arXiv subjects

M. E. Gusakov

Publications and source records attributed to M. E. Gusakov.

At least 19 recordsLinked to original sources

arXiv2025

Instability windows of relativistic r-modes in stably stratified neutron stars with hyperonic cores

(abridged) $R$-modes are oscillations in rotating stars, primarily restored by the Coriolis force. These oscillations are the most susceptible to the Chandrasekhar-Friedman-Schutz (CFS) instability driven by gravitational wave emission, which makes them promising targets for current and future gravitational wave searches. In order to develop, the instability must overcome dissipative processes within the star. As a result, $r$-modes become unstable only for certain combinations of stellar angular velocity $Ω$ and (redshifted) temperature $T^\infty$, defining the so-called instability window on the $(Ω, T^\infty)$ plane. At high temperatures, bulk viscosity $ζ$, arising from out-of-equilibrium chemical reactions, is the dominant dissipative agent. Dissipation due to $ζ$ can be greatly enhanced by two independent mechanisms: (1) the presence of hyperons, which significantly increases the bulk viscosity, and (2) the distinctive properties of relativistic $r$-modes in nonbarotropic matter, which further amplify dissipation beyond Newtonian predictions. In this work, we present the first investigation of the combined impact of these mechanisms on $r$-mode instability windows. Our calculations also account for the fact that chemical reactions modify the adiabatic index, in addition to producing bulk viscosity. We further estimate the influence of nucleon pairing effects on the instability windows. By comparing our predictions with recent observations of neutron stars in low-mass X-ray binaries, we find that bulk viscosity in hyperonic matter may provide the necessary dissipation to stabilize $r$-modes in the fastest-spinning and moderately hot stars, even when nucleon superfluidity and superconductivity are taken into account. These results have important implications for the interpretation of observations and for the broader understanding of relativistic $r$-mode physics.

astro-ph.HE↗

arXiv2025

Magnetothermal evolution of neutron star cores in the `weak-coupling' regime: implications of ambipolar diffusion for the quiescent X-ray luminosity of magnetars

The high quiescent X-ray luminosity observed in some magnetars is widely attributed to the decay and evolution of their ultra-strong magnetic fields. Several dissipation mechanisms have been proposed, each operating with different efficiencies depending on the region of the star. In this context, ambipolar diffusion, i.e., the relative motion of charged particles with respect to neutrons in the neutron star core, has been proposed as a promising candidate due to its strong dependence on magnetic field strength and its capacity to convert magnetic energy into heat. We perform axisymmetric magnetohydrodynamic simulations to study the long-term magnetic evolution of a NS core composed of normal (non-Cooper paired) matter under the influence of ambipolar diffusion. The core is modeled as a two-fluid system consisting of neutrons and a charged-particle fluid (protons and electrons), coupled to the magnetic field. Simulations are performed both at constant and variable temperatures. In the latter case, a strategy that decouples the magnetic and thermal evolution is employed, enabling efficient thermal modeling across a range of initial magnetic field strengths. At constant temperature, we obtained the expected result where neutrons reach diffusive equilibrium, the Lorentz force is balanced by chemical potential gradients of charged particles, and the magnetic field satisfies a non-linear Grad-Shafranov equation. When thermal evolution is included, fields $B \gtrsim 5 \times 10^{15} \,\text{G}$ can balance ambipolar heating and neutrino cooling, delaying the evolution over $\sim 10^{3} \,[B/(5 \times 10^{15}\,\text{G})]^{-6/5}$ yr. Although the surface luminosity is enhanced compared to passive cooling, the heating from ambipolar diffusion alone is insufficient to fully explain the persistent X-ray emission observed in magnetars.

astro-ph.HE↗

arXiv2025

Validating and improving two-fluid simulations of the magnetic field evolution in neutron star cores

This paper addresses the evolution of an axially symmetric magnetic field in the core of a neutron star. The matter in the core is modeled as a system of two fluids, namely neutrons and charged particles, with slightly different velocity fields, controlled by their mutual collisional friction. This problem was addressed in our previous work through the so-called ``fictitious friction'' approach. We study the validity of our previous work and improve it by comparing the fictitious friction approach to alternatives, making approximations that allow it to be applied to arbitrary magnetic field strengths and using realistic equations of state. We assume the neutron star crust to be perfectly resistive, so its magnetic field reacts instantaneously to changes in the core, in which we neglect the effects of Cooper pairing. We explore different approaches to solve the equations to obtain the velocities and chemical potential perturbations induced by a given, fixed magnetic field configuration in the core. We also present a new version of our code to perform time-evolving simulations and discuss the results obtained with it. Our calculations without fictitious friction further confirm that bulk velocity is generally much greater than ambipolar velocity, leading to faster evolution. These findings align with those with fictitious friction, validating this approach. We also find that, in the long term, the star evolves towards a barotropic ``Grad-Shafranov equilibrium,'' where the magnetic force is fully balanced by charged particle fluid forces. Qualitatively, the evolution and the final equilibrium are independent of the magnetic field strength $B$ and the equation of state considered. The timescale to reach this equilibrium is proportional to $B^{-2}$ and becomes shorter for equations of state with a smaller gradient of the ratio between the densities of protons and neutrons.

