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Sergei B. Popov

Publications and source records attributed to Sergei B. Popov.

At least 19 recordsLinked to original sources
arXiv2026

Dependences of radio pulsar parameters on the kick velocity

According to several studies, analysis of observational data and theoretical modeling favor a bimodal distribution of the natal velocity kick of neutron stars. We analyze this proposal by using available data on radio pulsars. For $\sim200$ normal isolated radio pulsars with well-measured spin and kinematic parameters, we determine if they belong to the low- or high-velocity mode of such a distribution by applying the parametrization proposed by Igoshev (2020). Our results demonstrate that about $23\%$ belong to the low-velocity mode. We then analyze the differences in the properties of the two sets of pulsars belonging to the two modes. For some parameters (characteristic ages and distances), we see a clear difference between the two modes. However, for these quantities, it can be attributed to selection bias. For those parameters that are not subject to strong selection, such as pulse width, we do not observe any difference. Interestingly, we detect a notable difference in the magnetic field distribution between the two modes. Lower-field pulsars ($B\lesssim 10^{12}$~G) are overabundant among objects from the low-velocity mode. Among pulsars with low fields ($\lesssim 10^{11}$~G), we do not identify any objects from the high-velocity mode of the kick distribution. The origin of this discrepancy is not clear, and we discuss several possibilities. Our analysis demonstrates that, most probably, this feature can be explained by selection effects. Thus, we conclude that there is no robust bimodality in physical parameters of radio pulsars that can be related to the proposed bimodality of the kick velocity. This can be considered as an indirect argument against the hypothetical bimodality in the NS kick distribution. (abridged)

astro-ph.HE↗
arXiv2026

Transient discs around isolated accreting neutron stars

Mature, isolated neutron stars can accrete from the interstellar medium. Due to turbulence, the accreted gas can have a substantial angular momentum and form a disc around the compact object. In this paper, we perform a population synthesis of isolated neutron stars in the Milky Way that specifically tracks the possibility of disc formation, which typically requires a low spatial velocity of the compact object ($\lesssim 40$ km s$^{-1}$). In general, weak magnetic fields favour disc formation as the magnetosphere occupies a smaller volume. Still, even in the case of fields decaying exponentially over a characteristic timescale of $\sim 1.5$ Gyr, we find that for several realistic models of propeller spin-down, only a small fraction of accretors (down to $\sim 0.02$ %) attain discs. However, disc-accreting isolated neutron stars are relatively numerous among the brightest sources. We estimate that their number can reach a few hundred at X-ray fluxes of $\gtrsim 10^{-14}$ erg s$^{-1}$ cm$^{-2}$. We speculate that disc-accreting isolated neutron stars can manifest as long-period radio transient sources via electron cyclotron maser emission, predicting spectra and critical conditions for quenching.

astro-ph.HE↗
arXiv2026

A phenomenological model of the magnetic field re-emergence in magnetars and discrepancy between the kinematic and characteristic ages

Robust age measurements for isolated neutron stars (NSs) are not easily available. That is why, often the characteristic age $τ_\mathrm{ch}=P/2\dot P$ is used as a proxy. Here $P$ is the spin period of the NS and $\dot P$ is the time derivative of $P$. Additional assumptions related to the initial properties and spin-down evolution are made to derive $τ_\mathrm{ch}$. As a result, it is expected that $τ_\mathrm{ch}$ is an upper limit for the real age $τ_\mathrm{real}$. Recently, Chrimes et al. presented measurements of kinematic ages $τ_\mathrm{kin}$ for several magnetars. Surprisingly, for the majority of these sources $τ_\mathrm{kin}>τ_\mathrm{ch}$. We present a simple model including a realistic approximation for the magnetic field decay in magnetars and a simple phenomenological description of the field re-emergence after an episode of fallback after the birth of a NS. We demonstrate that this simple model can explain the observed relation $τ_\mathrm{kin}>τ_\mathrm{ch}$ for realistic sets of parameters.

astro-ph.HE↗
arXiv2026

Disc accretion onto a binary black hole in a hierarchical triple system as an origin of the most luminous hyper-soft sources

We propose that the recently discovered luminous hypersoft X-ray sources can be explained by accretion onto a binary black hole in a hierarchical triple system. For black hole masses $\sim 15 M_\odot$, the orbital separation of the internal binary might be $\sim 0.01 $~AU. If the donor provides $\gtrsim 10^{-8} M_\odot$ yr$^{-1}$, then the circumbinary accretion disc can explain the observed properties of the most luminous supersoft sources.

astro-ph.HE↗
arXiv2025

Low-frequency observations of low-mass binary systems with neutron star candidates

