arXiv subjects

Pavel Abolmasov

Publications and source records attributed to Pavel Abolmasov.

At least 19 recordsLinked to original sources

arXiv2026

Pulse profiles of tall accretion columns in X-ray pulsars

Accreting X-ray pulsars (XRPs) are strongly magnetized neutron stars (NSs) distinguished by a periodic variability pattern related to the rotation of their magnetospheres (pulsations). Many of these objects show complex asymmetric pulse profiles that are difficult to explain in the framework of existing models. We calculate the pulsations of X-ray radiation from a magnetized NS in the regime of supercritical accretion, when the radiation shock wave is high above the NS surface, and the observed radiation comes from two optically thick accretion columns located downstream of the shocks. We show that the accretion column beam pattern calculated from the first principles has a potential of reproducing the variety of XRP pulse shapes. The surfaces of the columns are assumed to radiate locally as blackbodies with the temperature distribution following the solution of Basko and Sunyaev (1976). The adopted geometry of the field is an inclined dipole. It is taken into account that the two accretion flows above the shocks attenuate and scatter the radiation of the accretion columns. Using the ray-tracing technique, we obtain a vast variety of pulse shapes controlled by the mass accretion rate, magnetic angle, the direction towards the observer, and the parameters that determine the particular set of magnetic field lines being loaded with accreting matter. We propose a universal dimensionless parameter describing the asymmetry of a pulse quantitatively. The model naturally produces asymmetric pulse profiles observed in many XRPs. Its applicability, however, is limited to the brightest objects like pulsating ultraluminous Xray sources and strong flares of Galactic XRPs.

astro-ph.HE↗

arXiv2026

Shock-free super-Eddington accretion onto magnetized neutron star

Strong magnetic fields allow accreting neutron stars to form magnetically controlled funnel flows. At sufficiently high accretion rates, such flows become radiation-pressure-supported. In the conventional picture, the magnetospheric flow contains a nearly free-falling transonic outer region, a radiation-mediated shock, and a subsonic accretion column. We investigate the regime in which the height of the column is expected to exceed the magnetospheric radius. In this case a self-consistent transonic solution with a standing shock is no longer possible, and the entire magnetospheric flow remains subsonic. We construct a semi-analytical hydrodynamical solution for a fully subsonic, radiation-supported magnetospheric flow in the low-Mach-number limit. The magnetic field is treated in the force-free approximation, while the flow structure is determined by mass conservation, energy conservation, and the momentum equation along the field lines. We test the solution using one-dimensional time-dependent simulations with the code HACol. The shock-free solution is highly advective. Its enthalpy profile is close to the adiabatic limit, while the energy density is regulated by leakage of mass and heat through the sides of the accretion channel. Most of the observable power is expected to be released by plasma leaving the magnetospheric flow above the neutron-star surface. Such systems should be associated with super-Eddington accretion discs and low pulse fractions due to visibility selection and scattering in the disc wind. We suggest that many non-pulsating ultraluminous X-ray sources containing neutron stars may accrete in this shock-free regime.

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arXiv2026

Variable opacity and accretion-column regimes of magnetized neutron stars

Radiation-supported accretion columns of magnetized neutron stars can operate in several distinct regimes. As the accretion rate increases, magnetospheric accretion may proceed through hot-spot emission, efficiently cooling radiative shocks, and advective sinking columns. The structure of the accretion flow is determined by the global parameters such as magnetic field strength and mass accretion rate, but is also affected by opacity variations in strong magnetic fields. A decrease of opacity for photons with energy below the cyclotron energy is able to substantially augment the classical critical accretion rate separating efficiently cooling solutions from advective solutions. We propose an approach to determine the critical accretion rate above which the Basko-Sunyaev sinking solution becomes unavoidable. We also use the Basko-Sunyaev sinking solution to estimate the plasma temperature and compare the characteristic photon energy with the local cyclotron energy. For a combination of moderate magnetic fields and high accretion rates, the characteristic photon energy in advective columns typically exceeds the cyclotron energy. In this case, the opacity is not reduced by the magnetic field, and tall accretion columns are formed. Strong magnetic-field systems may retain short, efficiently cooling accretion columns over a wide range of super-Eddington accretion rates. We identify a region in accretion rate and magnetic-field strength where both efficiently cooling and advective solutions may coexist. In this region, the column may switch or oscillate between the regimes and geometries. The typical time of such oscillations are close to the replenishment time of the column, or to the thermal time near its bottom. The quasi-periodic oscillations, observed in some super-Eddington objects in the 1-100 mHz frequency range, may be related to such relaxation cycles.

