arXiv subjects

D. G. Yakovlev

Publications and source records attributed to D. G. Yakovlev.

At least 19 recordsLinked to original sources

arXiv2025

Zeeman effect in oscillations of magnetars with toroidal magnetic fields

Magnetars are neutron stars with superstrong magnetic fields. Some of them (soft-gamma repeaters, SGRs) demonstrate gigantic flares which nature is still unclear. At decay phase of such flares one often observes quasi-periodic oscillations (QPOs) which are treated as stellar oscillations triggered by the flares. We study, for the first time, magneto-elastic oscillations of magnetars possessing toroidal magnetic fields confined in the stellar crust, without imposing axial symmetry of perturbations. We show that the Zeeman effect makes the oscillation spectrum much richer than for axially symmetric oscillations. The main properties of theoretical QPO spectra are discussed as well as their potential to interpret observations and explore magnetar physics.

hep-ph↗

arXiv2024

Powerful flares and magneto-elastic oscillations of magnetars

Magnetars are neutron stars with superstrong magnetic fields which can exceed 1e15 G. Some magnetars (the so-called soft gamma-repeaters) demonstrate occasionally very powerful processes of energy release, which result in exceptionally strong flares of electromagnetic radiation. It is believed that these flares are associated with the presence of superstrong magnetic fields. Despite many hypotheses, the mechanism of these flares remains a mystery. In afterglows of the flares, one has often observed quasi-periodic oscillations (QPOs) of magnetar emission. They are interpreted as stellar vibrations, excited by the flares, which are useful for exploring the nature of magnetar activity. The incompleteness of theories employed to interpret magnetar QPOs is discussed.

astro-ph.HE↗

arXiv2024

Zeeman splitting of torsional oscillation frequencies of magnetars

Magnetars form a special class of neutron stars possessing superstrong magnetic fields and demonstrating power flares triggered likely by these fields. Observations of such flares reveal the presence of quasi-periodic oscillations (QPOs) at certain frequencies; they are thought to be excited in the flares. QPOs carry potentially important information on magnetar structure, magnetic field, and mechanisms of magnetar activity. We calculate frequencies of torsional (magneto-elastic) oscillations of the magnetar crust treating the magnetic field effects in the first order of perturbation theory. The theory predicts splitting of non-magnetic oscillation frequencies into Zeeman components. Zeeman splitting of torsional oscillation spectrum of magnetars was suggested, clearly described and estimated by Shaisultanov and Eichler (2009) but their work has not been given considerable attention. To extend it we suggest the technique of calculating oscillation frequencies including Zeeman splitting at not too strong magnetic fields for arbitrary magnetic field configuration. Zeeman splitting enriches the oscillation spectrum and simplifies theoretical interpretation of observations. We calculate several low-frequency oscillations of magnetars with pure dipole magnetic field in the crust. The results qualitatively agree with low-frequency QPOs detected in the hyperflare of SGR 1806--20, and in the giant flare of SGR 1900+14.

astro-ph.HE↗

arXiv2023

Neutrino emission of neutron-star superbursts

Superbursts of neutron stars are rare but powerful events explained by the explosive burning of carbon in the deep layers of the outer envelope of the star. In this paper we perform a simulation of superbursts and propose a simple method for describing the neutrino stage of their cooling, as well as a method for describing the evolution of the burst energy on a scale of several months. We note a universal relation for the temperature distribution in the burnt layer at its neutrino cooling stage, as well as the unification of bolometric light curves and neutrino heat loss rates for deep and powerful bursts. We point out the possibility of long-term retention of the burst energy in the star's envelope. The results can be useful for interpretation of superburst observations.

astro-ph.HE↗

arXiv2023

Nearly Forgotten Cosmological Concept of E. B. Gliner

E. B. Gliner started his scientific career in 1963 at the age of 40. In 1965, when the existence of the cosmological constant $λ$ seemed unnecessary to most cosmologists, he renewed interest in the problem by emphasizing a material interpretation of de Sitter space (i.e., the space curved in the presence of $λ$). According to that interpretation, the curvature is produced by a cosmological vacuum (now identified as dark energy of the universe). In 1970, Gliner proposed a description of exponential expansion (or contraction) of the universe at the early (or late) evolution stage dominated by cosmological vacuum. In 1975, Gliner (with I.G. Dyminikova) suggested a model of the early universe free of Big Bang singularity, and developed a scenario of nonsingular Friedmann cosmology. Many of these findings were used in the modern inflation scenarios of the universe, first proposed by A.A. Starobinsky (1979) and A. Guth (1981) and greatly multiplied later. However, these inflation scenarios differ from the scenario of Gliner and Dymnikova, and Gliner's contribution to cosmology is nearly forgotten. The history and the essence of this contribution are outlined, as well the difference from the inflation theories.