Recently, astrometric and spectroscopic observations resulted in the discovery of several low-mass binaries with invisible components, which are expected to be compact objects. In about two dozen cases, the masses of these components are consistent with neutron stars. We use low-frequency archival data obtained with the Large Phased Array in Pushchino to search for radio emission from five of these systems. For all the systems, we do not detect persistent or periodic emission. In one case (2MASS J1527+3536), we identify a single radio burst with a flux of 13 Jy and a duration of 0.13 s. However, the dispersion measure of the burst does not correspond to an expected value for the source. We discuss several possibilities to explain the properties of this burst.

astro-ph.HE↗
arXiv2025

Supernova explosions of runaway stars and young neutron stars above the Galactic plane

Several supernova remnants and young neutron stars were recently discovered relatively high above the Galactic plane. One possibility is that they originate from runaway OB stars born in the Galactic disk. Understanding their origin will provide key insights into the properties of the Galactic halo. This paper aims to determine the spatial distribution of supernova explosions from runaway OB stars and to assess whether this model can explain certain observed neutron stars. We map the distribution of supernova events produced by runaway OB stars by incorporating their birth rate, initial spatial distribution, lifetime, ejection mechanisms, and velocity distributions. By tracking their motion in the Galactic potential, we determine their final distribution right before the explosion. We show that the neutron star Calvera, which is found at z around 2.2 kpc, could have originated from a runaway OB star. In addition, we compare the probabilities of finding a supernova remnant originating from Type Ia and core-collapse supernovae high above the Galactic plane, showing that supernova remnants related to core-collapse supernovae outnumber those related to Type Ia supernovae.

astro-ph.SR↗
arXiv2024

Planetary migration in wind-fed non-stationary accretion disks in binary systems

An accretion disk can be formed around a secondary star in a binary system when the primary companion leaves the Main sequence and starts to lose mass at an enhanced rate. We study the accretion disk evolution and planetary migration in wide binaries. We use a numerical model of a non-stationary alpha-disk with a variable mass inflow. We take into account that the low-mass disk has an extended region that is optically thin along the rotation axis. We consider irradiation by both the host star and the donor. The migration path of a planet in such a disk is determined by the migration rate varying during the disk evolution. Giant planets may open/close the gap several times over the disk lifetime. We identify the new type of migration specific to parts of the growing disk with a considerable radial gradient of an aspect ratio. Its rate is enclosed between the type 2 and the fast type 1 migration rates being determined by the ratio of time and radial derivatives of the disk aspect ratio. Rapid growth of the wind rate just before the envelope loss by the donor leads to the formation of a zone of decretion, which may lead to substantial outward migration. In binaries with an initial separation $a\lesssim 100$\,AU migration becomes most efficient for planets with 60--80 Earth masses. This results in approaching a short distance from the host star where tidal forces become non-negligible. Less massive Neptune-like planets at the initial orbits $r_\mathrm{p} \lesssim 2$\,AU can reach these internal parts in binaries with $a \lesssim 30$\,AU. In binaries, mass loss by the primary component at late evolutionary stages can significantly modify the structure of a planetary system around the secondary component resulting in mergers of relatively massive planets with a host star.

astro-ph.EP↗
arXiv2024

Indication of rapid magnetic field decay in X-ray Dim Isolated Neutron Star RX J0720.4-3125

Magnetic field evolution of neutron stars is a long-standing debate. The rate of magnetic field decay for isolated, non-accreting neutron stars can be quantified by measuring the negative second derivative of the spin period. Alternatively, this rate can be estimated by observing an excess of thermal emission with respect to the standard cooling without additional heating mechanisms involved. One of the nearby cooling isolated neutron stars -- RX J0720.4-3125, -- offers a unique opportunity to probe the field decay as for this source there are independent measurements of the surface X-ray luminosity, the second spin period derivative, and magnetic field. We demonstrate that the evolution rate of the spin period derivative is in correspondence with the rate of dissipation of magnetic energy of the dipolar field if a significant part of the released energy is emitted in X-rays. The instantaneous time scale for the magnetic field decay is $\sim 10^4$ years.

astro-ph.HE↗
arXiv2023

FRBs and magnetar activity statistics

With simple estimates based on recent observational data and the assumption that among known FRB sources most repeaters with extremely high rates of repetition (super-repeaters) are already identified, we demonstrate that the hypothesis that super-repeaters and one-off events come from the same population of magnetars is not self-contradictory. In this toy model, the super-repeater stage has a duration of about a few years and the period when one-off events are mostly emitted corresponds to the active life of a magnetar $\sim $few$\times 10^3$~years. Intervals between strong events (observed as one-off FRBs) from the same source are $\sim10$~years, corresponding to the expected time interval between giant flares.

