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Konstantin Postnov

Publications and source records attributed to Konstantin Postnov.

At least 37 records · Page 2Linked to original sources
arXiv2020

X-ray variability of the HMXB Cen X-3: evidence for inhomogeneous accretion flows

Cen X-3 is a compact high mass X-ray binary likely powered by Roche lobe overflow. We present a phase-resolved X-ray spectral and timing analysis of two pointed XMM-Newton observations. The first one took place during a normal state of the source, when it has a luminosity $L_{\rm X}\sim 10^{36}$ erg s$^{-1}$. This observation covered orbital phases $ϕ= 0.00-0.37$, i.e. the egress from the eclipse. The egress lightcurve is highly structured, showing distinctive intervals. We argue that different intervals correspond to the emergence of different emitting structures. The lightcurve analysis enables us to estimate the size of such structures around the compact star, the most conspicuous of which has a size $\sim 0.3R_{*}$, of the order of the Roche lobe radius. During the egress, the equivalent width of Fe emission lines, from highly ionized species, decreases as the X-ray continuum grows. On the other hand, the equivalent width of the Fe K$α$ line, from near neutral Fe, strengthens. This line is likely formed due to the X-ray illumination of the accretion stream. The second observation was taken when the source was 10 times X-ray brighter and covered the orbital phases $ϕ= 0.36-0.80$. The X-ray lightcurve in the high state shows dips. These dips are not caused by absorption but can be due to instabilities in the accretion stream. The typical dip duration, of about 1000~s, is much longer than the timescale attributed to the accretion of the clumpy stellar wind of the massive donor star, but is similar to the viscous timescale at the inner radius of the accretion disk.

astro-ph.HE↗
arXiv2020

Advances in Understanding High-Mass X-ray Binaries with INTEGRAL and Future Directions

High mass X-ray binaries are among the brightest X-ray sources in the Milky Way, as well as in nearby Galaxies. Thanks to their highly variable emissions and complex phenomenology, they have attracted the interest of the high energy astrophysical community since the dawn of X-ray Astronomy. In more recent years, they have challenged our comprehension of physical processes in many more energy bands, ranging from the infrared to very high energies. In this review, we provide a broad but concise summary of the physical processes dominating the emission from high mass X-ray binaries across virtually the whole electromagnetic spectrum. These comprise the interaction of stellar winds with the high gravitational and magnetic fields of compact objects, the behaviour of matter under extreme magnetic and gravity conditions, and the perturbation of the massive star evolutionary processes by presence in a binary system. We highlight the role of the INTEGRAL mission in the discovery of many of the most interesting objects in the high mass X-ray binary class and its contribution in reviving the interest for these sources over the past two decades. We show how the INTEGRAL discoveries have not only contributed to significantly increase the number of high mass X-ray binaries known, thus advancing our understanding of the population as a whole, but also have opened new windows of investigation that stimulated the multi-wavelength approach nowadays common in most astrophysical research fields. We conclude the review by providing an overview of future facilities being planned from the X-ray to the very high energy domain that will hopefully help us in finding an answer to the many questions left open after more than 18 years of INTEGRAL scientific observations.