astro-ph.HE↗

arXiv2024

Cooling of neutron stars in soft X-ray transients with realistic crust composition

Thermal radiation of neutron stars in soft X-ray transients (SXTs) in a quiescent state is believed to be powered by the heat deposited in the stellar crust due to nuclear reactions during accretion. Confronting observations of this radiation with simulations helps to verify theoretical models of the dense matter in neutron stars. We simulate the thermal evolution of the SXTs with theoretical models of the equation of state and composition of the accreted crust. The new family of such models were recently developed within a thermodynamically consistent approach by modeling the nuclear evolution of an accreted matter as it sinks toward the stellar center, starting from representative thermonuclear ash compositions. The crust cooling curves computed with the traditional and modern theory are compared with observations of SXTs MXB 1659-29 and IGR J17480-2446. We show that the new and traditional models of the accreted neutron star crusts are similar in their capability to explain the thermal evolution of neutron stars in SXTs. Both kinds of models require inclusion of additional ingredients not supplied by the current theory, such as the shallow heating and variation of thermal conductivity, to fit observations.

astro-ph.HE↗

arXiv2023

Thermal evolution of neutron stars in soft X-ray transients with thermodynamically consistent models of the accreted crust

Thermal emission of neutron stars in soft X-ray transients (SXTs) in a quiescent state is believed to be powered by the heat deposited in the stellar crust due to nuclear reactions during accretion (deep crustal heating paradigm). Confronting observations of SXTs with simulations helps to verify theoretical models of the dense matter in the neutron stars. Usually, such simulations were carried out assuming that the free neutrons and nuclei in the inner crust move together. A recently proposed thermodynamically consistent approach allows for independent motion of the free neutrons. We simulate the thermal evolution of the SXTs within the thermodynamically consistent approach and compare the results with the traditional approach and with observations. For the latter, we consider a collection of quasi-equilibrium thermal luminosities of the SXTs in quiescence and the observed neutron star crust cooling in SXT MXB 1659$-$29. We test different models of the equation of state and baryon superfluidity and take into account additional heat sources in the shallow layers of neutron-star crust (the shallow heating). We find that the observed quasi-stationary thermal luminosities of the SXTs can be equally well fitted using the traditional and thermodynamically consistent models, provided that the shallow heat diffusion into the core is taken into account. The observed crust cooling in MXB 1659$-$29 can also be fitted in the frames of both models, but the choice of the model affects the derived parameters responsible for the thermal conductivity in the crust and for the shallow heating.

astro-ph.HE↗

arXiv2022

Observability of HOFNARs with SRG/eROSITA

Neutron stars can appear as sources of different nature. In this paper we address observability of a hypothetical class of neutron stars -- HOt and Fast Non Accreting Rotators, HOFNARs. These objects are heated due to the r-mode instability. With surface temperatures $\sim 10^6$~K they are expected to be thermal soft X-ray emitters. We perform a population synthesis modeling of HOFNARs to predict the number of potentially detectable sources in the eROSITA all-sky survey. For surface temperatures $\sim 10^6$~K we obtain $\sim 500$ sources above the detection limit 0.01~cts~s$^{-1}$ and $\sim 100$ easier identifiable sources with $>0.1$~cts~s$^{-1}$. Temperatures $\gtrsim 1.2\times 10^6$~K start to be in contradiction with non-detection of HOFNARs by ROSAT. Only for $T\lesssim 5\times 10^5$~K numbers predicted for eROSITA turn out to be so low that identification does not look possible. We conclude that eROSITA has good chances to discover HOFNARs, if they exist. Non-detection will put very stringent limits on the properties of this type of neutron stars.