physics.hist-ph↗

arXiv2022

Selfsimilarity relations for torsional oscillations of neutron stars

Selfsimilarity relations for torsional oscillation frequencies of neutron star crust are discussed. For any neutron star model, the frequencies of fundamental torsional oscillations (with no nodes of radial wave function, i.e. at n=0, and at all possible angular wave numbers l >= 2) is determined by a single constant. Frequencies of ordinary torsional oscillations (at any n>0 with l >= 2) are determined by two constants. These constants are easily calculated through radial integrals over the neutron star crust, giving the simplest method to determine full oscillation spectrum. All constants for a star of fixed mass can be accurately interpolated for stars of various masses (but the same equation of state). In addition, the torsional oscillations can be accurately studied in the flat space-time approximation within the crust. The results can be useful for investigating magneto-elastic oscillations of magnetars which are thought to be observed as quasi-periodic oscillations after flares of soft-gamma repeaters.

astro-ph.SR↗

arXiv2021

A Simple Model of Radiation from a Magnetized Neutron Star: Accreted Matter and Polar Hotspots

A simple and well known model for thermal radiation spectra from a magnetized neutron star is further studied. The model assumes that the star is internally isothermal and possesses dipole magnetic field (B <= 1e14 G) in the outer heat-insulating layer. The heat transport through this layer makes the surface temperature distribution anisotropic; any local surface element is assumed to emit a blackbody (BB) radiation with a local effective temperature. It is shown that this thermal emission is nearly independent of the chemical composition of insulating envelope (at the same taken averaged effective surface temperature). Adding a slight extra heating of magnetic poles allows one to be qualitatively consistent with observations of some isolated neutron stars.

astro-ph.HE↗

arXiv2021

Two-blackbody portraits of radiation from magnetized neutron stars

We study a simple model describing thermal radiation spectra from magnetized neutron stars. The model assumes that a star is nearly spherical and isothermal inside and possesses dipole magnetic fields ($B \lesssim 10^{14}$ G) near the surface, which make the surface temperature distribution non--uniform. We assume further that any surface element emits a blackbody (BB) spectrum with a local effective temperature. We show that such thermal spectra (including phase--resolved) are accurately approximated by simple equivalent two--BB (2BB) models. We introduce and study phase--space maps of 2BB parameters and show that these maps can be useful for interpreting neutron star observations, in which 2BB spectral fits have been done.

astro-ph.HE↗

arXiv2021

Heat blanketing envelopes of neutron stars

Near the surface of any neutron star there is a thin heat blanketing envelope that produces substantial thermal insulation of warm neutron star interiors and that relates the internal temperature of the star to its effective surface temperature. Physical processes in the blanketing envelopes are reasonably clear but the chemical composition is not. The latter circumstance complicates inferring physical parameters of matter in the stellar interiors from observations of the thermal surface radiation of the stars and urges one to elaborate the models of blanketing envelopes. We outline physical properties of these envelopes, particularly, the equation of state, thermal conduction, ion diffusion and others. Various models of heat blankets are reviewed, such as composed of separate layers of different elements, or containing diffusive binary ion mixtures in or out of diffusion equilibrium. The effects of strong magnetic fields in the envelopes are outlined as well as the effects of high temperatures which induce strong neutrino emission in the envelopes themselves. Finally, we discuss how the properties of the heat blankets affect thermal evolution of neutron stars and the ability to infer important information on internal structure of neutron stars from observations.

astro-ph.SR↗

arXiv2020

Model of heat diffusion in the outer crust of bursting neutron stars

We study heat diffusion after an energy release in a deep spherical layer of the outer neutron star crust (10^7 < ρ< 4 x 10^{11} g/cm^3). We demonstrate that this layer possesses specific heat-accumulating properties, absorbing heat and directing it mostly inside the star. It can absorb up to about 10^{43}-10^{44} erg due to its high heat capacity, until its temperature exceeds T ~ 3 x 10^9 K and triggers a rapid neutrino cooling. A warm layer with T ~ 10^8 - 3 x 10^9 K can serve as a good heat reservoir, which is thermally decoupled from the inner crust and the stellar core for a few months. We present a toy model to explore the heat diffusion within the heat-accumulating layer, and we test this model using numerical simulations. We formulate some generic features of the heat propagation which can be useful, for instance, for the interpretation of superbursts in accreting neutron stars. We present a self-similar analysis of late afterglow after such superbursts, which can be helpful to estimate properties of bursting stars.

astro-ph.HE↗

arXiv2020

Deformed crystals and torsional oscillations of neutron star crust

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

astro-ph.HE↗

arXiv2020

Thermal luminosities of cooling neutron stars

Ages and thermal luminosities of neutron stars, inferred from observations, can be interpreted with the aid of the neutron star cooling theory to gain information on the properties of superdense matter in neutron-star interiors. We present a survey of estimated ages, surface temperatures and thermal luminosities of middle-aged neutron stars with relatively weak or moderately strong magnetic fields, which can be useful for these purposes. The catalogue includes results selected from the literature, supplemented with new results of spectral analysis of a few cooling neutron stars. The data are compared with the theory. We show that overall agreement of theoretical cooling curves with observations improves substantially for models where neutron superfluidity in stellar core is weak.