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arXiv2019

Parkes Pulsar Timing Array constraints on ultralight scalar-field dark matter

It is widely accepted that dark matter contributes about a quarter of the critical mass-energy density in our Universe. The nature of dark matter is currently unknown, with the mass of possible constituents spanning nearly one hundred orders of magnitude. The ultralight scalar field dark matter, consisting of extremely light bosons with $m \sim 10^{-22}$ eV and often called "fuzzy" dark matter, provides intriguing solutions to some challenges at sub-Galactic scales for the standard cold dark matter model. As shown by Khmelnitsky and Rubakov, such a scalar field in the Galaxy would produce an oscillating gravitational potential with nanohertz frequencies, resulting in periodic variations in the times of arrival of radio pulses from pulsars. The Parkes Pulsar Timing Array (PPTA) has been monitoring 20 millisecond pulsars at two to three weeks intervals for more than a decade. In addition to the detection of nanohertz gravitational waves, PPTA offers the opportunity for direct searches for fuzzy dark matter in an astrophysically feasible range of masses. We analyze the latest PPTA data set which includes timing observations for 26 pulsars made between 2004 and 2016. We perform a search in this data set for evidence of ultralight dark matter in the Galaxy using Bayesian and Frequentist methods. No statistically significant detection has been made. We therefore place upper limits on the local dark matter density. Our limits, improving on previous searches by a factor of two to five, constrain the dark matter density of ultralight bosons with $m \leq 10^{-23}$ eV to be below $6\,\text{GeV}\,\text{cm}^{-3}$ with 95\% confidence in the Earth neighborhood. Finally, we discuss the prospect of probing the astrophysically favored mass range $m \gtrsim 10^{-22}$ eV with next-generation pulsar timing facilities.

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arXiv2019

Spins of primordial binary black holes before coalescence

Primordial stellar-mass black holes, which may contribute to dark matter and to the observed LIGO binary black hole coalescences, are expected to be born with very low spins. Here we show that accretion mass gain by the components of a primordial black hole binary from the surrounding matter could lead to noticeable spins of the components prior to the coalescence provided high initial orbital eccentricities.

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arXiv2019

SS433: a massive X-ray binary at advanced evolutionary stage

INTEGRAL IBIS/ISGRI 18-60 keV observations of SS433 performed in 2003-2011 enabled the hard X-ray phase-resolved orbital and precessional light curves and spectra to be constructed. The spectra can be fitted by a power-law with photon index $\simeq 3.8$ and remain almost constant while the X-ray flux varies by a factor of a few. This suggests that the hard X-ray emission is produced in an extended quasi-isothermal hot 'corona' surrounding central parts of a supercritical accretion disc. A joint analysis of the broadband 18-60 keV orbital and precessional light curves was performed in the model assuming a significant Roche lobe overfilling by the optical star, up to its filling the outer Lagrangian surface enabling mass loss through the outer Lagrangian L$_2$ point. From this modeling, the relativistic-to-optical component mass ratio $q=M_x/M_v\gtrsim0.4÷0.8$ is estimated. An analysis of the observed long-term stability of the orbital period of SS433 with an account of the recent observations of SS433 by the VLTI GRAVITY interferometer enabled an independent mass ratio estimate $q>0.6$. This estimate in combination with the radial velocity semi-amplitude for stationary He II emission, $K_x=168\pm 18$ km/s (Hillwig et al 2004) suggests the optical component mass in SS433 $M_v>12 M_\odot$. Thus, the mass of the relativistic component in SS433 is $M_x>7 M_\odot$, which is close to the mean mass of black holes in X-ray binaries ($\sim 8 M_\odot$). The large binary mass ratio in SS433 allows us to understand why there is no common envelope in this binary at the secondary mass transfer evolutionary stage and the system remains semi-detached (van den Heuvel et al. 2017). We also discuss unsolved issues and outline prospects for further study of SS433.

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arXiv2019

Wind-accreting Symbiotic X-ray Binaries

We present a new model of the population of symbiotic X-ray binaries (SyXBs) that takes into account non-stationary character of quasi-spherical sub-sonic accretion of the red giant's stellar wind onto slowly rotating neutron stars. Updates of the earlier models are given, which include more strict criteria of slow NS rotation for plasma entry into the NS magnetosphere via Rayleigh-Taylor instability, as well as more strict conditions for settling accretion for slow stellar winds, with an account of variations in the specific angular momentum of captured stellar wind in eccentric binaries. These modifications enabled a more adequate description of the distributions of observed systems over binary orbital periods, NS spin periods and their X-ray luminosity in the $\sim 10^{32}-10^{36}$~erg s$^{-1}$ range and brought their model Galactic number into reasonable agreement with the observed one. Reconciliation of the model and observed orbital periods of SyXBs requires a low efficiency of matter expulsion from common envelopes during the evolution that results in the formation of NS-components of symbiotic X-ray systems.

