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

We propose alternative processes of generation of positrons producing 511 keV annihilation line in the Galactic bulge by cosmic rays. Since relativistic cosmic rays produce both positrons and gamma rays and the flux of the latter is limited by observations, we consider sub-relativistic particles. Particles with energies below the threshold of charged pions production can generate positrons in two processes: direct pair production in electromagnetic interactions (ultraperipheral collisions) and by production of unstable isotopes by spallation and proton capture. Cross-sections of these processes are very small at non-relativistic energies and rapidly increase with particles velocities. Optimal protons velocities are about (0.1{-}0.3) of luminal velocity. In this velocity range, the cross-sections are high enough while gamma rays production is low. Thus, to produce positrons a special class of cosmic ray sources is necessary. These sources should produce large amount of sub-relativistic particles in the specified velocity range. We consider fast blue optical transients and stellar tidal disruptions by central black hole as these type of sources. Characteristic outflow velocities in these objects are of order of tenth of luminal velocity. If acceleration of relativistic particles in these outflows is low enough, they can produce the required amount of positrons. We also show that energy produced by stellar tidal disruptions events is large enough to produce the required amount of positrons.

By combining data from several X-ray telescopes (Swift/XRT, NICER, Chandra) we reconstructed a profile of the 2019 outburst of accreting X-ray pulsar 4U 1901+03 from its peak down to the return to the ‘‘low’’ state. Softening of the X-ray spectrum and disappearance of the pulsations at a later stages of the outburst tentatively indicates that the source transitioned to the ‘‘propeller’’ state at luminosity about (10^{36}) erg s({}^{-1}), which corresponds to a magnetic field strength of (Blesssim 10^{12}) G at the neutron star surface. We also investigated the fast X-ray variability that was observed during the peak of 2003 outburst. The shape of the power spectrum could be described with a broken power law, with a break frequency of 7.5 Hz. Assuming that this frequency corresponds to a viscous frequency at a magnetospheric boundary we estimated magnetic field strength as (Bapprox 7times 10^{11}) G, which is similar to the estimate obtained from observed transition to the ‘‘propeller’’ regime.

We have analyzed the change in the period of the ultracompact binary system ZTF J213056.71+442046.5, a source of potentially detectable mHz gravitational waves for planned space laser interferometers. The photometric data cover a time interval of almost 6.5 years and include our own observations carried out with the RC600 telescope at the Caucasus Mountain Observatory of the Sternberg Astronomical Institute of the Moscow State University and data from the ZTF sky survey. The (O-C) diagram can be described by quadratic light elements that correspond to a rate of period decrease (dP/dt=(-2.66pm 0.62)times 10^{-12}) s s({}^{-1}). This value leads to an almost twofold increase in the expected signal-to-noise ratio for the observations of gravitational waves from this binary system with space laser interferometers.

Observational manifestations of disturbances in a protoplanetary disk caused by a collision with a massive planet are studied. It is assumed that the planet moves along a parabolic trajectory that intersects the disk plane near the star. Gas-dynamic simulation is performed using the finite volume method on a long time scale. On its basis, images of the disk observed from the pole and edge-on are constructed in the infrared and submillimeter ranges. A wide range of planet orbit parameters is considered. The approach of the planet was considered both prograde and retrograde with the respect to the disk rotation. Calculations have shown that in the images of the disk seen pole-on, two spiral arms can be observed in case of the prograde fall, and one with retrograde case. In the case of observations of a disk whose plane is inclined at a small angle to the line of sight, distortions of the disk plane can be noticeable. In addition, a gas tail is extended from the disk in the direction of the planet’s motion, which can also be identified in observations.

We present new results of our photometric and spectroscopic monitoring of the WTTS star V715 Per that is located in the young cluster IC 348 and exhibits a complex pattern of variability. We show that the variability of the H(alpha) line equivalent width is modulated with the rotation period of the star and is in antiphase with its brightness variations. This serves as a confirmation of the previously proposed model for the photometric variability of V715 Per based on variable circumstellar extinction in the inner disk at the corotation radius. According to our estimates, the radical change in the pattern of variability occurred after 2010 is attributable with a high probability to changes in the rate of accretion in the system. Our new photometric data have shown the end of the long fading of the star observed in the period 2017–2023 and caused by the occultation of an extended dusty structure in the outer disk. The refined period of the low-amplitude variability component is (5.22^{textrm{d}}) and showed stability over the entire series of observations

In this paper we analyse the measurements of the brightness of the night sky above the CMO SAI MSU in the visible and near-infrared range made in 2019–2014. In 2023–2024 the median zenith brightness of 1 arcsec({}^{2}) of the moonless night sky was (21.31^{m}) in the (B) band, (20.63^{m}) in the (V) band, (20.15^{m}) in the (R_{c}) band, and (19.11^{m}) in the (I_{c}) band. In 5 years the sky brightness had increased by (0.7^{m}) in (B) and (V), by (0.45^{m}) in (R_{c}), and ({sim}0.1^{m}) in (I_{c}). We found that the brightness growth is mostly (up to ({sim}85%)) due to the increasing light pollution from nearby cities, while the remainder can be attributed to the increase of solar activity after the 2019 minimum. We discuss how the sky brightness is influenced by such factors as the airmass and the location of the Sun and the Moon in the sky. A qualitative analysis of the sky emission spectrum has demonstrated the growing role of LED lamps in light pollution. These changes in sky brightness which are only going to get harder favour observations that are less sensitive to the degree of light pollution—IR photometry and spectroscopy and high-resolution optical spectroscopy.