astro-ph.HE↗

arXiv2021

Diffusion as a leading dissipative mechanism in superconducting neutron stars

Despite the fact that different particle species can diffuse with respect to each other in neutron star (NS) cores, the effect of particle diffusion on various phenomena associated with NS oscillations is usually ignored. Here we demonstrate that the diffusion can be extremely powerful dissipative mechanism in superconducting NSs. In particular, it can be much more efficient than the shear and bulk viscosities. This result has important implications for the damping times of NS oscillations, development and saturation of dynamical instabilities in NSs, and for the excitation and coupling of oscillation modes during the late inspiral of binary NSs.

astro-ph.HE↗

arXiv2019

Bulk viscosity in neutron stars with hyperon cores

It is well-known that r-mode oscillations of rotating neutron stars may be unstable with respect to the gravitational wave emission. It is highly unlikely to observe a neutron star with the parameters within the instability window, a domain where this instability is not suppressed. But if one adopts the `minimal' (nucleonic) composition of the stellar interior, a lot of observed stars appear to be within the r-mode instability window. One of the possible solutions to this problem is to account for hyperons in the neutron star core. The presence of hyperons allows for a set of powerful (lepton-free) non-equilibrium weak processes, which increase the bulk viscosity, and thus suppress the r-mode instability. Existing calculations of the instability windows for hyperon NSs generally use reaction rates calculated for the $Σ^-Λ$ hyperonic composition via the contact $W$ boson exchange interaction. In contrast, here we employ hyperonic equations of state where the $Λ$ and $Ξ^-$ are the first hyperons to appear (the $Σ^-$'s, if they are present, appear at much larger densities), and consider the meson exchange channel, which is more effective for the lepton-free weak processes. We calculate the bulk viscosity for the non-paired $npeμΛΞ^-$ matter using the meson exchange weak interaction. A number of viscosity-generating non-equilibrium processes is considered (some of them for the first time in the neutron-star context). The calculated reaction rates and bulk viscosity are approximated by simple analytic formulas, easy-to-use in applications. Applying our results to calculation of the instability window, we argue that accounting for hyperons may be a viable solution to the r-mode problem.

astro-ph.HE↗

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

A note on the ambipolar diffusion in superfluid neutron stars

We address the problem of magnetic field dissipation in the neutron star cores, focusing on the role of neutron superfluidity. Contrary to the results in the literature, we show that in the finite-temperature superfluid matter composed of neutrons, protons, and electrons, magnetic field dissipates exclusively due to Ohmic losses and non-equilibrium beta-processes, and only an admixture of muons restores (to some extent) the role of particle relative motion for the field dissipation. The reason for this discrepancy is discussed.

astro-ph.HE↗

arXiv2018

Evolution of the magnetic field in neutron stars

We propose a general method to self-consistently study the quasistationary evolution of the magnetic field in the cores of neutron stars. The traditional approach to this problem is critically revised. Our results are illustrated by calculation of the typical timescales for the magnetic field dissipation as functions of temperature and the magnetic field strength.

astro-ph.HE↗

arXiv2018

Fast magnetic field evolution in neutron stars: the key role of magnetically induced fluid motions in the core

In [Gusakov et al.\ PRD, 96, 103012, (2017)], we proposed a self-consistent method to study the quasistationary evolution of the magnetic field in neutron-star cores. Here we apply it to calculate the instantaneous particle velocities and other parameters of interest, which are fixed by specifying the magnetic field configuration. Interestingly, we found that the magnetic field can lead to generation of a macroscopic fluid motion with the velocity, significantly exceeding the diffusion particle velocities. This result calls into question the standard view on the magnetic field evolution in neutron stars and suggests a new, shorter timescale for such evolution.

astro-ph.HE↗

arXiv2015

Oscillations of superfluid hyperon stars: decoupling scheme and g-modes

We analyse the oscillations of general relativistic superfluid hyperon stars, following the approach suggested by Gusakov & Kantor and Gusakov et al. and generalizing it to the nucleon-hyperon matter. We show that the equations governing the oscillations can be split into two weakly coupled systems with the coupling parameters $s_{\rm e}$, $s_{\rm μ}$, and $s_{\rm str}$. The approximation $s_{\rm e} = s_{\rm μ} = s_{\rm str} = 0$ (decoupling approximation) allows one to drastically simplify the calculations of stellar oscillation spectra. An efficiency of the presented scheme is illustrated by the calculation of sound speeds in the nucleon-hyperon matter composed of neutrons (n), protons (p), electrons (e), muons ($μ$), as well as $\rm Λ$, ${\rm Ξ}^-$, and ${\rm Ξ}^0$-hyperons. However, the gravity oscillation modes (g-modes) cannot be treated within this approach, and we discuss them separately. For the first time we study the composition g-modes in superfluid hyperon stars with the $\rm npeμΛ$ core and show that there are two types of g-modes (`muonic' and `$Λ$--hyperonic') in such stars. We also calculate the g-mode spectrum and find out that the eigenfrequencies $ν$ of the superfluid g-modes can be exceptionally large (up to $ν\approx 742~{\rm Hz}$ for a considered stellar model).