astro-ph.HE↗
arXiv2018

Black hole spins in coalescing binary black holes

The possible formation mechanisms of massive close binary black holes (BHs) that can merge in the Hubble time to produce powerful gravitational wave bursts detected during advanced LIGO O1 and O2 science runs include the evolution from field low-metallicity massive binaries, the dynamical formation in dense stellar clusters and primordial BHs. Different formation channels produce different source distributions of total masses ${M_\mathrm{tot}}$ and effective spins $χ_\mathrm{eff}$ of coalescing binary BHs. Using a modified \textsc{bse} code, we carry out extensive population synthesis calculations of the expected effective spin and total mass distributions from the standard field massive binary formation channel for different metallicities of BH progenitors (from zero-metal Population III stars up to solar metal abundance), different initial rotations of the binary components, stellar wind mass loss prescription, different BH formation models and a range of common envelope efficiencies. The stellar rotation is treated in two-zone (core-envelope) approximation using the effective core-envelope coupling time and with an account of the tidal synchronization of stellar envelope rotation during the binary system evolution. The results of our simulations, convolved with the metallicity-dependent star-formation history, show that the total masses and effective spins of the merging binary black holes detected during LIGO O1-O2 runs but the heaviest one (GW170729) can be simultaneously reproduced by the adopted BH formation models. Noticeable effective spin of GW170729 requires additional fallback from the rotating stellar envelope.

astro-ph.HE↗
arXiv2018

X-ray binaries with neutron stars at different accretion stages

Different accretion regimes onto magnetized NSs in HMXBs are considered: wind-fed supersonic (Bondi) regime at high accretion rates $\dot M\gtrsim 4\times 10^{16}$~[g s$^{-1}$] , subsonic settling regime at lower $\dot M$ and supercritical disc accretion during Roche lobe overflow. In wind-fed stage, NSs in HMXBs reach equilibrium spin periods $P^*$ proportional to binary orbital period $P_b$. At supercritical accretion stage, the system may appear as a pulsating ULX. Population synthesis of Galactic HMXBs using standard assumptions on the binary evolution and NS formation is presented. Comparison of the model $P^*$ -- $P_b$ (the Corbet diagram), $P^*$ -- $L_x$ and $P_b$ -- $L_x$ distributions with those for the observed HMXBs (including Be X-ray binaries) and pulsating ULXs suggests the importance of the reduction of $P^*$ in non-circular orbits, explaining the location of Be X-ray binaries in the model Corbet diagram, and the universal parameters of pulsating ULXs depending only on the NS magnetic fields.

astro-ph.HE↗
arXiv2017

Electromagnetic Radiation Accompanying Gravitational Waves from Black Hole Binaries

The transformation of powerful gravitational waves, created by the coalescence of massive black hole binaries, into electromagnetic radiation in external magnetic fields is revisited. In contrast to the previous calculations of the similar effect, we study the realistic case of the gravitational radiation frequency below the plasma frequency of the surrounding medium. The gravitational waves propagating in the plasma constantly create electromagnetic radiation dragging it with them, despite the low frequency. The plasma heating by the unattenuated electromagnetic wave may be significant in a hot rarefied plasma with strong magnetic field and can lead to a noticeable burst of electromagnetic radiation with higher frequency. The graviton-to-photon conversion effect in plasma is discussed in the context of possible electromagnetic counterparts of GW150914 and GW170104.