Consistent stellar evolution and nonlinear radial stellar pulsation calculations were carried out for models of asymptotic giant branch stars with initial masses (1.5leq M_{textrm{ZAMS}}leq 3;M_{odot}) and initial metal abundance (Z=0.006). All the models are shown to be either the fundamental mode or the first overtone pulsators. The lower limit of the first overtone period increases with increasing mass of the Mira model from (Pi_{1,textrm{min}}approx 80) days for (M=1.3;M_{odot}) to (Pi_{1,textrm{min}}approx 120) days for (M=2.6;M_{odot}). The upper limit of the first overtone period and lower limit of the fundamental mode period depend on the stellar structure during mode switching and range from (Pi_{1,textrm{max}}=130) days, (Pi_{0,textrm{min}}=190) days for (M=0.96;M_{odot}) to (Pi_{1,textrm{max}}=210) days, (Pi_{0,textrm{min}}=430) days for (M=2.2;M_{odot}). The slope of the theoretical period–luminosity relation of Mira variables perceptibly increases with decreasing (Z). Fourier spectra of the kinetic energy of twelve hydrodynamic models show a split of the fundamental mode maximum into several equidistant components. Frequency intervals between split components fall within the range (0.03leqDeltanu/nu_{0}leq 0.1). The superposition of radial oscillations with the fundamental mode splitting leads to the long–term amplitude variations with the cycle length from 10 to 30 times longer than the fundamental mode period. A more thorough analysis of hydrodynamic models is required for understanding the origin of the principal pulsation mode splitting.

We investigate the possibility of experimentally determining the PPN (gamma) parameter by measuring the gravitational frequency shift of signals exchanged by two satellites in heliocentric orbits. The satellites are supposed to be equipped with highly stable atomic clocks and the Gravity Probe A non-relativistic Doppler compensation system. We demonstrate that the Doppler compensation system significantly lowers the requirements to the satellite velocity determination accuracy but, at the same time, cancels out the leading contribution, of (O(c^{-3})), to the frequency shift due to (gamma). We derive an equation for the Doppler-compensated frequency shift due to (gamma) in the next-to-leading order, (O(c^{-4})), and show that it is greatly enhanced by numerical factors that rapidly grow in magnitude for signals that propagate close to the gravitational field source. Due to these ‘‘enhanced’’ factors, the accuracy of the proposed experiment with the best of the currently available clocks, such as the JILA SrI, can reach (1.7times 10^{-7}) after 5 yr of data accumulation, if performed in the optimal orbital configuration. This is an order of magnitude worse than our earlier estimate for the accuracy of a similar experiment that does not rely on the Doppler compensation system but 2 orders of magnitude better than the current best result obtained with the Cassini interplanetary probe. Finally, we discuss aspects of the practical realization of the proposed experiment, including prospects for realizing it as part of a multi-science mission that also targets other kinds of gravitational experiments.

The models of force-free magnetic flux ropes differ in their internal current structure: strong electric currents are concentrated on the rope axis in one case and in a thin peripheral shell in the other one. In this paper we perform a comparative analysis of three new models of the second type. All force-free magnetic flux ropes have one common physical property leading to flare energy release: as the top of the loop rope exits into the solar chromosphere and corona, the external pressure keeping the rope from lateral expansion drops steadily; at some critical decrease in this pressure the longitudinal magnetic field of the rope approaches zero on the current inversion surface. At the same time, the azimuthal current (j_{varphi}(r)) and the force-free parameter (alpha(r)), while approaching the discontinuity on this surface, begin to increase indefinitely near it. This leads to the excitation of plasma ion-sound instability, a sharp decrease in the plasma conductivity, rapid magnetic energy dissipation in the rope, and the generation of super-Dreicer electric fields. The set of such processes in combination with the Parker effect, i.e., torque equalization along the rope axis with the Alfv’en speed, describes well the main manifestations of a solar flare.

Based on low-resolution ((Rapprox 3000)) spectra, we have determined some parameters of the star Gaia DR3 5877303483506681472 ((G=19.4) mag), the closest object to the X-ray source [GV2003]N toward the supernova remnant G315.4–2.30: its effective temperature (T_{textrm{eff}}=4830) K, surface gravity log (g=4.3), and metallicity (textrm{[Fe/H]}=-0.05). We have analyzed its radial velocity for variability and found no variations within the error limits ((sigma V_{textrm{rad}}approx 5) km s({}^{-1})). An overabundance of the elements Ca, Ti, V, and Mn has been revealed. The color excess of the star has been estimated from the intensity of diffuse interstellar bands to be (E(B-V)=0.68pm 0.08). Our estimate of the photometric distance (d=2.5pm 0.9) kpc points to a possible connection with the supernova remnant.