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arXiv2026

Theoretical Constraints on Neutron Star Superfluidity from Her X-1 Precession

Recent IXPE observations of Her X-1 reveal correlations between flux, polarization degree, and polarization angle with its 35-day superorbital cycle. These measurements have been interpreted as strong evidence that the 35-day period is driven by nearly free precession of the neutron star. We show that this interpretation carries far-reaching implications for the dynamics of the crustal superfluid. In particular, maintaining precession over the $\sim 50$-year observational baseline of Her X-1 would require that superfluid vortices remain unpinned for centuries and experience extremely weak mutual friction while traversing the heavy-ion lattice of the inner crust -- conditions that challenge conventional wisdom and standard models of glitch dynamics.

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arXiv2025

Rotational evolution of deformed magnetized neutron stars: implications for obliquity distribution and braking indices statistics

The rotational evolution of a strongly magnetized neutron star (NS), accreting or isolated, is driven by external torques of different nature. In addition to the torques, even the tiniest deformations of the NS crust can affect its rotation through asymmetries in its inertia tensor. Several factors may be responsible for the deformations, including strong magnetic fields, internal stresses, or local heating. The main effect produced by the deformations is the so-called free precession: the motion of the rotational axis with respect to the crust. We consider the evolution of a triaxially deformed isolated NS with a strong dipolar magnetic field for a broad range of parameters, taking into account the magnetic field decay. We show that the combination of pulsar torques and free precession results in a considerable broadening of the distribution of magnetic obliquity angles (the angle between the magnetic and rotational axes) and creates a population of objects where the rotational axis does not align with the magnetic axis at all but enters a limit-cycle regime. The combination of free precession and magnetic torques can also explain the observed distribution in pulsar braking indices by creating a periodic oscillation in the magnetic obliquity.

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arXiv2025

Tidal disruption of a magnetized star

Tidal disruptions of stars by supermassive black holes in galactic centers (TDEs) are now being actively studied both theoretically and observationally. They are observed throughout the electromagnetic spectrum, from radio to gamma-rays. It is still unclear how the emission is produced and, in particular, what is the role of the magnetic field of the disrupted star. There are many ways how magnetic fields might affect the dynamics of a TDE. They are likely responsible for the angular momentum transfer in the accretion disk formed at later stages and thus affect the radiation associated with the disk. Magnetic fields are also an important requirement for the formation of relativistic jets, that are seen in some TDEs. The goal of our study is to connect the field within the star to the fields that develop during the fallback and disk accretion. Using the fluid-dynamic code Athena++, we perform a large-scale three-dimensional adaptive-mesh magnetohydrodynamic simulation of a tidal disruption of a magnetized star. The fallback stream returning to the black-hole vicinity after the disruption contains smooth magnetic fields aligned with the stream lines. Formation of a nozzle shock near the pericenter of the initial orbit leads to a turbulent eccentric disk-like structure where the field is amplified and entangled on the local dynamic time scales up to approximate equipartition. The resulting field is mildly anisotropic and has a typical length several times smaller than the pericenter distance. The properties of the field are consistent with the early stages of turbulent dynamo.

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arXiv2025

Numerical approach to compressible shallow-water dynamics of neutron-star spreading layers

A weakly magnetized neutron star (NS) undergoing disk accretion should release about a half of its power in a compact region known as the accretion boundary layer. Latitudinal spread of the accreted matter and efficient radiative cooling justify the approach to this flow as a two-dimensional spreading layer (SL) on the surface of the star. Numerical simulations of SLs are challenging because of the curved geometry and supersonic nature of the problem. We develop a new two-dimensional hydrodynamics code that uses the multislope second-order MUSCL scheme in combination with an HLLC+ Riemann solver on an arbitrary irregular mesh on a spherical surface. The code is suitable and accurate for Mach numbers at least up to 5-10. Adding sinks and sources to the conserved variables, we simulate constant-rate accretion onto a spherical NS. During the early stages of accretion, heating in the equatorial region triggers convective instability that causes rapid mixing in latitudinal direction. One of the outcomes of the instability is the development of a two-armed `tennis ball' pattern rotating as a rigid body. From the point of view of a high-inclination observer, its contribution to the light curve is seen as a high-quality-factor quasi-periodic oscillation mode with a frequency considerably smaller than the rotation frequency of the matter in the SL. Other variability modes seen in the simulated light curves are probably associated with low-azimuthal-number Rossby waves.