astro-ph.HE↗
arXiv2017

XMM-Newton spectroscopy of the accreting magnetar candidate 4U0114+65

4U0114$+$65 is one of the slowest known X-ray pulsars. We present an analysis of a pointed observation by the XMM-Newton X-ray telescope. The energy-resolved light curve, the time-resolved X-ray spectra provided by the EPIC cameras on board and the first high-resolution spectrum of this source, provided by the Reflection Grating Spectrometer, were analysed. An X-ray pulse of $9350\pm 160$ s was measured, confirming the secular spin up of this source. We also fit the pulse-phase-resolved spectra with Comptonisation models. These models imply a very small ($r\sim 3$ km) and hot ($kT\sim 2-3$ keV) emitting region and therefore point to a hot spot over the neutron star (NS) surface as the most reliable explanation for the X-ray pulse. The long NS spin period, the spin-up rate, and persistent X-ray emission can be explained within the theory of quasi-spherical settling accretion, which may indicate that the magnetic field is in the magnetar range. Thus, 4U 0114$+$65 could be a wind-accreting magnetar. We also observed two episodes of low luminosity. The first can be explained as an absorption by an over-dense structure in the wind of the donor. The second episode may be due to temporal cessation of accretion onto one magnetic pole caused by non-spherical matter capture from the structured stellar wind. The light curve displays two types of dips seen during the high-flux intervals. The short dips, with durations of tens of seconds, are produced through absorption by wind clumps. The long dips might be associated with the rarefied interclump medium. We also found evidence of emission lines in the X-ray photoionised wind of the B1Ia donor. The Fe K$α$ line is highly variable and much weaker than in other X-ray binaries with supergiant donors. The degree of wind clumping, measured through the covering fraction, is much lower than in supergiant donor stars with earlier spectral types.

astro-ph.HE↗
arXiv2017

Progenitors of binary black hole mergers detected by LIGO

Possible formation mechanisms of massive close binary black holes that can merge in the Hubble time to produce powerful gravitational wave bursts detected during advanced LIGO O1 science run are briefly discussed. The pathways include the evolution from field low-metallicity massive binaries, the dynamical formation in globular clusters and primordial black holes. Low effective black hole spins inferred for LIGO GW150914 and LTV151012 events are discussed. Population synthesis calculations of the expected spin and chirp mass distributions from the standard field massive binary formation channel are presented for different metallicities (from zero-metal Population III stars up to solar metal abundance). We conclude that that merging binary black holes can contain systems from different formation channels, discrimination between which can be made with increasing statistics of mass and spin measurements from ongoing and future gravitational wave observations.

astro-ph.HE↗
arXiv2017

Low-luminosity stellar wind accretion onto neutron stars in HMXBs

Features and applications of quasi-spherical settling accretion onto rotating magnetized neutron stars in high-mass X-ray binaries are discussed. The settling accretion occurs in wind-fed HMXBs when the plasma cooling time is longer than the free-fall time from the gravitational capture radius, which can take place in low-luminosity HMXBs with $L_x\lesssim 4\times 10^{36}$ erg/s. We briefly review the implications of the settling accretion, focusing on the SFXT phenomenon, which can be related to instability of the quasi-spherical convective shell above the neutron star magnetosphere due to magnetic reconnection from fast temporarily magnetized winds from OB-supergiant. If a young neutron star in a wind-fed HMXB is rapidly rotating, the propeller regime in a quasi-spherical hot shell occurs. We show that X-ray spectral and temporal properties of enigmatic $γ$ Cas Be-stars are consistent with failed settling accretion regime onto a propelling neutron star. The subsequent evolutionary stage of $γ$ Cas and its analogs should be the X Per-type binaries comprising low-luminosity slowly rotating X-ray pulsars.

astro-ph.HE↗
arXiv2016

Discovery and Modeling of a Flattening of the Positive Cyclotron Line/Luminosity Relation in GX 304-1 with RXTE

