Astronomy Letters-A Journal of Astronomy and Space Astrophysics最新文献

We present the results of our identification of 11 X-ray sources detected on the half of the sky (0^{circ}<l<180^{circ}) in the 4–12 keV energy band on the combined map of the first five all-sky surveys with the Mikhail Pavlinsky ART-XC telescope onboard the SRG observatory. All these sources were also detected by the SRG/eROSITA telescope in the 0.2–8 keV energy band, whose data have allowed us to improve their positions and to investigate their X-ray spectra. Five of them have been detected in X-rays for the first time, while the remaining ones have already been known previously, but their nature has remained unknown. We have taken optical spectra for nine sources with the 1.6-m AZT-33IK telescope at the Sayan Observatory (the Institute of Solar–Terrestrial Physics, the Siberian Branch of the Russian Academy of Sciences); for two more objects we have analyzed the archival spectra from SDSS and the 6dF survey. The objects are classified as Seyfert galaxies (seven Sy1, three Sy1.9, and one Sy2) at redshifts (z=0.029{-}0.258). Our analysis of the X-ray spectra has revealed a noticeable intrinsic absorption ((N_{textrm{H}}sim 10^{22}) cm({}^{-2})) in two of the four Seyfert 2 galaxies (Sy1.9–2). The spectrum of one more of them (SRGA J000132.9+240237) cannot be described within the model of an absorbed Comptonization continuum, which may point to a strong absorption and a significant contribution of the reflected radiation. However, the available SRG all-sky survey data are not enough to obtain reliable constraints on the absorption column density in this object, which is also interesting in that it is radio loud. Longer X-ray observations are required to refine the physical properties of this active galactic nucleus.

We present the results of our optical study of the eclipsing polar Gaia23cer. We analyze the orbital brightness variability in high ((langle rrangleapprox 16.5^{m})) and low ((langle rrangleapprox 19.2^{m})) states. The system has an orbital period (P_{textrm{orb}}=102.0665pm 0.0015) min and exhibits deep eclipses with a duration (Delta t_{textrm{ecl}}=401.30pm 0.81) s. The spectra have a red cyclotron continuum with the Zeeman H(alpha) absorption triplet forming in a magnetic field with a strength (B=15.2pm 1.1) MG. The source of emission lines has a high radial velocity semi-amplitude ((Kapprox 450) km s({}^{-1})), and its eclipse lags behind the eclipse of the white dwarf. The mass (M_{1}=0.79pm 0.03;M_{odot}) and temperature (T=11,350pm 650) K of the white dwarf have been estimated by modelling the spectral energy distribution. The eclipse duration corresponds to a donor mass (M_{2}=0.10{-}0.13;M_{odot}) and an orbital inclination (i=84.3^{circ}{-}87.0^{circ}). The donor temperature was estimated to be (Tapprox 2900) K by modelling the elliptical variability and eclipse depth.

We consider the final evolutionary stages of a neutron star–black hole pair. According to the current paradigm, such systems eventually coalesce, which in some cases is accompanied by neutron-star tidal disruption. Using analytical methods, we show that the scenario of slow (of the order of several seconds) neutron-star stripping by the black hole is also possible, depending on the system parameters (the initial masses and intrinsic angular momenta of the components, the equation of state for the neutron star). Reaching the lower mass limit (about one tenth of the solar mass), the neutron star explodes to produce a comparatively powerful electromagnetic transient. Our population calculations show that the stripping mechanism is possible in 50–90(%) of the cases among all coalescing neutron star–black hole pairs, depending on the model assumptions about the evolution of close binary systems (the common-envelope efficiency parameter, the supernova explosion mechanism) and the initial metallicity of the stellar population. Because of the large mass of the ejected material, the kilonova emission in this scenario has good prospects of detection.

Based on spectrophotometric observations, we have investigated the envelope of Nova V1405 Cas. The spectrophotometric evolution has shown that it belongs in its spectral characteristics to the rare type of hybrid novae. At the optically thick phase in the Balmer lines the profiles had a P Cyg shape; the radial velocities of the absorption components in the profiles pointed to ejecta with velocities reaching (RV=-1800) km s({}^{-1}). We have determined the maximum envelope expansion velocity from the half-width of the line profiles at the continuum level, which reached 3300 km s({}^{-1}) from the H({alpha}) line. We have obtained approximate estimates of the electron density (N_{e}=10^{7}) cm({}^{-3}), temperature (T_{e}=2times 10^{4}) K, and the abundances of some elements. We show that the neon and iron abundances for such electron density and temperature can differ from the solar ones insignificantly. The low neon abundance does not allow one to assert with confidence that the white dwarf in this nova is an oxygen–neon one. The mass of the envelope has been estimated to be ({sim}6times 10^{-5};M_{odot}).

Greenwich type 0 sunspot groups, i.e., single sunspots, are considered. It is shown that in addition to the previously noted properties of sunspot groups separating into two populations, large long-lived (LLG) and small short-lived (SSG) ones, such groups, i.e., single sunspots, differ by the relationship between the total sunspot area (S) and the umbra area (U). The ratio (S/U) for the entire LLG population of this class may be deemed constant and is (Q_{textrm{LLG}}=5.756pm 0.039). The ratio of (S) and (U) for the SSG population is determined by a power law, (S=(5.569pm 0.093)U^{0.8957pm 0.0040}). The difference of the structural properties of sunspots in the populations, along with other previously found differences of their physical properties, is consistent with the hypothesis about the formation of the magnetic flux of two different sunspot populations in different zones: the deep tachocline and the near-surface leptocline.