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arXiv2023

Simulating the shock dynamics of a neutron star accretion column

Accretion onto a highly-magnetised neutron star runs through a magnetospheric flow, where the plasma follows the magnetic field lines in the force-free regime. The flow entering the magnetosphere is accelerated by the gravity of the star and then abruptly decelerated in a shock located above the surface of the star. For large enough mass accretion rates, most of the radiation comes from the radiation-pressure-dominated region below the shock, known as accretion column. Though the one-dimensional, stationary structure of this flow has been studied for many years, its global dynamics was hardly ever considered before. Considering the time-dependent structure of an accretion column allows us to test the stability of the existing stationary analytic solution, as well as its possible variability modes, and check the validity of its boundary conditions. Using a conservative scheme, we perform one-dimensional time-dependent simulations of an ideal radiative MHD flow inside an aligned dipolar magnetosphere. Whenever thermal pressure locally exceeds magnetic pressure, the flow is assumed to lose mass. Position of the shock agrees well with the theoretical predictions below a limit likely associated with advection effects: if more than $2/3$ of the released power is advected with the flow, the analytic solution becomes self-inconsistent, and the column starts leaking at a finite height. Depending on the geometry, this breakdown may broaden the column, mass-load the field lines, and produce radiation-driven, mildly relativistic ejecta. Evolving towards the equilibrium position, the shock front experiences damped oscillations at a frequency close to the inverse sound propagation time.

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arXiv2023

Interface instabilities in hydrodynamic relativistic jets

Both the dynamics and the observational properties of relativistic jets are determined by their interaction with the ambient medium. A crucial role is played by the contact discontinuity at the jet boundary, which in the presence of jet collimation may become subject to Rayleigh-Taylor instability (RTI) and Richtmyer-Meshkov instability (RMI). Here, we study the evolution of these instabilities in non-magnetized relativistic jets using special relativistic three-dimensional hydrodynamic simulations. We show that the growth of initial perturbations is consistent with relativistic RTI operating in the jet collimation region. The contribution of RMI becomes important downstream from the collimation shock in agreement with theoretical expectations. Both instabilities reach non-linear scales above the shock convergence point and trigger strong turbulence, mixing jet with ambient matter. We devise an analytic solution for the mixing rate and show that it is sensitive to the external density gradients. Our results may be applied to different types of astrophysical objects. In particular, different contribution of interface instabilities is a natural explanation for the observed dichotomy between FR-I and FR-II radiogalaxies. The rapid slow-down in the jet of M87 is consistent with baryon entrainment from the circumnuclear matter with the observed density distribution. In microquasars, baryon loading triggered by interface instabilities is a probable reason for the low observed Lorentz factors. We show that the observed variability in gamma-ray bursts cannot come from mixing driven by interface instabilities and likely originates from the engine, suggesting the presence of magnetic fields in the jet.

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arXiv2021

Magnetic angle evolution in accreting neutron stars

The rotation of a magnetised accreting neutron star (NS) in a binary system is described by its spin period and two angles: spin inclination $α$ with respect to the orbital momentum and magnetic angle $χ$ between the spin and the magnetic moment. Magnetospheric accretion spins the NS up and adjusts its rotation axis, decreasing $α$ to nearly perfect alignment. Its effect upon the magnetic angle is more subtle and relatively unstudied. In this work, we model the magnetic angle evolution of a rigid spherical accreting NS. We find that the torque spinning the NS up may affect the magnetic angle while both $α$ and $χ$ significantly deviate from zero, and the spin-up torque varies with the phase of the spin period. As the rotation axis of the NS is being aligned with the spin-up torque, the magnetic axis becomes misaligned with the rotation axis. Under favourable conditions, magnetic angle may increase by $Δχ\sim 15^\circ-20^\circ$. This orthogonalisation may be an important factor in the evolution of millisecond pulsars, as it partially compensates the $χ$ decrease potentially caused by pulsar torques. If the direction of the spin-up torque changes randomly with time, as in wind-fed high-mass X-ray binaries, both the rotation axis of the NS and its magnetic axis become involved in a non-linear random-walk evolution. The ultimate attractor of this process is a bimodal distribution in $χ$ peaking at $χ=0^\circ$ and $χ= 90^\circ$.

