arXiv subjects

Mikhail V. Beznogov

Publications and source records attributed to Mikhail V. Beznogov.

16 recordsLinked to original sources

arXiv2026

Thermal evolution of neo-neutron stars. II. Temperature-dependent crusts

The neo-neutron star phase is an intermediate stage in the evolution from a hot lepton-rich compact object, which is formed in the aftermath of a core-collapse supernova or an accretion-induced collapse of a white dwarf, to a cold deleptonized neutron star (NS). Alternatively, this phase can occur after a binary NS merger if the final compact object does not collapse immediately into a black hole. Radial temperature profiles provided by numerical simulations of proto-NS evolution suggest that the composition and mechanical structure of the star's core at the beginning of the neo-NS phase are, as a good approximation, identical to those of a cold NS. In contrast, the composition and mechanical structure of the outer layers (i.e., the crust) are expected to change as the star cools. The thermal evolution of neo-NSs with temperature-dependent (inner) envelopes was considered by Beznogov et al. (2020). Here, we investigated what happens if the equation of state (EoS) of the entire crust, including its composition, depends on temperature. We analyzed the thermal, mechanical, and chemical evolution of neo-NSs by further developing and employing neo-NSCool, our NS thermal evolution code. First, we proved that thermally dripped neutrons slightly slow down the cooling during the early thermal relaxation stage. Then, we showed that the EoSs with exotic light species in the deepest layers of the crust result in significantly slower cooling in the photon cooling era compared with the EoSs that favor massive nuclei. The crust's composition also impacts the crystallization process as well as the way in which the crust contracts while it cools. The EoSs with light nuclei cause the innermost layers to remain liquid for a longer time than the EoSs with heavy nuclei.

astro-ph.HE↗

arXiv2026

Bayesian inference of the dense matter equation of state built upon extended Skyrme interactions: A generalization

The nonrelativistic theory of nuclear matter (NM) based on Brussels-Skyrme interactions is employed to develop models for dense and neutron-rich matter within a Bayesian framework. We employ the following set of constraints: the four best-known nuclear empirical parameters, density dependence of the energy per particle in pure neutron matter (PNM), density dependence of the Landau effective mass ($m_\mathrm{eff}$) of neutrons in PNM and symmetric NM, and a lower limit on the maximum gravitational mass that neutron stars (NSs) can sustain. In addition, a number of ``sanity checks'' are added: the values of the speed of sound, neutron and proton Landau effective masses and Fermi velocities are constrained up to the central density of the most massive NS configuration and for isospin asymmetries $δ=(n_n-n_p)/(n_n+n_p)$ ranging from 0 to 1. Our ensemble of models \emph{fully} explores the capacity of non-relativistic Brussels-Skyrme effective interactions to describe NM at densities exceeding several times the nuclear saturation density. This is a necessary step toward a better understanding of the properties of dense matter and possible correlations between the parameters of NSs and the parameters of NM. Due to pronounced U-shaped density-dependencies of $m_\mathrm{eff}$, all our models exhibit a non-monotonic ``rise-and-fall'' behavior of the thermal pressure ($p_\mathrm{th}$) as a function of density, which in extreme cases leads to $p_\mathrm{th} < 0$. This work is a generalization of [Beznogov and Raduta, Phys. Rev. C 110, 035805 (2024)].

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arXiv2025

On the Inner Crusts of Neo-Neutron Stars: exotic light nuclei, diffusional and thermodynamical stability

Based on an extended nuclear statistical equilibrium model, we investigate the properties of non-accreted crusts of young and warm neo-neutron stars, i.e., of finite-temperature inhomogeneous dense matter in beta equilibrium. An interesting feature is the appearance, in the deep inner crust, of an extensive and almost pure layer of neutron-rich light nuclei that extends up to the density of the transition to homogeneous matter. Most probably, this layer emerges due to translational degrees of freedom of the nuclei. If confirmed, it will significantly impact the transport and elastic properties of the crust and its crystallization process. Then, we demonstrate that our inner crust is stable with respect to the diffusion of ions, which is in contrast with some of the predictions made in the literature for cold crusts. Finally, we show that clusterization completely exhausts the density instabilities that affect sub-saturated nuclear matter.

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arXiv2025

New ab initio constrained extended Skyrme equations of state for simulations of neutron stars, supernovae and binary mergers: II. Thermal response in the suprasaturation density domain

Numerical simulations of core-collapse supernovae, mergers of binary neutron stars and formation of stellar black holes, which employed standard Skyrme interactions, established clear correlations between the evolution of these processes, characteristics of the hot compact objects, as well as neutrino and gravitational wave signals, and the value of effective nucleon mass at the saturation density. Unfortunately, the density dependence of the effective mass of nucleons in these models does not align with the predictions of ab initio models with three body forces. In this work, we investigate the thermal response for a set of extended Skyrme interactions that feature widely different density dependencies of the effective mass of the nucleons. Thermal contributions to the energy density and pressure are studied along with a few thermal coefficients over wide domains of density, temperature and isospin asymmetry, relevant for the physics of hot compact objects. For some of the effective interactions, the thermal pressure is negative at high densities. This results in a situation where hot compact stars can support less mass before collapsing into a black hole compared to their cold counterparts. Moreover, the higher the temperature, the lower the maximum mass that the hot star can support.