We investigate the possibility of experimentally determining the value of the PPN (gamma) parameter, which characterizes possible deviations from general relativity, by measuring the gravitational frequency shift of signals exchanged by two satellites in geo- and heliocentric orbits. We demonstrate that with modern optical clocks the accuracy of an experiment based on our concept can reach an accuracy of at least (1.4times 10^{-8}), which is 3 orders of magnitude better than the current best result achieved with the Cassini interplanetary probe.

In 2015–2021 we carried out observations of the Seyfert 1 galaxy (SyG 1) NGC 7469 in Bessell (UBVRI) filters with the 1.5-m telescope of the Maidanak Observatory, which have continued the monitoring of NGC 7469 since 1990. We took (UBVRI) CCD frames and performed standard frame processing, including bias correction, sky background subtraction, flat fielding, cosmic-ray particle removal, etc. All our data were obtained on CCD cameras with deep cooling and subarcsecond seeing. We carried out the calibrations using several standard stars in the same frame as the galaxy. The results of our photometry in a 13.5 arcsec aperture are presented graphically in the visibility windows of the observed period. A slow (S) flare was detected on the constructed variability curves in the (UBVRI) filters; a slow increase in brightness is observed since 2015, reaching a maximum in 2019. The amplitude of the S flare is 0.5 mag in the (U) filter and decreases to zero in the (I) filter. On the color–color ((U-B)/(B-V)) diagrams the colors in a 5 arcsec aperture are bluer than those in a 30 arcsec aperture.

The optical characteristics of a magnetocentrifugal disk wind from T Tauri stars that we calculated based on the MHD models of Safier (1993a, 1993b) are presented. For an outflow rate in the range (10^{-10}{-}10^{-7} M_{odot}) yr({}^{-1}) we calculated the critical angles at which the wind becomes opaque in the optical, ultraviolet, and X-ray parts of the spectrum. The illumination of the outer regions of protoplanetary disks involved in creating a photoevaporating wind by the star and the conditions for observing young stars in different wavelength ranges depend on these angles. We show that at the early evolutionary stages of T Tauri stars the disk wind is capable of completely shielding the star and preventing the direct illumination of the peripheral disk regions in both optical and X-ray ranges. By absorbing the bulk of the stellar radiation, the disk wind itself becomes a radiation source capable of heating the disk. We show that at an outflow rate ({geq}10^{-9} M_{odot}) yr({}^{-1}) the fraction of the radiation absorbed by the wind can reach (60%). Hot accretion spots can contribute significantly to the absorbed radiation. This allows the disk wind to be considered as an important infrared radiation source of T Tauri stars.

We have investigated the formation of optical radiation and refined the set of parameters for the young pre-cataclysmic binary with an sdB-subdwarf V1828 Aql. We have obtained moderate-resolution spectra with the 6-m BTA telescope and multiband light curves with the Zeiss-1000 telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences. The method of model stellar atmospheres with an accurate allowance for the gravitational and radiative interaction of the components has been used to calculate the optical radiation from the system. We have found that, despite the high temperature of the primary component, (T_{textrm{eff}}=41,200pm 800) K, the amplitude of the photometric reflection effects limited by the small sizes of the secondary component does not exceed (Delta m_{V}=0overset{m}{.}16). The emission features in the HI lines forming on the irradiated part of the surface of the cool star, when added with the absorption lines in the spectrum of the sdB-subdwarfs, weaken them by 10–12(%) and shift them by (Delta V_{r}<10) km s({}^{-1}) in a narrow range of phases adjacent to the time of the secondary eclipse. As a result, the distortions of the observed radial velocity curves under the action of reflection effects turn out to be much smaller than the measurement errors and cannot be used to determine the component mass ratio from a model analysis. Therefore, to determine the set of parameters for V1828 Aql, we have used the assumption that the mass of the primary component is close to (M_{1}=0.47 M_{odot}) found previously for a large group of sdB-subdwarfs. Based on our model analysis of the observed spectra and light curves for V1828 Aql, we have estimated the atmospheric parameters of the subdwarf and the fundamental parameters of both components. We have found that using the method of model atmospheres in analyzing the reflection effects reduces the estimate of the radius of the cool star by 20(%) compared to the calculations in the blackbody approximation.

Based on moderate-resolution spectra in the range 4200–5200 Å, we have studied late O–early B stars in the very young open star cluster IC 1805. The temperatures of the stars under study have been derived by the differential method in which simple parameters were chosen for a number of spectral lines and were then compared with the analogous parameters from an extensive sample of O–B stars. Accurate (T_{textrm{eff}}) estimates have been obtained for them from model atmospheres. Applying this method for the sample of objects under study has allowed the cluster age (t=2pm 0.5) Myr and distance modulus ((v-V)_{0}=11.5^{m}) to be determined with confidence. We have studied the spectroscopic variability of the emission spectrum for the sole Be star of early spectral type B MWC 50 that is a member of the cluster. We show that it is a classical Be star probably seen pole-on and has no long-term variability of the profiles typical for many classical Be stars.