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Of all publications in the section: 66
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Article
Lutovinov A. Monthly Notices of the Royal Astronomical Society. 2016. Vol. 462. No. 4. P. 3823-3829.

We report the results of a comprehensive analysis of X-ray (Chandra and Swift observatories), optical (Southern African Large Telescope, SALT) and near-infrared (the VVV survey) observations of the Be/X-ray binary pulsar 2S 1553-542. Accurate coordinates for the X-ray source are determined and are used to identify the faint optical/infrared counterpart for the first time. Using VVV and SALTICAM photometry, we have constructed the spectral energy distribution (SED) for this star and found a moderate NIR excess that is typical for Be stars and arises due to the presence of circumstellar material (disc). A comparison of the SED with those of known Be/X-ray binaries has allowed us to estimate the spectral type of the companion star as B1-2V and the distance to the system as >15 kpc. This distance estimation is supported by the X-ray data and makes 2S 1553-542 one of the most distant X-ray binaries within the Milky Way, residing on the far side in the Scutum-Centaurus arm or even further.

Article
Lukin V. V., Malanchev K. L., Shakura N. I. et al. Monthly Notices of the Royal Astronomical Society. 2017. Vol. 467. No. 3. P. 2934-2942.

We present the results of 3D-hydrodynamical simulations of accretion flow in the eclipsing dwarf nova V1239 Her in quiescence. The model includes the optical star filling its Roche lobe, a gas stream emanating from the inner Lagrangian point of the binary system, and the accretion disc structure. A cold hydrogen gas stream is initially emitted towards a point-like gravitational centre. A stationary accretion disc is formed in about 15 orbital periods after the beginning of accretion. The model takes into account partial ionization of hydrogen and uses realistic cooling function for hydrogen. The light curve of the system is calculated as the volume emission of optically thin layers along the line of sight up to the optical depth τ = 2/3 calculated using Planck-averaged opacities. The calculated eclipse light curves show good agreement with observations, with the changing shape of pre-eclipse and post-eclipse light curves being explained entirely due to ˜50 per cent variations in the mass accretion rate through the gas stream.

Article
Chalov S. V., Malama Y. G., Alexashov D. et al. Monthly Notices of the Royal Astronomical Society. 2016. Vol. 455. No. 1. P. 431-437.

Fluxes of energetic protons in the range from 30 keV up to several MeV measured at the Voyager 1/2spacecraft downstream of the heliospheric termination shock can be explained by shock-drift acceleration theory, which includes variations of the magnetic field direction in a vicinity of the shock. The variations can be connected with the sector structure of the interplanetary magnetic field near the solar equatorial plane. Theoretical fluxes of accelerated protons are calculated numerically in the framework of a 3D kinetic-magnetohydrodynamic model of the interaction of the solar wind and local interstellar medium.

Article
Pandey S., Castro-Tirado A., Hu Y. et al. Monthly Notices of the Royal Astronomical Society. 2019. Vol. 485. No. 4. P. 5294-5318.

We investigate the prompt emission and the afterglow properties of short-duration gamma-ray burst (sGRB) 130603B and another eight sGRB events during 2012-2015, observed by several multiwavelength facilities including the Gran Canarias Telescope 10.4 m telescope. Prompt emission high energy data of the events were obtained by INTEGRAL-SPI-ACS, Swift-BAT, and Fermi-GBM satellites. The prompt emission data by INTEGRAL in the energy range of 0.1-10 MeV for sGRB 130603B, sGRB 140606A, sGRB 140930B, sGRB 141212A, and sGRB 151228A do not show any signature of the extended emission or precursor activity and their spectral and temporal properties are similar to those seen in case of other short bursts. For sGRB 130603B, our new afterglow photometric data constrain the pre-jet-break temporal decay due to denser temporal coverage. For sGRB 130603B, the afterglow light curve, containing both our new and previously published photometric data is broadly consistent with the ISM afterglow model. Modeling of the host galaxies of sGRB 130603B and sGRB 141212A using the LePHARE software supports a scenario in which the environment of the burst is undergoing moderate star formation activity. From the inclusion of our late-time data for eight other sGRBs we are able to: place tight constraints on the non-detection of the afterglow, host galaxy, or any underlying kilonova' emission. Our late-time afterglow observations of the sGRB 170817A/GW170817 are also discussed and compared with the sub-set of sGRBs.

Article
Katushkina O., Alexashov D., Gvaramadze V. et al. Monthly Notices of the Royal Astronomical Society. 2018. Vol. 473. No. 2. P. 1576-1588.

