso note chaotic orbital phase changes of the signal with maximum amplitude of about 25 seconds, corresponding to thelinear shifts of about 3000 kilometers.

Over the past decades, the achievements in astronomical instrumentation have given rise to a number of novel advanced studies related to the analysis of large arrays of observational data. One of the most famous of these studies is a study of transient events in the near and far space and a search for exoplanets. The main requirements for such kinds of projects are a simultaneous coverage of the largest possible field of view with the highest possible detection limits and temporal resolution. In this study, we present a similar project aimed at creating an extensive, continuously updated survey of transient events and exoplanets. To date, the core of the project incorporates several 0.07–2.5 m optical telescopes and the 6-m BTA telescope of the Special Astrophysical Observatory of RAS (Russia), a number of other Russian observatories and the Bonhyunsan observatory of the Korea Astronomy and Space Science Institute (South Korea). Our attention is mainly focused on the description of two groups of small, wide-angle optical telescopes for primary detection. All the telescopes are originally designed for the goals of the project and may be of interest to the scientific community. A description is also given for a new, high-precision optical spectrograph for the Doppler studies of transient and exoplanet events detected within the project. We present here the philosophy, expectations and first results obtained during the first year of running the project.

A hardware and software complex of the MANIA experiment designed to search for and study the photometric variability of astrophysical objects with a temporal resolution of 10−6 s is described. The panoramic photospectropolarimeter uses interchangeable optical units, which allow the observations to be performed in four modes—spectroscopic, spectropolarimetric, photometric, and polarimetric. A coordinate—sensitive detector equipped with a set of microchannel plates is capable of recording flux fluctuations from objects and comparison stars simultaneously in different photometric bands and in different polarization planes. The Quantochron 4–48 data acquisition facility operating on line with a computer acquires observational data with a temporal resolution of 1microsecond. The methods of analysis of panoramic data with high temporal resolution are discussed.

We report some results of optical observations of the transitional millisecond pulsar PSR J1023+0038, performed with the multi-channel panoramic photometer-polarimeter mounted on the 6-meter BTA telescope. The setup allowed us to register photons simultaneously in two channels (``red'' and ``blue'') with the effective temporal resolution of about 1 $\mu$s. We detected optical pulsations at the pulsar frequency with characteristics similar to the ones reported in other studies. In addition, during one of the nights we observed dramatic change in the pulse profile structure, mainly characterized by fast transition from double-peaked to single-peaked shape and back, and accompanied by flaring activity of the system. We analyse the results and discuss some aspects of their physical interpretation regarding the origin of the pulsing emission.

This paper presents the functional design of "Quantochron 5-48" high temporal resolution system for multidimensional chronometration of stochastic fluxes. The device can determine the photon arrival times by array detectors and the accompanying spatial, spectral, and polarization characteristics of these photons, with a sampling time of 10 nanoseconds and the accuracy of UTC calibration provided by the time server. The peak intensity of detected fluxes is determined by the frequency of the internal high-frequency generator and may amount to 10(8) counts/s. The average intensity is determined by the input/output computer bus bandwidth and may exceed 10(6) counts/s. The simplicity of design and use allows the device to be mass replicated and widely used.

— The results of polarimetric observations of blazar S5 0716+714, carried out with the 6-meter BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences, are presented. The polarized emission microvariability of blazar S5 0716+714 was observed in the R band with an initial time resolution of 3–5 seconds. Exposures were integrated at 1-minute intervals with a total observation time of several hours over two nights. Microvariability of the polarization degree with amplitude of 3–4% was detected during 7–11 minutes and flux oscillations within 9–10% during 25 minutes. A partial synchronism of the flux oscillations and the polarization degree of this blazar was found. Possible interpretations of the detected microvariability are briefly described.