astro-ph.HE↗

arXiv2019

Quantum ion thermodynamics in liquid interiors of white dwarfs

We present an accurate analytic approximation for the energy of a quantum one-component Coulomb liquid of ions in a uniform electron background which has been recently calculated from first principles (Baiko 2019). The approximation enables us to develop in an analytic form a complete thermodynamic description of quantum ions in a practically important range of mass densities at temperatures above crystallization. We show that ionic quantum effects in liquid cores of white dwarfs (WDs) affect heat capacity, cooling, thermal compressibility, pulsation frequencies and radii of sufficiently cold WDs, especially with relatively massive helium and carbon cores.

astro-ph.SR↗

arXiv2018

Bulk viscosity in a neutron star mantle

We study the bulk viscosity in two (anti-spaghetti and Swiss cheese) phases of non- spherical nuclei in the mantle of an oscillating neutron star near the boundary with the stellar core. The bulk viscosity is produced by non-equilibrium Urca neutrino emis- sion processes. In the mantle, the direct Urca process may be open (Gusakov et al., 2004) if neutrons and protons move in a periodic potential created by a lattice of non- spherical nuclei (which allows the nucleons to have large quasi-momenta and satisfy direct Urca momentum-conservation). This bulk viscosity can dominate over that due to the modified Urca process in the outer stellar core and over the shear viscosity. The bulk viscosity depends strongly on temperature, oscillation frequency and nucleon superfluidity. The enhanced bulk viscosity in the mantle can control propagation and damping of neutron star oscillations.

astro-ph.HE↗

arXiv2018

Afterburst thermal relaxation in neutron star crusts

We study thermal relaxation in a neutron star after internal heating events (outbursts) in the crust. We consider thin and thick spherically symmetric heaters, superfluid and non-superfluid crusts, stars with open and forbidden direct Urca processes in their cores. In particular, we analyze long-term thermal relaxation after deep crustal heating produced by nuclear transformations in fully or partly accreted crusts of transiently accreting neutron stars. This long-term relaxation has a typical relaxation time and an overall finite duration time for the crust to thermally equilibrate with the core. Neutron superfluidity in the inner crust greatly affects the relaxation if the heater is located in the inner crust. It shortens and unifies the time of emergence of thermal wave from the heater to the surface. This is important for the interpretation of observed outbursts of magnetars and transiently accreting neutron stars in quasi-persistent low-mass X-ray binaries.

astro-ph.HE↗

arXiv2017

Neutrino luminosities and heat capacities of neutron stars in analytic form

We derive analytic approximations for the neutrino luminosities and the heat capacities of neutron stars with isothernal nucleon cores as functions of the mass and radius of stars. The neutrino luminosities are approximated for the three basic neutrino emission mechanisms, and the heat capacities for the five basic combinations of the partial heat capacities. The approximations are valid for for a wide class of equations of state of dense nucleonmatter. The results significantly simplify the theoretical interpretation of observations of cooling neutron stars as well as of quasistationary thermal states of neutron stars in X-ray transients. For illustration, we present an analysis of the neutrino cooling functions of nine isolated neutron stars taking into account the effects of their magnetic fields and of the presence of light elements in their heat blanketing envelopes. These results allow one to investigate the superfluid properties of neutron star cores.

astro-ph.HE↗

arXiv2016

Analytic approximations of neutrino luminosities and heat capacities of neutron stars with nucleon cores

We derive analytic approximations of neutrino luminosities and heat capacities of neutron stars with nucleon cores valid for a wide class of equations of state of dense nucleon matter. The neutrino luminosities are approximated for the three cases in which they are produced by (i) direct Urca or (ii) modified Urca processes in non-superfluid matter, or (iii) neutrino-pair bremsstrahlung in neutron-neutron collisions (when other neutrino reactions are suppressed by strong proton superfluidity). The heat capacity is approximated for the two cases of (i) non-superfluid cores and (ii) the cores with strong proton superfluidity. The results can greatly simplify numerical simulations of cooling neutron stars with isothermal interiors at the neutrino and photon cooling stages as well as simulations of quasi-stationary internal thermal states of neutron stars in X-ray transients. For illustration, a model-independent analysis of thermal states of the latter sources is outlined.

astro-ph.HE↗

arXiv2016

Cooling of neutron stars with diffusive envelopes

We study the effects of heat blanketing envelopes of neutron stars on their cooling. To this aim, we perform cooling simulations using newly constructed models of the envelopes composed of binary ion mixtures (H--He, He--C, C--Fe) varying the mass of lighter ions (H, He or C) in the envelope. The results are compared with those calculated using the standard models of the envelopes which contain the layers of lighter (accreted) elements (H, He and C) on top of the Fe layer, varying the mass of accreted elements. The main effect is that the chemical composition of the envelopes influences their thermal conductivity and, hence, thermal insulation of the star. For illustration, we apply these results to estimate the internal temperature of the Vela pulsar and to study the cooling of neutron stars of ages of 0.1 - 1 Myr at the photon cooling stage. The uncertainties of the cooling models associated with our poor knowledge of chemical composition of the heat insulating envelopes strongly complicate theoretical reconstruction of the internal structure of cooling neutron stars from observations of their thermal surface emission.

astro-ph.HE↗