High-resolution mid-infrared observations carried out by the Spitzer Space Telescope allowed one to resolve the fine structure of many astrospheres. In particular, they showed that the astrosphere around the B0.7 Ia star κ Cas (HD 2905) has a clear-cut arc structure with numerous cirrus-like filaments beyond it. Previously, we suggested a physical mechanism for the formation of such filamentary structures. Namely, we showed theoretically that they might represent the non-monotonic spatial distribution of the interstellar dust in astrospheres (viewed as filaments) caused by interaction of the dust grains with the interstellar magnetic field disturbed in the astrosphere due to colliding of the stellar and interstellar winds. In this paper, we invoke this mechanism to explain the structure of the astrosphere around κ Cas. We performed 3D magnetohydrodynamic modelling of the astrosphere for realistic parameters of the stellar wind and space velocity. The dust dynamics and the density distribution in the astrosphere were calculated in the framework of a kinetic model. It is found that the model results with the classical MRN (Mathis, Rumpl & Nordsieck 1977) size distribution of dust in the interstellar medium do not match the observations, and that the observed filamentary structure of the astrosphere can be reproduced only if the dust is composed mainly of big (μm-sized) grains. Comparison of the model results with observations allowed us to estimate parameters (number density and magnetic field strength) of the surrounding interstellar medium.

Article
Bespalov P. A., Savina O. N. Monthly Notices of the Royal Astronomical Society. 2018. Vol. 480. No. 4. P. 4761-4765.

A beam pulsed amplifier mechanism responsible for effective amplification of short electromagnetic pulses by relativistic electrons is proposed. Effective pulse amplification inside the rarefied magnetosphere is considered. A conditional growth rate of short electromagnetic pulses is calculated. The amplification takes place for an extraordinary mode in a frequency band close to half the local electron cyclotron frequency. The obtained results based on the conditional instability concept can explain some important features of the radio emissions typical for brown dwarfs, such as the high brightness temperature of its source, circular polarization with handedness switching, periodicity with a period of rotation of the object, and narrow beams of radiation. The proposed mechanism of radio-emission excitation is effective without strong anisotropy of the relativistic electrons distribution function.

Article
Pruzhinskaya M., Malanchev K.L., Kornilov M.V. et al. Monthly Notices of the Royal Astronomical Society. 2019. Vol. 489. No. 3. P. 3591-3608.

In the upcoming decade, large astronomical surveys will discover millions of transients raising unprecedented data challenges in the process. Only the use of the machine learning algorithms can process such large data volumes. Most of the discovered transients will belong to the known classes of astronomical objects. However, it is expected that some transients will be rare or completely new events of unknown physical nature. The task of finding them can be framed as an anomaly detection problem. In this work, we perform for the first time an automated anomaly detection analysis in the photometric data of the Open Supernova Catalog (OSC), which serves as a proof of concept for the applicability of these methods to future large-scale surveys. The analysis consists of the following steps: (1) data selection from the OSC and approximation of the pre-processed data with Gaussian processes, (2) dimensionality reduction, (3) searching for outliers with the use of the isolation forest algorithm, and (4) expert analysis of the identified outliers. The pipeline returned 81 candidate anomalies, 27 (33 per cent) of which were confirmed to be from astrophysically peculiar objects. Found anomalies correspond to a selected sample of 1.4 per cent of the initial automatically identified data sample of approximately 2000 objects. Among the identified outliers we recognized superluminous supernovae, non-classical Type Ia supernovae, unusual Type II supernovae, one active galactic nucleus and one binary microlensing event. We also found that 16 anomalies classified as supernovae in the literature are likely to be quasars or stars. Our proposed pipeline represents an effective strategy to guarantee we shall not overlook exciting new science hidden in the data we fought so hard to acquire. All code and products of this investigation are made publicly available.1

Article
Malanchev K., Pruzhinskaya M. V., Korolev V. S. et al. Monthly Notices of the Royal Astronomical Society. 2021. Vol. 502. No. 4. P. 5147-5175.

