arXiv subjects

E. M. Kantor

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

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↗

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↗

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↗

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↗

arXiv2012

Dissipation in relativistic superfluid neutron stars

We analyze damping of oscillations of general relativistic superfluid neutron stars. To this aim we extend the method of decoupling of superfluid and normal oscillation modes first suggested in [Gusakov & Kantor PRD 83, 081304(R) (2011)]. All calculations are made self-consistently within the finite temperature superfluid hydrodynamics. The general analytic formulas are derived for damping times due to the shear and bulk viscosities. These formulas describe both normal and superfluid neutron stars and are valid for oscillation modes of arbitrary multipolarity. We show that: (i) use of the ordinary one-fluid hydrodynamics is a good approximation, for most of the stellar temperatures, if one is interested in calculation of the damping times of normal f-modes; (ii) for radial and p-modes such an approximation is poor; (iii) the temperature dependence of damping times undergoes a set of rapid changes associated with resonance coupling of neighboring oscillation modes. The latter effect can substantially accelerate viscous damping of normal modes in certain stages of neutron-star thermal evolution.

astro-ph.SR↗

arXiv2012

Velocity-dependent energy gaps and dynamics of superfluid neutron stars

We show that suppression of the baryon energy gaps, caused by the relative motion of superfluid and normal liquid components, can substantially influence dynamical properties and evolution of neutron stars. This effect has been previously ignored in the neutron-star literature.

astro-ph.SR↗

arXiv2012

Decoupling of superfluid and normal modes in pulsating neutron stars

We show that equations governing pulsations of superfluid neutron stars can be splitted into two sets of weakly coupled equations, one describing the superfluid modes and another one -- the normal modes. The coupling parameter s is small, |s| ~ 0.01-0.05, for realistic equations of state. Already an approximation s=0 is sufficient to calculate the pulsation spectrum within the accuracy of a few percents. Our results indicate, in particular, that emission of gravitational waves from superfluid pulsation modes is suppressed in comparison to that from normal modes. The proposed approach allows to drastically simplify modeling of pulsations of superfluid neutron stars.

astro-ph.SR↗

arXiv2011

Temperature effects in pulsating superfluid neutron stars

We study the effects of finite stellar temperatures on the oscillations of superfluid neutron stars. The importance of these effects is illustrated with a simple example of a radially pulsating general relativistic star. Two main effects are taken into account: (i) temperature dependence of the entrainment matrix and (ii) the variation of the size of superfluid region with temperature. Four models are considered, which include either one or both of these two effects. Pulsation spectra are calculated for these models, and asymptotes for eigenfrequencies at temperatures close to critical temperature of neutron superfluidity, are derived. It is demonstrated that models that allow for the temperature effect (ii) but disregard the effect (i), yield unrealistic results. Eigenfunctions for the normal- and superfluid-type pulsations are analyzed. It is shown that superfluid pulsation modes practically do not appear at the neutron-star surface and, therefore, can hardly be observed by measuring the modulation of the electromagnetic radiation from the star. The e-folding times for damping of pulsations due to the shear viscosity and nonequilibrium modified Urca processes are calculated and their asymptotes at temperatures close to the neutron critical temperature, are obtained. It is demonstrated that superfluid pulsation modes are damped by 1--3 orders of magnitude faster than normal modes.

astro-ph.SR↗

arXiv2009

The relativistic entrainment matrix of a superfluid nucleon-hyperon mixture. II. Effect of finite temperatures

We calculate the important quantity of superfluid hydrodynamics, the relativistic entrainment matrix for a nucleon-hyperon mixture at arbitrary temperature. In the nonrelativistic limit this matrix is also termed the Andreev-Bashkin or mass-density matrix. Our results can be useful for modeling the pulsations of massive neutron stars with superfluid nucleon-hyperon cores and for studies of the kinetic properties of superfluid baryon matter.

astro-ph.SR↗

arXiv2009

The relativistic entrainment matrix of a superfluid nucleon-hyperon mixture at zero temperature

We calculate the relativistic entrainment matrix Y_ik at zero temperature for nucleon-hyperon mixture composed of neutrons, protons, Lambda and Sigma^- hyperons, as well as of electrons and muons. This matrix is analogous to the entrainment matrix (also termed mass-density matrix or Andreev-Bashkin matrix) of non-relativistic theory. It is an important ingredient for modelling the pulsations of massive neutron stars with superfluid nucleon-hyperon cores. The calculation is done in the frame of the relativistic Landau Fermi-liquid theory generalized to the case of superfluid mixtures; the matrix Y_ik is expressed through the Landau parameters of nucleon-hyperon matter. The results are illustrated with a particular example of the sigma-omega-rho mean-field model with scalar self-interactions. Using this model we calculate the matrix Y_ik and the Landau parameters. We also analyze stability of the ground state of nucleon-hyperon matter with respect to small perturbations.

astro-ph.HE↗

arXiv2008

Bulk viscosity of superfluid hyperon stars

We calculated bulk viscosity due to non-equilibrium weak processes in superfluid nucleon-hyperon matter of neutron stars. For that, the dissipative relativistic hydrodynamics, formulated in paper [1] for superfluid mixtures, was extended to the case when both nucleons and hyperons are superfluid. It was demonstrated that in the most general case (when neutrons, protons, Lambda, and Sigma^{-} hyperons are superfluid), non-equilibrium weak processes generate sixteen bulk viscosity coefficients, with only three of them being independent. In addition, we corrected an inaccuracy in a widely used formula for the bulk viscosity of non-superfluid nucleon-hyperon matter.

astro-ph↗