astro-ph.SR↗

arXiv2015

Rotation-induced deep crustal heating of millisecond pulsars

The spin-down of a neutron star, e.g. due to magneto-dipole losses, results in compression of the stellar matter and induces nuclear reactions at phase transitions between different nuclear species in the crust. We show that this mechanism is effective in heating recycled pulsars, in which the previous accretion process has already been compressing the crust, so it is not in nuclear equilibrium. We calculate the corresponding emissivity and confront it with available observations, showing that it might account for the likely thermal ultraviolet emission of PSR J0437-4715.

astro-ph.HE↗

arXiv2014

Quasi-normal modes of superfluid neutron stars

We study non-radial oscillations of neutron stars with superfluid baryons, in a general relativistic framework, including finite temperature effects. Using a perturbative approach, we derive the equations describing stellar oscillations, which we solve by numerical integration, employing different models of nucleon superfluidity, and determining frequencies and gravitational damping times of the quasi-normal modes. As expected by previous results, we find two classes of modes, associated to superfluid and non-superfluid degrees of freedom, respectively. We study the temperature dependence of the modes, finding that at specific values of the temperature, the frequencies of the two classes of quasi-normal modes show avoided crossings, and their damping times become comparable. We also show that, when the temperature is not close to the avoided crossings, the frequencies of the modes can be accurately computed by neglecting the coupling between normal and superfluid degrees of freedom. Our results have potential implications on the gravitational wave emission from neutron stars.

gr-qc↗

arXiv2014

Composition temperature-dependent g-modes in superfluid neutron stars

We demonstrate a possibility of existence of a peculiar temperature-dependent composition $g$-modes in superfluid neutron stars. We calculate the Brunt-V$\ddot{\rm a}$is$\ddot{\rm a}$l$\ddot{\rm a}$ frequency for these modes, as well as their eigenfrequencies. The latter turn out to be rather large, up to $\sim 500$ Hz for a chosen model of a neutron star. This result indicates, in particular, that use of the barotropic equation of state may be not a good approximation for calculation of inertial modes even in most rapidly rotating superfluid neutron stars.

astro-ph.SR↗

arXiv2014

Physics input for modelling superfluid neutron stars with hyperon cores

Observations of massive ($M \approx 2.0~M_\odot$) neutron stars (NSs), PSRs J1614-2230 and J0348+0432, rule out most of the models of nucleon-hyperon matter employed in NS simulations. Here we construct three possible models of nucleon-hyperon matter consistent with the existence of $2~M_\odot$ pulsars as well as with semi-empirical nuclear matter parameters at saturation, and semi-empirical hypernuclear data. Our aim is to calculate for these models all the parameters necessary for modelling dynamics of hyperon stars (such as equation of state, adiabatic indices, thermodynamic derivatives, relativistic entrainment matrix, etc.), making them available for a potential user. To this aim a general non-linear hadronic Lagrangian involving $σωρϕσ^\ast$ meson fields, as well as quartic terms in vector-meson fields, is considered. A universal scheme for calculation of the $\ell=0,1$ Landau Fermi-liquid parameters and relativistic entrainment matrix is formulated in the mean-field approximation. Use of this scheme allow us to obtain numerical tables with the equation of state, Landau quasiparticle effective masses, adiabatic indices, the $\ell=0,1$ Landau Fermi-liquid parameters, and the relativistic entrainment matrix for the selected models of nucleon-hyperon matter. These data are available on-line and suitable for numerical implementation in computer codes modelling various dynamical processes in NSs, in particular, oscillations of superfluid NSs and their cooling.

astro-ph.HE↗

arXiv2012

New class of g-modes and unexpected convection in neutron stars

We suggest a specific new class of low-frequency g-modes in superfluid neutron stars. We determine the Brunt-Vaisala frequency for these modes and demonstrate that they can be unstable with respect to convection. The criterion for the instability onset (analogue of the well known Schwarzschild criterion) is derived. It is very sensitive to equation of state and a model of nucleon superfluidity. In particular, convection may occur for both positive and negative temperature gradients. Our results have interesting implications for neutron star cooling and seismology.

astro-ph.SR↗