arXiv subjects

Konstantin Postnov

Publications and source records attributed to Konstantin Postnov.

At least 19 recordsLinked to original sources

arXiv2026

Spin-down of the accreting magnetar candidate 4U 0114+65: possible first evidence for a strong coupling regime

4U~0114+65 is a high-mass X-ray binary composed of the B1\,Ia supergiant V*~V662~Cas and one of the slowest known accreting neutron stars, with a spin period of $\sim$9.4 ks. In 2025, its long X-ray pulsations became undetectable in \textit{Swift}/BAT monitoring, motivating a Director's Discretionary Time observation with \textit{XMM-Newton}. We compare this observation with a 2015 \textit{XMM-Newton} observation, when the source was brighter and clearly pulsed, and analyze the long-term spin evolution using \textit{Swift}/BAT data. We performed average and pulse-phase-resolved spectroscopy using the same model as in previous work. The 2025 observation still reveals weak pulsations, with a period of about 9.3 ks, despite their non-detection in \textit{Swift}/BAT. The overall spectral shape remains similar in both epochs, but the luminosity decreased by about one order of magnitude, mainly due to strong suppression of the bulk-motion Comptonization component. Although the absorbing column is higher in 2025, the inferred wind properties remain broadly compatible with those from 2015, suggesting that no major global change in the donor wind is required. Instead, the results point to a substantial reduction in accretion efficiency close to the neutron-star magnetosphere. We propose that 4U~0114+65 may be evolving toward partial centrifugal inhibition in the strong-coupling regime, where the toroidal magnetic-field component is comparable to the poloidal one. If confirmed, this would represent the first observational evidence of this state. Accretion would become progressively less efficient and more intermittent without reaching a fully developed propeller regime. The apparent disappearance of the pulse in long-term hard X-ray monitoring would then result from the lower luminosity and reduced absolute pulsed flux, rather than from the loss of the underlying spin modulation.

astro-ph.HE↗

arXiv2025

Double white dwarf ZTF J0538+1953 as the brightest verification binary for space laser interferometers

A decrease in the orbital period of the ultrashort-period binary white dwarf \ZTF, which is one of the Galactic verification binaries in the millihertz frequency range for planned space laser interferometers, has been measured. Based on photometric observations carried out on the 2.5-m telescope of the Caucasian Mountain Observatory of the Sternberg Astronomical Institute of Moscow State University (CMO SAI MSU), a diagram \textit{O-C} is constructed. It can be described by quadratic elements of the brightness variation, which correspond to a decrease rate of the orbital period of the system of $dP/dt=-(1.16\pm 0.22)\times 10^{-11}$ s/s. The decrease rate of the orbital period in the quadrupole approximation for the emission of gravitational waves by a binary system corresponds to its chirp mass $\mathcal{M}=0.434\pm 0.05 M_\odot$, which turned out to be $\sim 30\%$ higher than the value obtained earlier from spectroscopic mass determination. The chirp mass of \ZTF inferred from the measured orbital decay rate makes this system the brightest Galactic verification binary for LISA and TianQin space interferometers with a signal-to-noise ratio of $\approx 119$ and $\approx 30$ over 5 years and 2.5 years of observations, respectively.

astro-ph.HE↗

arXiv2025

Empirical correlation between masses of black holes and Wolf-Rayet stars derived from their mass distributions in spectroscopic binaries

We present observationally determined mass distributions of Wolf-Rayet (WR) stars in WR+OB binaries and black holes (BH) in spectroscopic binaries. Both WR and BH mass probability distributions can be well approximated by unbiased log-normal functions. Assuming that all WR stars with $M_\mathrm{WR}\gtrsim 6 M_\odot$ after core collapse are progenitors of the BHs, the similar shape of their mass distributions before and after collapse suggests a power-law relation between them $M_{\mathrm{BH}} \simeq (0.39\pm0.09) {M_{\mathrm{WR}}}^{1.13\pm0.09}$. Using the relation between masses of a WR star and its CO-core, we obtain the empirical relation between the BH mass and CO-core of the collapsing WR star $M_\mathrm{BH}\sim 0.9 M_\mathrm{CO}$, which can be used in the population synthesis calculations.