astro-ph.HE↗
arXiv2023

The zoo of isolated neutron stars

In this brief review I summarize our basic knowledge about different types of isolated neutron stars. I discuss radio pulsars, central compact objects in supernova remnants, magnetars, near-by cooling neutron stars (aka the Magnificent seven), and sources of fast radio bursts. Several scenarios of magneto-rotational evolution are presented. Recent observational data, in the first place -- discovery of long period radio pulsar, require non-trivial evolution of magnetics fields or/and spin periods of neutron stars. In some detail I discuss different models of magnetic field decay and interaction of young neutron stars with fallback matter.

astro-ph.HE↗
arXiv2022

Formation of periodic FRB in binary systems with eccentricity

Long-term periodicity in the rate of flares is observed for two repeating sources of fast radio bursts (FRBs). In this paper We present a hydrodynamical modeling of a massive binary consisting of a magnetar and an early-type star. We model the interaction of the pulsar wind from the magnetar with an intense stellar wind. It is shown that only during a fraction of the orbital period radio emission can escape the system. This explains the duty cycle of the two repeating FRB sources with periodic activity. The width of the transparency window depends on the eccentricity, stellar wind properties, and the viewing angle. To describe properties of the known sources it is necessary to assume large eccentricities $\gtrsim 0.5$. We apply the maser cyclotron mechanism of the radio emission generation to model spectral properties of the sources. The produced spectrum is not wide: $Δν/ν\sim 0.2$ and the typical frequency depends on the radius of the shock where the emission is generated. The shock radius changes along the orbit. This, together with changing parameters of the medium, allows us to explain the frequency drift during the phase of visibility. Frequency dependence of the degree of polarization at few GHz can be a consequence of a small scale turbulence in the shocked stellar wind. It is much more difficult to explain huge ($\sim 10^5$ [rad/m$^2$]) and variable value of the rotation measure observed for FRB 121102. We suggest that this can be explained if the supernova explosion which produced the magnetar happened near a dense interstellar cloud with $n \sim100$ cm$^{-3}$.

astro-ph.HE↗
arXiv2022

Initial periods and magnetic fields of neutron stars

Initial distributions of pulsar periods and magnetic fields are essential components of multiple modern astrophysical models. Not enough work has been done to properly constrain these distributions using direct measurements. Here we aim to fill this gap by rigorously analysing properties of young neutron stars associated to supernova remnants. In order to perform this task, we compile a catalogue of 56 neutron stars uniquely paired to supernova remnants with known age estimate. Further, we analyse this catalogue using multiple statistical techniques. We found that distribution of magnetic fields and periods for radio pulsars are both well described using the log-normal distribution. The mean magnetic field is $\log_{10} [B/\mathrm{G}] = 12.44$ and standard deviation is $σ_B = 0.44$. Magnetars and central compact objects do not follow the same distribution. The mean initial period is $\log_{10} P_0 [P / \mathrm{s}] = -1.04_{-0.2}^{+0.15}$ and standard deviation is $σ_p = 0.53_{-0.08}^{+0.12}$. We show that the normal distribution does not describe the initial periods of neutron stars sufficiently well. Parameters of the initial period distribution are not sensitive to the exact value of the braking index.

astro-ph.HE↗
arXiv2022

Model-independent classification of events from the first CHIME/FRB Fast Radio Burst catalog

The CHIME/FRB collaboration has recently published a catalog containing about half a thousand fast radio bursts (FRBs) including their spectra and several reconstructed properties, like signal widths, amplitudes, etc. We have developed a model-independent approach for a classification of these bursts using cross-correlation and clustering algorithms applied to one-dimensional intensity profiles of the bursts (i.e., to amplitudes as a function of time averaged over the frequency). Using this algorithm we identified two major classes of FRBs featuring different waveform morphology, and, simultaneously, different distribution of brightness temperature. Bursts from one of the identified cluster have lower brightness temperatures and larger widths than events from the other cluster. Both groups include bursts from the repeaters and one-off events.

astro-ph.HE↗
arXiv2021

Evolution of neutron star magnetic fields

Neutron stars are natural physical laboratories allowing us to study a plethora of phenomena in extreme conditions. In particular, these compact objects can have very strong magnetic fields with non-trivial origin and evolution. In many respects its magnetic field determines the appearance of a neutron star. Thus, understanding the field properties is important for interpretation of observational data. Complementing this, observations of diverse kinds of neutron stars enable us to probe parameters of electro-dynamical processes at scales unavailable in terrestrial laboratories. In this review we first briefly describe theoretical models of formation and evolution of magnetic field of neutron stars, paying special attention to field decay processes. Then we present important observational results related to field properties of different types of compact objects: magnetars, cooling neutron stars, radio pulsars, sources in binary systems. After that, we discuss which observations can shed light on obscure characteristics of neutron star magnetic fields and their behaviour. We end the review with a subjective list of open problems.