We present results from applying the SNAD anomaly detection pipeline to the third public data release of the Zwicky Transient Facility (ZTF DR3). The pipeline is composed of three stages: feature extraction, search of outliers with machine learning algorithms, and anomaly identification with followup by human experts. Our analysis concentrates in three ZTF fields, comprising more than 2.25 million objects. A set of four automatic learning algorithms was used to identify 277 outliers, which were subsequently scrutinized by an expert. From these, 188 (68 per cent) were found to be bogus light curves – including effects from the image subtraction pipeline as well as overlapping between a star and a known asteroid, 66 (24 per cent) were previously reported sources whereas 23 (8 per cent) correspond to non-catalogued objects, with the two latter cases of potential scientific interest (e.g. one spectroscopically confirmed RS Canum Venaticorum star, four supernovae candidates, one red dwarf flare). Moreover, using results from the expert analysis, we were able to identify a simple bi-dimensional relation that can be used to aid filtering potentially bogus light curves in future studies. We provide a complete list of objects with potential scientific application so they can be further scrutinised by the community. These results confirm the importance of combining automatic machine learning algorithms with domain knowledge in the construction of recommendation systems for astronomy. Our code is publicly available.

Article
Sazonov S. Monthly Notices of the Royal Astronomical Society. 2019. Vol. 489. No. 1. P. 1127-1138.

There is a hope that looking into the early Universe with next-generation telescopes, one will be able to observe the early accretion growth of supermassive black holes (BHs) when their masses were ∼104-106 M☉. According to the standard accretion theory, the bulk of the gravitational potential energy released by radiatively efficient accretion of matter on to a BH in this mass range is expected to be emitted in the extreme UV-ultrasoft X-ray bands. We demonstrate that such a miniquasar' at z ∼ 15 should leave a specific, localized imprint on the 21 cm cosmological signal. Namely, its position on the sky will be surrounded by a region with a fairly sharp boundary of several arcmin radius, within which the 21 cm brightness temperature quickly grows inwards from a background value of (∼-250) mK to (∼+30) mK. The size of this region is only weakly sensitive to the BH mass, so that the flux density of the excess 21 cm signal is expected to be ∼0.1-0.2 mJy at z ∼ 15 and should be detectable by the Square Kilometer Array. We argue that an optimal strategy would be to search for such signals from high-z miniquasar candidates that can be found and localized with a next-generation X-ray mission such as Lynx. A detection of the predicted 21 cm signal would provide a measurement of the growing BH's redshift to within ∆z/(1 + z) ≲ 0.01.

Article
Igoshev A., Popov S. Monthly Notices of the Royal Astronomical Society. 2020. Vol. 499. No. 2. P. 2826-2835.

Recently, Parthsarathy et al. analysed long-term timing observations of 85 young radio pulsars. They found that 15 objects have absolute values of braking indices ranging ∼10–3000, far from the classical value n = 3. They also noted a mild correlation between measured value of n and characteristic age of a radio pulsar. In this article, we systematically analyse possible physical origin of large braking indices. We find that a small fraction of these measurements could be caused by gravitational acceleration from an unseen ultra-wide companion of a pulsar or by precession. Remaining braking indices cannot be explained neither by pulsar obliquity angle evolution, nor by complex high-order multipole structure of the poloidal magnetic field. The most plausible explanation is a decay of the poloidal dipole magnetic field which operates on a time-scale ∼104−105 yr in some young objects, but has significantly longer time-scale in other radio pulsars. This decay can explain both amplitude of measured n and some correlation between n and characteristic age. The decay can be caused by either enhanced crystal impurities in the crust of some isolated radio pulsars or, more likely, by enhanced resistivity related to electron scattering off phonons due to slow cooling of low-mass neutron stars. If this effect is indeed the main cause of the rapid magnetic field decay manifesting as large braking indices, we predict that pulsars with large braking indices are hotter in comparison to those with n ≈ 3.

Article
Sazonov S. Monthly Notices of the Royal Astronomical Society. 2017. Vol. 466. No. 1. P. 1019-1051.

Using a spectral analysis of bright Chandra X-ray sources located in 27 nearby galaxies and maps of star-formation rate (SFR) and interstellar medium surface densities for these galaxies, we constructed the intrinsic X-ray luminosity function (XLF) of luminous high-mass X-ray binaries (HMXBs), taking into account absorption effects and the diversity of HMXB spectra. The XLF per unit SFR can be described by a power-law dN/dlog LX,unabs ≈ 2.0(LX,unabs/1039 erg s-1)-0.6 (M⊙ yr-1)-1 from LX,unabs = 1038 to 1040.5 erg s-1, where LX,unabs is the unabsorbed luminosity at 0.25-8 keV. The intrinsic number of luminous HMXBs per unit SFR is a factor of ˜2.3 larger than the observed number reported before. The intrinsic XLF is composed of hard, soft and supersoft sources (defined here as those with the 0.25-2 keV to 0.25-8 keV flux ratio of <0.6, 0.6-0.95 and >0.95, respectively) in ˜ 2:1:1 proportion. We also constructed the intrinsic HMXB XLF in the soft X-ray band (0.25-2 keV). Here, the numbers of hard, soft and supersoft sources prove to be nearly equal. The cumulative present-day 0.25-2 keV emissivity of HMXBs with luminosities between 1038 and 1040.5 erg s-1 is ˜5 × 1039 erg s-1(M⊙ yr-1)-1, which may be relevant for studying the X-ray preheating of the early Universe.