astro-ph.HE↗

arXiv2025

Fundamental Physics and Cosmology with TianQin

The exploration of the surrounding world and the universe is an important theme in the legacy of humankind. The detection of gravitational waves is adding a new dimension to this grand effort. What are the fundamental physical laws governing the dynamics of the universe? What is the fundamental composition of the universe? How has the universe evolved in the past and how will it evolve in the future? These are the basic questions that press for answers. The space-based gravitational wave detector TianQin will tune in to gravitational waves in the millihertz frequency range ($10^{-4} \sim 1$ Hz, to be specific), opening a new gravitational wave spectrum window to explore many of the previously hidden sectors of the universe. TianQin will discover many astrophysical systems, populating the universe at different redshifts: some will be of new types that have never been detected before, some will have very high signal-to-noise ratios, and some will have very high parameter estimation precision. The plethora of information collected will bring us to new fronts on which to search for the breaking points of general relativity, the possible violation of established physical laws, the signature of possible new gravitational physics and new fundamental fields, and to improve our knowledge on the expansion history of the universe. In this white paper, we highlight the advances that TianQin can bring to fundamental physics and cosmology.

gr-qc↗

arXiv2025

Progress of the TianQin project

TianQin is a future space-based gravitational wave observatory targeting the frequency window of $10^{-4}$ Hz $\sim 1$ Hz. A large variety of gravitational wave sources are expected in this frequency band, including the merger of massive black hole binaries, the inspiral of extreme/intermediate mass ratio systems, stellar-mass black hole binaries, Galactic compact binaries, and so on. TianQin will consist of three Earth orbiting satellites on nearly identical orbits with orbital radii of about $10^5$ km. The satellites will form a normal triangle constellation whose plane is nearly perpendicular to the ecliptic plane. The TianQin project has been progressing smoothly following the ``0123" technology roadmap. In step ``0", the TianQin laser ranging station has been constructed and it has successfully ranged to all the five retro-reflectors on the Moon. In step ``1", the drag-free control technology has been tested and demonstrated using the TianQin-1 satellite. In step ``2", the inter-satellite laser interferometry technology will be tested using the pair of TianQin-2 satellites. The TianQin-2 mission has been officially approved and the satellites will be launched around 2026. In step ``3", i.e., the TianQin-3 mission, three identical satellites will be launched around 2035 to form the space-based gravitational wave detector, TianQin, and to start gravitational wave detection in space.

gr-qc↗

arXiv2025

Cyclical accretion regime change in the slow X-ray pulsar 4U 0114+65 observed with Chandra

4U 0114+65 is a high-mass X-ray binary system formed by the luminous supergiant B1Ia, known as V{*} V662 Cas, and one of the slowest rotating neutron stars (NS) with a spin period of about 2.6 hours. This fact provides a rare opportunity to study interesting details of the accretion within each individual pulse of the compact object. In this paper, we analyze 200 ks of Chandra grating data, divided into 9 uninterrupted observations around the orbit. The changes in the circumstellar absorption column through the orbit suggest an orbital inclination of $\sim$ $40^{\circ}$ with respect to the observer and a companion mass-loss rate of $\sim$ 8.6 10$^{-7}$ solar masses yr$^{-1}$. The peaks of the NS pulse show a large pulse-to-pulse variability. Three of them show an evolution from a brighter regime to a weaker one. We propose that the efficiency of Compton cooling in this source fluctuates throughout an accumulation cycle. After significant depletion of matter within the magnetosphere, since the settling velocity is $\sim \times$ 2 times lower than the free-fall velocity, the source gradually accumulates matter until the density exceeds a critical threshold. This increase in density triggers a transition to a more efficient Compton cooling regime, leading to a higher mass accretion rate and consequently to an increased brightness.

astro-ph.HE↗

arXiv2024

Stripping of a neutron star in a close binary system in a pair with a black hole

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.

astro-ph.HE↗

arXiv2024

Revealing the state transition of Cen X-3 at high spectral resolution with Chandra

Cen X-3 is a compact, high-mass X-ray binary (HMXRB), likely powered by Roche lobe overflow. We present a phase-resolved X-ray spectral and timing analysis of a target of opportunity \textit{Chandra} observation made during a low-flux to high-flux transition. The high-resolution spectra allow us to delve into the events that occurred during this episode. The spectrum is described by a single black body absorbed by a local column density of the order of $10^{23-24}$ cm$^{-2}$, which is one to two orders of magnitude higher than found for previous analyses of data taken at similar orbital phases. Such a large column produces a Compton shoulder in the Fe K$\alpha$ line. The transition appears to be caused by the onset of efficient cooling, which cools the plasma by 10 million degrees in just 10 ks, allowing matter to enter the magnetosphere. This happens after a major disturbance, probably the arrival of a train of wind clumps with individual masses in the range $10^{19-20}$ g. This train moves ballistically in an eccentric orbit around the NS, producing a distinctive Doppler modulation in the \ion{Fe}{xxv} line.

astro-ph.HE↗

arXiv2024

Primordial Intermediate-mass Binary Black Holes as Targets for Space Laser Interferometers