astro-ph.HE↗
arXiv2021

X-ray Emission from Isolated Neutron Stars revisited: 3D magnetothermal simulations

X-ray emission from the surface of isolated neutron stars (NSs) has been now observed in a variety of sources. The ubiquitous presence of pulsations clearly indicates that thermal photons either come from a limited area, possibly heated by some external mechanism, or from the entire (cooling) surface but with an inhomogeneous temperature distribution. In a NS the thermal map is shaped by the magnetic field topology, since heat flows in the crust mostly along the magnetic field lines. Self-consistent surface thermal maps can hence be produced by simulating the coupled magnetic and thermal evolution of the star. We compute the evolution of the neutron star crust in three dimensions for different initial configurations of the magnetic field and use the ensuing thermal surface maps to derive the spectrum and the pulse profile as seen by an observer at infinity, accounting for general-relativistic effects. In particular, we compare cases with a high degree of symmetry with inherently 3D ones, obtained by adding a quadrupole to the initial dipolar field. Axially symmetric fields result in rather small pulsed fractions ($\lesssim 5\%$), while more complex configurations produce higher pulsed fractions, up to $\sim25\%$. We find that the spectral properties of our axisymmetric model are close to those of the bright isolated NS RX~J1856.5-3754 at an evolutionary time comparable with the inferred dynamical age of the source.

astro-ph.HE↗
arXiv2019

The Rate of Planet-star Coalescences Due to Tides and Stellar Evolution

Orbits of close-in planets can shrink significantly due to dissipation of tidal energy in a host star. This process can result in star-planet coalescence within the Galactic lifetime. In some cases, such events can be accompanied by an optical or/and UV/X-ray transient. Potentially, these outbursts can be observed in near future with new facilities such as LSST from distances about few Mpc. We use a population synthesis model to study this process and derive the rate of star-planet mergers of different types. Mostly, planets are absorbed by red giants. However, these events, happening with the rate about 3 per year, mostly do not produce detectable transients. The rate of mergers with main sequence stars depends on the effectiveness of tidal dissipation; for reasonable values of stellar tidal quality factor, such events happen in a Milky Way-like galaxy approximately once in 70 yrs or more rarely. This rate is dominated by planets with low masses. Such events do not produce bright transients having maximum luminosities $\lesssim 10^{36.5}$erg s$^{-1}$. Brighter events, related to massive planets, with maximum luminosity $\sim 10^{37.5}$--$10^{38}$erg s$^{-1}$, have the rate nearly five times smaller.

astro-ph.EP↗
arXiv2018

Discovery of X-rays from the old and faint pulsar J1154--6250

We report on the first X-ray observation of the 0.28 s isolated radio pulsar PSR J1154--6250 obtained with the XMM-Newton observatory in February 2018. A point-like source is firmly detected at a position consistent with that of PSR J1154--6250. The two closest stars are outside the 3$σ$ confidence limits of the source position and thus unlikely to be responsible for the observed X-ray emission. The energy spectrum of the source can be fitted equally well either with an absorbed power-law with a steep photon index $Γ\approx 3.3$ or with an absorbed blackbody with temperature $kT=0.21\pm 0.04$~keV and emitting radius $R_\mathrm{BB} \approx 80$ m (assuming a distance of 1.36~kpc). The X-ray luminosity of $4.4\times 10^{30}$ erg s$^{-1}$ derived with the power-law fit corresponds to an efficiency of $η_X = L^\mathrm{unabs}_X/\dot E = 4.5\times 10^{-3}$, similar to those of other old pulsars. The X-ray properties of PSR J1154--6250 are consistent with an old age and suggest that the spatial coincidence of this pulsar with the OB association Cru OB1 is due to a chance alignment.

astro-ph.HE↗
arXiv2018

The merger of two compact stars: a tool for dense matter nuclear physics

We discuss the different signals, in gravitational and electromagnetic waves, emitted during the merger of two compact stars. We will focus in particular on the possible contraints that those signals can provide on the equation of state of dense matter. Indeed, the stiffness of the equation of state and the particle composition of the merging compact stars, strongly affect e.g. the life time of the post-merger remnant and its gravitational wave signal, the emission of the short gamma-ray-burst, the amount of ejected mass and the related kilonova. The first detection of gravitational waves from the merger of two compact stars in August 2017, GW170817, and the subsequent detections of its electromagnetic counterparts, GRB170817A and AT2017gfo, is the first example of the era of "multi-messenger astronomy": we discuss what we have learned from this detection on the equation of state of compact stars and we provide a tentative interpretation of this event, within the two families scenario, as due to the merger of a hadronic star with a quark star.

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