Article
Yakovlev D.G., Gusakov M., Haensel P. Monthly Notices of the Royal Astronomical Society. 2018. Vol. 481. No. 4. P. 4924-4930.

We study the bulk viscosity in two (antispaghetti and Swiss cheese) phases of non-spherical nuclei in the mantle of an oscillating neutron star near the boundary with the stellar core. The bulk viscosity is produced by non-equilibrium Urca neutrino emission processes. In the mantle, the direct Urca process may be open if neutrons and protons move in a periodic potential created by a lattice of non-spherical nuclei (which allows the nucleons to have large quasi-momenta and satisfy direct Urca momentum-conservation). This bulk viscosity can dominate over that due to the modified Urca process in the outer stellar core and over the shear viscosity. The bulk viscosity depends strongly on temperature, oscillation frequency, and nucleon superfluidity. The enhanced bulk viscosity in the mantle can control propagation and damping of neutron star oscillations.

Article
Lyskova N., Churazov E., Zhang C. et al. Monthly Notices of the Royal Astronomical Society. 2019. Vol. 485. No. 2. P. 2922-2934.

We study a merger of the NGC 4839 group with the Coma cluster using X-ray observations from the XMM–Newton and Chandra telescopes. X-ray data show two prominent features: (i) a long (∼600 kpc in projection) and bent tail of cool gas trailing (towards south-west) the optical centre of NGC 4839, and (ii) a ‘sheath’ region of enhanced X-ray surface brightness enveloping the group, which is due to hotter gas. While at first glance the X-ray images suggest that we are witnessing the first infall of NGC 4839 into the Coma cluster core, we argue that a post-merger scenario provides a better explanation of the observed features and illustrate this with a series of numerical simulations. In this scenario, the tail is formed when the group, initially moving to the south-west, reverses its radial velocity after crossing the apocenter, the ram pressure ceases and the ram pressure-displaced gas falls back towards the centre of the group and overshoots it. Shortly after the apocenter passage, the optical galaxy, dark matter, and gaseous core move in a north-east direction, while the displaced gas continues moving to the south-west. The ‘sheath’ is explained as being due to interaction of the re-infalling group with its own tail of stripped gas mixed with the Coma gas. In this scenario, the shock, driven by the group before reaching the apocenter, has already detached from the group and would be located close to the famous relic to the south-west of the Coma cluster.

Article
Lyskova N., Thomas J., Churazov E. et al. Monthly Notices of the Royal Astronomical Society. 2015. Vol. 450. No. 4. P. 3442-3457.

We compare the performance of mass estimators for elliptical galaxies that rely on the directly observable surface brightness and velocity dispersion profiles, without invoking computationally expensive detailed modelling. These methods recover the mass at a specific radius where the mass estimate is expected to be least sensitive to the anisotropy of stellar orbits. One uses the total luminosity-weighted velocity dispersion and evaluates the mass at a 3D half-light radius r1/2, i.e. it depends on the global galaxy properties. Another approach estimates the mass from the velocity dispersion at a radius R2 where the surface brightness declines as R−2, i.e. it depends on the localproperties. We evaluate the accuracy of the two methods for analytical models, simulated galaxies and real elliptical galaxies that have already been modelled by the Schwarzschild's orbit-superposition technique. Both estimators recover an almost unbiased circular speed estimate with a modest rms scatter (≲10 per cent). Tests on analytical models and simulated galaxies indicate that the local estimator has a smaller rms scatter than the global one. We show by examination of simulated galaxies that the projected velocity dispersion at R2 could serve as a good proxy for the virial galaxy mass. For simulated galaxies the total halo mass scales with σp(R2) as

<span data-mathml="hMvir[M⊙h−1]≈6×1012(σp(R2)200kms−1)4M_vir[M_⊙h^{−1}] ≈ 6×10^{12}(σ_p(R_2)/(200 km s^{−1}))^4

with rms scatter ≈40 per cent.

Article
Malanchev K. L., Postnov K. A., Shakura N. I. Monthly Notices of the Royal Astronomical Society. 2017. P. 410-417.