Primordial black holes (PBHs) with log-normal mass spectrum with masses up to $\sim 10^4-10^5 M_\odot$ can be created after QCD phase transition in the early Universe at $z\sim 10^{12}$ by the modified Affleck-Dine baryogenesis. Using a model binary PBH formation, the expected detection rate of such binary intermediate-mass PBHs by the TianQin space laser interferometer is calculated to be from a few to hundreds events per year for the assumed parameters of the PBH log-normal mass spectrum and abundance consistent with LIGO-Virgo-KAGRA results. Distinctive features of such primordial IMBH mergings are vanishingly small effective spins, possible high redshifts $z>20$ and lack of association with gas-rich regions or galaxies.

astro-ph.CO↗

arXiv2023

On the appearance of non-local MRI in Keplerian accretion discs

We revisit the modal analysis of small perturbations in Keplerian ideal gas flows leading to magneto-rotational instability (MRI) using the non-local approach. We consider the case of constant vertical background magnetic field, as well as the case of radially dependent background Alfvén velocity. In the case of constant Alfvén velocity, MRI modes are described by a Schrödinger-like differential equation with some effective potential including 'repulsive' ($1/r^2$) and 'attractive' ($-1/r^3$) terms. Taking into account the radial dependence of the background Alfvén speed leads to a qualitative change in the shape of the effective potential. It is shown that there are no stationary energy levels corresponding to unstable modes $ω^2 < 0$ in ``shallow'' potentials. In thin accretion disks, the wavelength of the disturbance $λ=2π/k_z$ is smaller than the half-thickness $h$ of the disk only in ``deep'' potentials. The limiting value of the background Alfvén speed $(c_A)_\mathrm{cr}$, above which the magnetorotational instability does not occur, is found. In thin accretion disks with low background Alfvén speed $c_A\ll (c_A)_\mathrm{cr}$, the increment of the magnetorotational instability $ω\approx -\sqrt{3}\mathrm{i}c_Ak_z$ is suppressed compared to the value obtained in the local perturbation analysis.

astro-ph.HE↗

arXiv2023

Astrophysical appearances of primordial black holes

Interest to astrophysical evidence for primordial black holes (PBHs) formed in the early Universe from initial cosmological perturbations has increased after the discovery of coalescing binary black holes with masses more than dozen solar ones by gravitational-wave (GW) observatories. We briefly discuss increasing evidence that PBHs can provide some fraction of detected merging binary BHs and can be related to an isotropic stochastic GW background recently discovered by pulsar timing arrays. We focus on PBHs with log-normal mass spectrum originated from isocurvature perturbations in the modified Affleck-Dine baryogenesis scenario by Dolgov and Silk (1993). We show that almost equal populations of astrophysical binary BHs from massive binary evolution and binary PBHs with log-normal mass spectrum can describe both the observed chirp mass distribution and effective spin -- mass ratio anti-correlation of the LVK binary BHs.

astro-ph.HE↗

arXiv2023

Antistars as possible sources of antihelium cosmic rays

A minor population of antistars in galaxies has been predicted by some of non-standard models of baryogenesis and nucleosynthesis in the early Universe, and their presence is not yet excluded by the currently available observations. Detection of an unusually high abundance of antinuclei in cosmic rays can probe the baryogenesis scenarios in the early Universe. Recent report of the \textit{AMS-02} collaboration on the tentative detection of a few antihelium nuclei in GeV cosmic rays provided a great hope on the progress in this issue. We discuss possible sources of antinuclei in cosmic rays from antistars which are predicted in a modified Affleck-Dine baryogenesis scenario by Dolgov and Silk (1993). The model allows us to estimate the expected fluxes and isotopic content of antinuclei in the GeV cosmic rays produced in scenarios involving antistars. We show that the flux of antihelium CRs reported by the \textit{AMS-02} experiment can be explained by Galactic anti-nova outbursts, thermonuclear anti-SN Ia explosions, a collection of flaring antistars or an extragalactic source with abundances not violating existing gamma-ray and microlensing constraints on the antistar population.

astro-ph.HE↗

arXiv2023

Evolutionary Increase of the orbital Separation and Change of the Roche Lobe Size in SS433