astro-ph.HE↗

arXiv2021

Mechanical model of a boundary layer for the parallel tracks of kilohertz quasi-periodic oscillations in accreting neutron stars

Kilohertz-scale quasi-periodic oscillations (kHz QPOs) are a distinct feature of the variability of neutron star low-mass X-ray binaries. Among all the variability modes, they are especially interesting as a probe for the innermost parts of the accretion flow, including the accretion boundary layer (BL) on the surface of the neutron star. All the existing models of kHz QPOs explain only part of their rich phenomenology. Here, we show that some of their properties may be explained by a very simple model of the BL that is spun up by accreting rapidly rotating matter from the disk and spun down by the interaction with the neutron star. In particular, if the characteristic time scales for the mass and the angular momentum transfer from the BL to the star are of the same order of magnitude, our model naturally reproduces the so-called parallel tracks effect, when the QPO frequency is correlated with luminosity at time scales of hours but becomes uncorrelated at time scales of days. The closeness of the two time scales responsible for mass and angular momentum exchange between the BL and the star is an expected outcome of the radial structure of the BL.

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arXiv2020

Inertial oscillation modes of an inclined dipolar magnetosphere as a source of band-limited noise in X-ray pulsars

Magnetic fields of strongly magnetized stars can trap conducting matter due to frozen-in condition. In the force-free regime, the motion of the matter along the field lines may be considered in the "bead on a wire" approximation. Such a motion, if gravity and centrifugal forces are taken into account, has equilibrium points, some of which are stable. In most cases, stability is possible in about several per cent of the possible locations. Corresponding oscillation frequencies span the range from zero to $\sqrt{3}$ of the spin frequency. We suggest that this variability mode may be excited in some X-ray pulsars during the outbursts and create the peaked broad-band noise component near the break frequency in the power density spectrum, as well as produce some of the quasi-periodic oscillation features in this frequency range. Existence of this variability does not require any changes in mass accretion rate and involves only a small amount of matter infiltrating from the disc and magnetic flow due to interchange instabilities.

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arXiv2020

Kilohertz quasi-periodic oscillations from neutron star spreading layers

When the accretion disc around a weakly magnetised neutron star (NS) meets the stellar surface, it should brake down to match the rotation of the NS, forming a boundary layer. As the mechanisms potentially responsible for this braking are apparently inefficient, it is reasonable to consider this layer as a spreading layer (SL) with negligible radial extent and structure. We perform hydrodynamical 2D spectral simulations of an SL, considering the disc as a source of matter and angular momentum. Interaction of new, rapidly rotating matter with the pre-existing, relatively slow material co-rotating with the star leads to instabilities capable of transferring angular momentum and creating variability on dynamical timescales. For small accretion rates, we find that the SL is unstable for heating instability that disrupts the initial latitudinal symmetry and produces large deviations between the two hemispheres. This instability also results in breaking of the axial symmetry as coherent flow structures are formed and escape from the SL intermittently. At enhanced accretion rates, the SL is prone to shearing instability and acts as a source of oblique waves that propagate towards the poles, leading to patterns that again break the axial symmetry. We compute artificial light curves of an SL viewed at different inclination angles. Most of the simulated light curves show oscillations at frequencies close to 1kHz. We interpret these oscillations as inertial modes excited by shear instabilities near the boundary of the SL. Their frequencies, dependence on flux, and amplitude variations can explain the high-frequency pair quasi-periodic oscillations observed in many low-mass X-ray binaries.

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arXiv2020

Discovery of a retrogradely rotating neutron star in the X-ray pulsar GX 301$-$2

We report on the analysis of the spin evolution of a slow X-ray pulsar GX 301$-$2 along the orbit using long-term monitoring by Fermi/GBM. Based on the observationally confirmed accretion scenario and an analytical model for the accretion of angular momentum we demonstrate that in this system, the neutron star spins retrogradely, that is, in a direction opposite to the orbital motion. This first-of-a-kind discovery of such a system proves the principal possibility of retrograde rotation in accreting systems with suitable accretion torque, and might have profound consequences for our understanding of the spin evolution of X-ray pulsars, estimates of their initial spin periods, and the ultimate result of their evolution.

