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In disk accretion of gas onto a neutron star, high densities and temperatures are expected. The objective of the quantitative accretion model is to obtain boundary layer gas parameters during the development of small-scale instability. For critical accretion of incident matter on a neutron star, we have discussed the possibility of nucleosynthesis.

We have performed timing of several of known second-period pulsars with poorly known coordinates and parameters of their own rotation. Data from the archive of round-the-clock monitoring observations with the third (uncontrolled) radiation pattern of the Large Phased Array telescope (LPA) of the Pushchino Radio Astronomy Observatory of the Lebedev Physical Institute (PRAO LPI) at a frequency of 111 MHz were used, which have an unsatisfactory time link of local quartz time standards to the reference scale (UTC). To compensate for the errors caused by this, we applied an algorithm previously developed by us, which uses an intermediate reference scale of pulsar time to calculate corrections at the Time of arrival (TOA) of pulsar pulses measured by local clocks and to switch to UTC. Based on the results of the analysis of a 10-year series of observations, we were able to significantly refine the rotational and astrometric parameters of 12 pulsars. The proper rotation frequencies ν and their first derivatives (dot {nu }) were determined with an accuracy of 10^–10 Hz and 10^–19 s^–2, respectively, which is 5–6 orders of magnitude higher than the accuracy given in the catalog. The coordinates are determined with an accuracy better than a few tens of arcseconds.

A statistical analysis of survey observations conducted by the global network of MASTER robotic telescopes over 20 years has been used to derive independent constraints on the optical emission associated with Fast Radio Bursts (FRBs). The method is based on simulating a synthetic catalog of FRBs uniformly distributed across the sky and in time and subsequently checking for the absence of random coincidences with optical observations. As a result, a constraint on the ratio of the burst energy in the optical band to the energy in the radio band has been obtained: (eta < 4.5 times {{10}^{4}}). Although this limit is less strict than that provided by the Gaia mission, it offers an independent test for FRB origin models and is less dependent on assumptions about the duration of the optical signal. The results place important constraints on theoretical models that predict significant optical emission from FRBs.

The approximate method, which was previously used to correct the results of detecting exoplanets with the radial velocity technique for observational selection effects, yielded a significant error in a range of low masses. In the present paper we have significantly refined and improved this method. With the Lomb–Scargle periodograms and with accounting for a different number of measurements of the radial velocities of host stars, we showed that, in a range of 2–14 Earth masses, the distribution of planets detected with the radial velocity technique follows a power law with an exponent of   –1: dN/dm ~ m^–1.

Since 2017, Institute of Astronomy of the Russian Academy of Sciences (INASAN) has been engaged in a collaborative effort with the Institute of Geophysics and Astronomy of the Republic of Cuba (IGA). During this period, an optical observation station of the international Russian–Cuban Observatory (RCO) has been established on the premises of the IGA in Havana. This station is equipped with a robotic 20-cm wide-field telescope. Observations are remotely operated in a collaborative mode by Russian and Cuban astronomers. In 2024, the Russian segment of the RCO has been established at the Kislovodsk Observatory of INASAN. The optical station includes a 50-cm telescope equipped with an astronomical camera for photometric observations and a medium-resolution spectrograph. Observations are carried out jointly by Russian and Cuban astronomers via remote access. In this paper, the optical complex’s equipment and the initial results of its operation in 2025 have been described.

The formation of large-scale structure of the Universe has been studied using a pressureless medium model. The evolution of initial perturbations and the formation of singularities, such as delta density functions, have been considered using the Euler–Poisson equations for isobaric media. It has been shown that different initial conditions lead to the emergence of different types of singularities, including point-like and curvilinear ones. Criteria for the formation of singularities depending on the initial conditions have been proposed.

Using two eruptive filaments as an example, it is shown that smooth axes of filaments can writhe into helical structures during eruption. This is a clear manifestation of kink instability, which, however, is not the trigger of eruption, but its result. The development of instability occurs at a late stage of eruption on a very large scale. This study is a continuation and development of work published earlier in The Astrophysical Journal, which focused only on the initial stage of eruption occurring at a relatively low height. To analyze the phenomena, observation data were used from the AIA (Atmospheric Imaging Assembly) and HMI (Heliospheric and Magnetic Imager) instruments on board SDO (Solar Dynamics Observatory), SECCHI EUVI (Sun Earth Connection Coronal and Heliospheric Investigation, Extreme UltraViolet Imager) on board STEREO (Solar Terrestrial Relations Observatory), LASCO C2 (Large Angle Spectrometric Coronagraph) on board SOHO (Solar and Heliospheric Observatory), as well as observations from ground-based telescopes. Comparison of images of filaments on the disk with calculations of the magnetic field in the corona gives a fairly reliable estimate of their initial height and convincingly indicates the proximity of the filaments to the threshold of eruptive instability, which probably causes eruptions as a result of the development of this instability. These examples contradict the fairly widespread opinion that kink instability usually initiates eruptions of magnetic flux ropes, and eruptive instability ensures their further development.