We present results of long-term photometric monitoring of SS433 which proves a secular evolutionary increase of the orbital period of SS433 at a rate of $(1.14\pm 0.25)\times 10^{-7}$ s~s$^{-1}$. Using a physical model of non-conservative mass transfer in SS433 through a supercritical accretion disc around the compact companion, we reliably confirm that the binary mass ratio in SS433, $q=M_x/M_v$ is $\gtrsim 0.8$. For an optical star mass $M_v\sim 10 M_\odot$ the compact object in SS433 is a black hole with mass $M_{BH}\gtrsim 8 M_\odot$. We discuss evolutionary implications of the found orbital period increase in SS433 -- a secular change in the orbital separation and a size of the Roche lobe of the optical star. We show that for the mass-loss rate $dM_v/dt\sim 10^{-4}-3\times 10^{-5} M_\odot$ per year and an optical star mass $M_v \sim 10-15 M_\odot$ the found orbital period increase implies the corresponding orbital separation increase while the Roche lobe size can shrink or expand around a mean constant value depending on the optical star mass-loss rate which may be modulated with the precessional period.

astro-ph.HE↗

arXiv2023

On the primordial binary black hole mergings in LIGO-Virgo-Kagra data

We briefly discuss a possible cosmological implication of the observed binary black hole mergings detected by LIGO-Virgo-Kagra collaboration (GWTC-3 catalogue) for the primordial black hole (PBH) formation in the early Universe. We show that the bumpy chirp mass distribution of the LVK BH+BH binaries can be fit with two distinct and almost equal populations: (1) astrophysical mergings from BH+BH formed in the modern Universe from evolution of massive binaries and (2) mergings of binary PBHs with initial log-normal mass distribution. We find that the PBH central mass ($M_c\simeq 30 M_\odot$) and distribution width derived from the observed LVK chirp masses are almost insensitive to the assumed double PBH formation model. To comply with the observed LVK BH+BH merging rate, the CDM PBH mass fraction should be $f_{pbh}\sim 10^{-3}$ but can be higher if PBH clustering is taken into account.

astro-ph.CO↗

arXiv2022

Gravitational wave astronomy: astrophysical and cosmological inferences

We briefly discuss the most prominent results and specific sources detected by gravitational-wave observatories LIGO-Virgo during first three O1-O3 runs, as well as possible astrophysical and cosmological channels of their formation. We show that it is possible to explain the observed correlation between the effective spin of coalescing binary black holes and mass ratio of the components by accretion from the ambient medium onto primordial binary black holes. We also briefly discuss the recent results of searches for stochastic gravitational-wave background in the nano-Hz frequency band by pulsar timing arrays.

gr-qc↗

arXiv2022

Evidence for neutron star triaxial free precession in Her X-1 from Fermi/GBM pulse period measurements

Her X-1/HZ Her is one of the best studied accreting X-ray pulsars. In addition to the pulsating and orbital periods, the X-ray and optical light curves of the source exhibit an almost periodic 35-day variability caused by a precessing accretion disk. The nature of the observed long-term stability of the 35-day cycle has been debatable. The X-ray pulse frequency of Her X-1 measured by the Fermi/GBM demonstrates periodical variations with X-ray flux at the Main-on state of the source. We explain the observed periodic sub-microsecond pulse frequency changes by the free precession of a triaxial neutron star with parameters previously inferred from an independent analysis of the X-ray pulse evolution over the 35-day cycle. In the Fermi/GBM data, we identified several time intervals with a duration of half a year or longer where the neutron star precession period describing the pulse frequency variations does not change. We found that the NS precession period varies within one per cent in different intervals. Such variations in the free precession period on a year time scale can be explained by <1% changes in the fractional difference between the triaxial neutron star's moments of inertia due to the accreted mass readjustment or variable internal coupling of the neutron star crust with the core.

astro-ph.HE↗

arXiv2021

Coalescing primordial binary black holes with log-normal mass spectrum

Primordial black holes created in the early Universe can constitute a substantial fraction of dark matter and serve as seeds for early galaxy formation. Binary primordial black holes with masses of the order of a few dozen solar masses can explain the observed LIGO/Virgo gravitational-wave events. In this Letter, we show that primordial black holes with log-normal mass spectrum centered at $M_0\simeq 15-17 M_\odot$ simultaneously explain both the chirp mass distribution of the detected LIGO/Virgo binary black holes and the differential chirp mass distribution of merging binaries as inferred from the LIGO/Virgo observations. The obtained parameters of log-normal mass spectrum of primordial black holes also give the fraction of seeds with $M\gtrsim 10^4 M_\odot$ required to explain the observed population of supermassive black holes at $z=6-7$.

astro-ph.HE↗

arXiv2020

Caucasian mountain observatory of Sternberg astronomical institute: six years of operation

The new SAI MSU observatory 2.5-meter telescope and capabilities of its current instrumentation are described. The facility operates actively since 2014 in parallel to the engineering works. It has delivered a number of prominent results in the field of optical and near-infrared photometry and spectroscopy as well as newly developed observational techniques like differential speckle polarimetry.

astro-ph.IM↗