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arXiv2019

Super-Eddington accretion discs with advection and outflows around magnetized neutron stars

We present a model for a super-Eddington accretion disc around a magnetized neutron star taking into account advection of heat and the mass loss by the wind. The model is semi-analytical and predicts radial profiles of all basic physical characteristics of the accretion disc. The magnetospheric radius is found as an eigenvalue of the problem. When the inner disc is in radiation-pressure-dominated regime but does not reach its local Eddington limit, advection is mild, and the radius of the magnetosphere depends weakly on the accretion rate. Once approaching the local Eddington limit, the disc becomes advection-dominated, and the scaling for the magnetospheric radius with the mass accretion rate is similar to the classical Alfven relation. Allowing for the mass loss in a wind leads to an increase of the magnetospheric radius. Our model may be applied to a large variety of magnetized neutron stars accreting close to or above their Eddington limits: ultra-luminous X-ray pulsars, Be/X-ray binaries in outbursts, and other systems. In the context of our model we discuss the observational properties of NGC 5907~X-1, the brightest ultra-luminous pulsar known so far, and NGC 300~ULX-1 which is apparently a Be/X-ray binary experiencing a very bright super-Eddington outburst.

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arXiv2017

Meridional circulation and reverse advection in hot thin accretion discs

In standard accretion discs, outward angular momentum transfer by viscous forces is compensated by the inward motion of the accreting matter. However, the vertical structure of real accretion discs leads to meridional circulation with comparable amplitudes of poloidal velocities. Using thin-disc approximation, we consider different regimes of disc accretion with different vertical viscosity scalings. We show that, while gas-pressure-dominated discs can easily have a midplane outflow, standard thin radiation-pressure-dominated disc is normally moving inwards at all the heights. However, quasi-spherical scaling for pressure ($p\propto \varpi^{-5/2}$) leads to a midplane outflow for a very broad range of parameters. It particular, this may lead to a reversed, outward heat advection in geometrically thick discs when the temperature decreases rapidly enough with height. While the overall direction of heat advection depends on the unknown details of vertical structure and viscosity mechanisms, existence of the midplane counterflow in quasi-spherical flows is a robust result weakly dependent on the parameters and the assumptions of the model. Future models of thick radiatively inefficient flows should take meridional circulation into account.

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arXiv2017

Super-Eddington accretion onto a magnetized neutron star

Most of ultraluminous X-ray sources are thought to be objects accreting above their Eddington limits. In the recently identified class of ultraluminous X-ray pulsars, accretor is a neutron star and thus has a fairly small mass with a small Eddington limit. The accretion disc structure around such an object affects important observables such as equilibrium period, period derivative and the size of the magnetosphere. We propose a model of a nearly-standard accretion disc interacting with the magnetosphere only in a thin layer near the inner disc rim. Our calculations show that the size of the magnetosphere may be represented as the classical Alfvén radius times a dimensionless factor $ξ$ which depends on the disc thickness only. In the case of radiation-pressure-dominated disc, the size of the magnetosphere does not depend on the mass accretion rate. In general, increasing the disc thickness leads to a larger magnetosphere size in units of the Alfvén radius. For large enough mass accretion rates and magnetic moments, it is important to take into account not only the pressure of the magnetic field and the radiation pressure inside the disc, but also the pressure of the radiation produced close to the surface of the neutron star in accretion column. The magnetospheric size may increase by up to factor of two as a result of the effects related to the disc thickness and the irradiation from the central source. Accounting for these effects reduces the estimate of the neutron star magnetic moment by a factor of several.

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arXiv2017

Apparent quasar disc sizes in the "bird's nest" paradigm

Quasar microlensing effects make it possible to measure the accretion disc sizes around distant supermassive black holes that are still well beyond the spatial resolution of contemporary instrumentation. The sizes measured with this technique appear inconsistent with the standard accretion disc model. Not only are the measured accretion disc sizes larger, but their dependence on wavelength is in most cases completely different from the predictions of the standard model. We suggest that these discrepancies may arise not from non-standard accretion disc structure or systematic errors, as it was proposed before, but rather from scattering and reprocession of the radiation of the disc. In particular, the matter falling from the gaseous torus and presumably feeding the accretion disc may at certain distances become ionized and produce an extended halo that is free from colour gradients. A simple analytical model is proposed assuming that a geometrically thick translucent inflow acts as a scattering mirror changing the apparent spatial properties of the disc. This inflow may be also identified with the broad line region or its inner parts. Such a model is able to explain the basic properties of the apparent disc sizes, primarily their large values and their shallow dependence on wavelength. The only condition required is to scatter significant portion of the luminosity of the disc. This can easily be fulfilled if the scattering inflow has large geometrical thickness and clumpy structure.

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