Abstract—We report the development of a photosensor device based on a position-sensitive detector with a gallium arsenide (GaAs) cathode and a 16-element anode. In the case of asymmetric heterostatic circuit (electrodes combined in fours) its working field was limited by 10 mm (for a 18-mm photocathode). Implementing a scheme of analog coding of the coordinates of the centroids of electron avalanches arriving to the anode in the photosensor device made it possible to increase the field size to 14 mm and achieve a spatial resolution of 50 μm. The resulting photosensor device is used as the principal component of a multimode field photopolarimeter in observations with a microsecond time resolution on the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. We report some of the results obtained in the process of this work.

New observations of the RS CVn variable UX Ari were carried out using Mini-MegaTORTORA (MMT-9) wide-field optical monitoring system and robotic wide-angle observation system in Zvenigorod observatory of INASAN. We defined more accurately the cycles of the long-period variability of UX Ari and noticed considerable changes in the shape of the power spectrum. In the long-term region we find three cycles at 5, 13.2 and 30 years. It is possible that some cycles are aliases but the most promising period of 30 years is clearly seen. We suppose that P(cycle) estimations for active stars over 10-50 years intervals should made it possible to build a new family of relations on diagrams P(cycle)/P(rot) versus 1/P(rot).

Here we present the results of our four years long observations of meteors with Mini-MegaTORTORA wide-field monitoring system with sub-second temporal resolution. Over this period, we detected and catalogued more than 175000 faint meteors with magnitudes down to 8-10 mag and angular velocities up to 40 deg/s. Recently, we started double-station observations using Mini-MegaTORTORA together with reconstructed FAVOR camera on 3.8 km baseline. This setup allows to observe tens of faint meteors per night. We present the preliminary results of such observations.

Funding information RFBR, 16-02-00604A, 17-52-45048; Russian Science Foundation, 14-50-00043; European Structural and Investment Fund and the Czech Ministry of Education, Youth and Sports, CZ.02.1.01/0.0/0.0/15 003/0000437 Abstract We observed the millisecond redback pulsar PSR J1023+0038 in its accreting regime on two nights in November 2017 on Russian 6-m telescope with a high-temporal resolution panoramic photometer-polarimeter in a two-channel (“blue” and “red”) setup. During 400 s (12% of nearly 3 hr of total observations), we detected coherent optical pulsations in both color bands with 1.69-ms period, corresponding to the rotational period of neutron star known from the radio data, with amplitudes of 2.1% (“red”) and 1.3% (“blue”). Corresponding luminosity of pulsed component is about 10 erg s−1 and may be caused by a synchrotron emission of electrons with moderate Lorentz factors close to a light cylinder during the interaction of accretion disk with ejected matter modulated with rotational period.

Here we present the summary of operation of a novel 9-channel wide-field optical monitoring system with subsecond temporal resolution, Mini-MegaTORTORA, which systematically surveys the sky since 2014 at Special Astrophysical Observatory on Russian Caucasus. The system is able to observe the sky simultaneously in either wide (∼900 square degrees) or narrow (∼100 square degrees fields of view, either in clear light or with any combination of color (Johnson-Cousins B, V or R) and polarimetric filters installed, with exposure times ranging from 0.1 s to hundreds of seconds. The real-time system data analysis pipeline performs automatic detection of rapid transient events, both near-Earth and extragalactic. The objects routinely detected by Mini-MegaTORTORA also include faint meteors and artificial satellites. The imaging survey performed by Mini-MegaTORTORA also allows to look for a slower variability of various kinds of objects.

Here we present the project of a Small Aperture Imaging Network Telescope (SAINT), which consists of 500 small telescopes - channels - with 1 sq.deg. fields of view each. The telescope operates either in wide-field monitoring mode, covering 500 square degrees of the sky simultaneously with 0.1 s temporal resolution and detection limit of about 22-23 mag in 1000 s, or in follow-up regime, upon detection of a transient. In the latter case, all the channels immediately repoint towards the transient for its detailed (multicolor, polarimetric, spectroscopic) investigation. In this regime, SAINT is equivalent to 8-m telescope and may be used to solve most of standard astro-physical problems.