The RXTE observed four outbursts of the accreting X-ray binary transient source, GX 304-1 in 2010 and 2011. We present results of detailed 3-100 keV spectral analysis of 69 separate observations, and report a positive correlation between cyclotron line parameters, as well as other spectral parameters, with power law flux. The cyclotron line energy, width and depth versus flux, and thus luminosity, correlations show a flattening of the relationships with increasing luminosity, which are well described by quasi-spherical or disk accretion that yield the surface magnetic field to be ~60 keV. Since HEXTE cluster A was fixed aligned with the PCA field of view and cluster B was fixed viewing a background region 1.5 degrees off of the source direction during these observations near the end of the RXTE mission, the cluster A background was estimated from cluster B events using HEXTEBACKEST. This made possible the detection of the ~55 keV cyclotron line and an accurate measurement of the continuum. Correlations of all spectral parameters with the primary 2-10 keV power law flux reveal it to be the primary driver of the spectral shape. The accretion is found to be in the collisionless shock braking regime.

astro-ph.HE↗
arXiv2016

A viscous-convective instability in laminar Keplerian thin discs

Using the anelastic approximation of linearized hydrodynamic equations, we investigate the development of axially symmetric small perturbations in thin Keplerian discs. Dispersion relation is found as a solution of general Sturm-Liouville eigenvalue problem for different values of relevant physical parameters (viscosity, heat conductivity, disc semithickness). The analysis reveals the appearance of overstable mode for Prandtl parameter higher than some critical value. These modes have a viscous-convective nature and can serve as a seed for turbulence in astrophysical discs even in the absence of magnetic fields.

astro-ph.HE↗
arXiv2015

A viscous instability in axially symmetric laminar shear flows

A viscous instability in shearing laminar axisymmetric hydrodynamic flows around a gravitating center is described. In the linearized hydrodynamic equations written in the Boussinesq approximation with microscopic molecular transport coefficients, the instability arises when the viscous dissipation is taken into account in the energy equation. Using the local WKB approximation, we derive a third-order algebraic dispersion equation with two modes representing the modified Rayleigh modes R+ and R-, and the third X-mode. We show that in thin accretion flows the viscosity destabilizes one of the Rayleigh modes in a wide range of wavenumbers, while the X-mode always remains stable. In Keplerian flows, the instability increment is found to be a few Keplerian rotational periods at wavelengths with $kr\sim 10-50$. This instability may cause turbulence in astrophysical accretion discs even in the absence of magnetic field.

astro-ph.HE↗
arXiv2014

The Evolution of Compact Binary Star Systems

We review the formation and evolution of compact binary stars consisting of white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Mergings of compact binary stars are expected to be the most important sources for the forthcoming gravitational-wave (GW) astronomy. In the first part of the review, we discuss observational manifestations of close binary stars with NS and/or black components and their merger rate, crucial points in the formation and evolution of compact stars in binary systems, including the treatment of the natal kicks which NSs and BHs acquire during the core collapse of massive stars and the common envelope phase of binary evolution, which are most relevant to the merging rates of NS-NS, NS-BH and BH-BH binaries. The second part of the review is devoted mainly to formation and evolution of binary WDs and their observational manifestations, including their role as progenitors of cosmologically important thermonuclear SN Ia. We also consider AM CVn-stars which are thought to be the best verification binary GW sources for future low-frequency GW space interferometers.

astro-ph.HE↗
arXiv2007

The Evolution of Compact Binary Star Systems

We review the formation and evolution of compact binary stars consisting of white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and BHs are thought to be the primary astrophysical sources of gravitational waves (GWs) within the frequency band of ground-based detectors, while compact binaries of WDs are important sources of GWs at lower frequencies to be covered by space interferometers (LISA). Major uncertainties in the current understanding of properties of NSs and BHs most relevant to the GW studies are discussed, including the treatment of the natal kicks which compact stellar remnants acquire during the core collapse of massive stars and the common envelope phase of binary evolution. We discuss the coalescence rates of binary NSs and BHs and prospects for their detections, the formation and evolution of binary WDs and their observational manifestations. Special attention is given to AM CVn-stars -- compact binaries in which the Roche lobe is filled by another WD or a low-mass partially degenerate helium-star, as these stars are thought to be the best LISA verification binary GW sources.

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