The vertical structure of stationary thin accretion discs is calculated from the energy balance equation with heat generation due to microscopic ion viscosity η and electron heat conductivity κ, both depending on temperature. In the optically thin discs it is found that for the heat conductivity increasing with temperature, the vertical temperature gradient exceeds the adiabatic value at some height, suggesting convective instability in the upper disc layer. There is a critical Prandtl number, Pr = 4/9, above which a Keplerian disc become fully convective. The vertical density distribution of optically thin laminar accretion discs as found from the hydrostatic equilibrium equation cannot be generally described by a polytrope but in the case of constant viscosity and heat conductivity. In the optically thick discs with radiation heat transfer, the vertical disc structure is found to be convectively stable for both absorption-dominated and scattering-dominated opacities, unless a very steep dependence of the viscosity coefficient on temperature is assumed. A polytropic-like structure in this case is found for Thomson scattering-dominated opacity.

Article
Yakovlev D., Kozhberov A. Monthly Notices of the Royal Astronomical Society. 2020. Vol. 498. No. 4. P. 5149-5158.

We study breaking stress of deformed Coulomb crystals in a neutron star crust, taking into account electron plasma screening of ion–ion interaction; calculated breaking stress is fitted as a function of electron screening parameter. We apply the results for analysing torsional oscillation modes in the crust of a non-magnetic star. We present exact analytical expression for the fundamental frequencies of such oscillations and show that the frequencies of all torsional oscillations are insensitive to the presence of the outer neutron star crust. The results can be useful in theoretical modelling of processes involving deformed Coulomb crystals in the crust of neutron stars, such as magnetic field evolution, torsional crustal, or magneto-elastic quasi-periodic oscillations of flaring soft gamma-ray repeaters, pulsar glitches. The applicability of the results to soft gamma-ray repeaters is discussed.

Article
Lipunova G. V., Malanchev K. L. Monthly Notices of the Royal Astronomical Society. 2017. Vol. 468. No. 4. P. 4735-4747.

We investigate the viscous evolution of the accretion disc in 4U 1543-47, a black hole binary system, during the first 30 d after the peak of the 2002 burst by comparing the observed and theoretical accretion rate evolution \dot{M}(t). The observed \dot{M}(t) is obtained from spectral modelling of the archival Proportional Counter Array aboard the RXTE observatory (RXTE/PCA) data. Different scenarios of disc decay evolution are possible depending on a degree of self-irradiation of the disc by the emission from its centre. If the self-irradiation, which is parametrized by factor Cirr, had been as high as ˜5 × 10-3, then the disc would have been completely ionized up to the tidal radius and the short time of the decay would have required the turbulent parameter α ˜ 3. We find that the shape of the \dot{M}(t) curve is much better explained in a model with a shrinking high-viscosity zone. If Cirr ≈ (2-3) × 10-4, the resulting α lie in the interval 0.5-1.5 for the black hole masses in the range 6-10 M⊙, while the radius of the ionized disc is variable and controlled by irradiation. For very weak irradiation, Cirr < 1.5 × 10-4, the burst decline develops as in normal outbursts of dwarf novae with α ˜ 0.08-0.32. The optical data indicate that Cirr in 4U 1543-47 (2002) was not greater than approximately (3-6) × 10-4. Generally, modelling of an X-ray nova burst allows one to estimate α that depends on the black hole parameters. We present the public 1D code freddi to model the viscous evolution of an accretion disc. Analytic approximations are derived to estimate α in X-ray novae using \dot{M}(t).

Article
Petrukhin N., Ruderman M. Monthly Notices of the Royal Astronomical Society. 2021. Vol. 501. P. 3017-3029.

We study kink oscillations of a straight magnetic tube with a transitional region at its boundary. The tube is homogeneous in the axial direction. The plasma density monotonically decreases in the transitional region from its value inside the tube to that in the surrounding plasma. The plasma motion is described by the linear magnetohydrodynamic equations in the cold plasma approximation. We use the ideal equations inside the tube and in the surrounding plasma, but take viscosity into account in the transitional region. We also use the thin tube and thin transitional or boundary layer (TTTB) approximation. Kink oscillations are assumed to be driven by a driver at the tube footpoint. We derive the equation describing the displacement in the fundamental mode and overtones. We use this equation to study kink oscillations in both the case of harmonic and random driving. In the case of random driving, we assume that the driver is described by a stationary random function. The displacements in the fundamental mode and overtones are also described by stationary random functions. We derive the relation between the power spectra of the fundamental mode and all overtones and the power spectrum of the driver. We suggest a new method of obtaining information on the internal structure of coronal magnetic loops based on the shape of graphs of the power spectrum of the fundamental mode