All three orbits for doubly eclipsing system (2+2) BU CMi were significantly refined using new TESS data and published spectral observations. New values of apsidal motion and accurate physical parameters of the system are obtained. Component A: ({{T}_{1}} = 9860 pm 50) K, ({{M}_{1}} = (2.7 pm 0.15){kern 1pt} {{M}_{ odot }}), ({{R}_{1}} = (2.19 pm 0.07){kern 1pt} {{R}_{ odot }}), ({{T}_{2}} = 10,000 pm 80) K, ({{M}_{2}} = (2.7 pm 0.15){kern 1pt} {{M}_{ odot }}), ({{R}_{2}} = (2.23 pm 0.07){kern 1pt} {{R}_{ odot }}), apsidal period ({{P}_{{{text{aps}}}}} = 25.06) years. Component B: ({{T}_{1}} = 10,260 pm 80) K, ({{M}_{1}} = (2.7 pm 0.15){kern 1pt} {{M}_{ odot }}), ({{R}_{1}} = (2.33 pm 0.02){kern 1pt} {{R}_{ odot }}), ({{T}_{2}} = 9700 pm 50) K, ({{M}_{2}} = (2.5 pm 0.15){kern 1pt} {{M}_{ odot }}), ({{R}_{2}} = (2.04 pm 0.02){kern 1pt} {{R}_{ odot }}), ({{P}_{{{text{aps}}}}} = 25.22) years. Orbiting period ({{P}_{{{text{orb}}}}} = (5.88 pm 0.03)) years. Photometric parallax exactly equals Gaia DR3 value (pi =0.00401'' ). The age of the system is determined to be 224 Myr at solar chemical composition. The work is partially based on a talk presented at the Modern Stellar Astronomy 2025 Conference.

In the paper, orbital dynamics, regular or chaotic, of globular clusters (GCs) in the central region of the Galaxy, which is subject to the greatest influence of the rotating bar, has been studied. Such methods for determining chaos as Poincaré sections and spectral methods have been compared. The relationship between the Poincaré sections and the spectral characteristics of the orbits has been estimated. The sample includes 45 globular clusters in the central region of the Galaxy with a radius of 3.5 kpc. To form the 6D-phase space required for integrating the orbits, the most accurate astrometric data to date from the Gaia satellite, as well as new refined average distances, have been used. The following, most realistic, bar parameters have been adopted: the mass is ({{10}^{{10}}}{kern 1pt} {{M}_{ odot }}), the length of the major semi-axis of the bar model in the form of a triaxial ellipsoid is 5 kpc, the rotation angle of the bar axis is (25^circ ), and the rotation velocity is 40 km/s/kpc. The result of the study is that a 100% correlation between the classification by Poincaré sections and the spectral characteristics of the orbits has been established. Consequently, the classification by Poincaré sections can be replaced by a more visual analysis of the amplitude spectra of the orbits. Thus, two lists of GCs: with regular and chaotic dynamics have been compiled. The GCs with varying degrees of orbital chaos have separately been distinguish.

In 2021–2022, photometric observations in the (Ic) filter of the faint X-ray nova in a quiescent state, GS 2000+25 (QZ Vul), have been carried out. Compared to 1995–1999, no significant long-term changes in the orbital period have been detected. There are also no any changes in the average brightness of the system, and the amplitude and the shape of the orbital light curve. The average (Ic)-light curve of the system (the total amplitude is (A sim ;{{0.3}^{m}})) has been constructed. The shape of the curve corresponds to the effect of ellipticity with a negligible effect of X-ray heating of the star and with a small difference in the heights of the maxima. The main contribution to the total flux from the system in the (Ic) filter is made by the secondary component K3 V–K6 V (81–87%). The contribution of the radiation from the disk with a hot spot is ( sim {kern 1pt} 10{-} 14% ). An interpretation of the (Ic) mean light curve of QZ Vul has been performed within the framework of a model of a close binary system consisting of an optical K4.5 V star that completely fills its Roche lobe and a compact (relativistic) object surrounded by a weakly elliptical accretion disk of complex shape with thin near the boundary layer and with a thick outer edge. The presence of a gas flow (hot line) and a hot spot on the lateral surface of the disk has been taken into account. Based on the dependences of the residuals ({{chi }^{2}}) on the mass ratio (q) and on the inclination of the orbit (i) at a significance level of 5%, confidence intervals have been obtained for (q) ((q = 26.2{-} 30.4), ({{q}_{{min }}} = {{M}_{x}}{text{/}}{{M}_{v}} = 28)) and (i) ((i = 61{kern 1pt} ^circ {-} 66^circ ), ({{i}_{{min }}} = 64^circ )). The masses of the stars in the system have been estimated based on the mass function of the optical star. The mass of the black hole is ({{M}_{x}} = (6.8{-} 8.2){kern 1pt} {{M}_{ odot }}) with the optimal value of (7.34{kern 1pt} {{M}_{ odot }}). The mass of the optical star lies in the range of ((0.21{-} 0.34){kern 1pt} {{M}_{ odot }}) with the optimal value of (0.265{kern 1pt} {{M}_{ odot }}). The radius of the optical star K4.5 V is ( sim {kern 1pt} 0.64{kern 1pt} {{R}_{ odot }}). Its mass, radius, and spectral type are inconsistent with the data for main-sequence stars and correspond to an evolved star that has lost part of its mass during a long-term mass exchange in the system.