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arXiv2025

New {\em ab initio} constrained extended Skyrme equations of state for simulations of neutron stars, supernovae and binary mergers: I. Subsaturation density domain

In numerical simulations of core-collapse supernova and binary neutron stars mergers, information about the energetics and composition of matter is implemented via external tables covering the huge ranges of thermodynamic conditions explored during the astrophysical evolution. More than 120 general-purpose equation of state (EOS) tables have been contributed so far. Unfortunately, not all of them comply with current constraints from theoretical and experimental nuclear physics and astrophysical observations of neutron stars. Systematic investigations of the role that dense matter properties play in the evolution of these astrophysical phenomena require that more EOS tables are provided. We build a set of general-purpose EOS tables. At zero temperature, they comply with all currently accepted constraints, including {\em ab initio} chiral effective field theory calculations of pure neutron matter. This set is designed to explore a wide variety of the behaviors of the effective masses as functions of density, which is reflected into a wide range of thermal behaviors. We employ Brussels extended Skyrme interactions generated by means of Bayesian inference techniques. An extended nuclear statistical equilibrium model is developed for modeling sub-saturated inhomogeneous nuclear matter (NM). Here, we study the properties of sub-saturated inhomogeneous NM over wide ranges of density, temperature, and proton fraction. We analyze in detail the mechanisms of transition to homogeneous matter and estimate the transition density. Our key results include a thick layer of neutron rich isotopes of He or H in the inner crusts of neo-neutron stars, significant abundance of exotic isotopes of H and He in warm and neutron-rich matter and a detailed study of the thermodynamic stability of cold stellar matter. The EOS tables are publicly available in the \textsc{CompOSE} online database.

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arXiv2024

Bayesian inference of the dense matter equation of state built upon extended Skyrme interactions

The non-relativistic model of nuclear matter with Brussels extended Skyrme interactions is employed in order to build, within a Bayesian approach, models for the dense matter equation of state (EOS). In addition to a minimal set of constraints on nuclear empirical parameters; the density behavior of the energy per particle in pure neutron matter (PNM); a lower limit on the maximum neutron star (NS) mass, we require that the Fermi velocity of neutrons ($v_{\mathrm{F;\,n}}$) in PNM and symmetric nuclear matter (SNM) with densities up to $0.8~\mathrm{fm}^{-3}$ (arbitrary) does not exceed the speed of light. The latter condition is imposed in order to cure a deficiency present in many Skyrme interactions [Duan and Urban, Phys. Rev. C 108, 025813 (2023)]. We illustrate the importance of this constraint for the posterior distributions. Some of our models are subjected to constraints on the density dependence of neutron (nucleon) Landau effective mass in PNM (SNM), too. The impact of various sets of constraints on the behaviors of nuclear matter and NSs is discussed in detail. Systematic comparison with results previously obtained by employing Skyrme interactions is done for posteriors of both nuclear matter (NM) and NS parameters. Special attention is given to the model and constraints dependence of correlations among various quantities.

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arXiv2024

Bayesian inference of thermal effects in dense matter within the covariant density functional theory

The high temperatures reached in a proto-neutron star or during the post-merger phase of a binary neutron star coalescence lead to non-negligible thermal effects on the equation of state (EOS) of dense nuclear matter. Here we study these effects within the covariant density functional theory employing the posteriors of a Bayesian inference, which encompasses a large sample of EOS models. Different densities and temperatures are considered. We find that for a number of quantities thermal effects are strongly correlated with the Dirac effective mass ($m^*$) of the nucleons and/or its logarithmic derivative as a function of density. These results can be explained within the low temperature approximation though they survive beyond this limit.

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arXiv2024

Bayesian Survey of the Dense Matter Equation of State built upon Skyrme effective interactions

The non-relativistic model of nuclear matter with zero-range Skyrme interactions is employed within a Bayesian approach in order to study the behavior of neutron stars (NSs) equation of state (EOS). A minimal number of constraints from nuclear physics and ab initio calculations of pure neutron matter (PNM) are imposed together with causality and a lower limit on the maximum mass of NS to all our models. Our key result is that accounting for correlations among the values that the energy per neutron in PNM takes at various densities, and that are typically disregarded, efficiently constrains the behavior of the EOS at high densities. A series of global NS properties, e.g., maximum mass, central density of the maximum mass configuration, minimum NS mass that allows for direct URCA, radii of intermediate and massive NS, appear to be correlated with the value of effective neutron mass in PNM at 0.16 fm$^{-3}$. Together with similar studies in the literature our work contributes to a better understanding of the NS EOS as well as its link with the properties of dense nuclear matter.

astro-ph.HE↗

arXiv2023

Frequencies of $f$- and $p$-oscillation modes in cold and hot compact stars

A large collection of equations of state (EOSs) built within the covariant density functional (CDF) theory of hadronic matter and allowing for density dependent (DD) couplings is employed to study polar $f$- and $p$- oscillations of cold and hot compact stars. Correlations between oscillation frequencies of cold purely nucleonic neutron stars (NSs), their global parameters as well as properties of nuclear matter (NM) are investigated by considering a set of models from Beznogov and Raduta, [Phys.~Rev.~C 107, 045803 (2023)], where a number of constraints on the saturation properties of NM, pure neutron matter (PNM) and the lower bound of the maximal NS mass were imposed within a Bayesian framework. The roles of finite temperature and exotic particle degrees of freedom, e.g., hyperons, $Δ$-resonances, anti-kaon condensates or a hadron to quark phase transition, are addressed by employing a family of models publicly available on \textsc{CompOSE} and assuming idealized profiles of temperature or entropy per baryon and charge fraction. We find that finite temperature effects reduce the oscillation frequencies of nucleonic stars while the opposite effect is obtained for stars with exotic particle degrees of freedom. When the $Γ$-law is employed to build finite temperature EOSs, errors in estimating oscillation modes frequencies are of the order of 10\% to 30\%, depending on the mass. Throughout this work the Cowling approximation is used.

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arXiv2023

Bayesian Inference of the Dense Matter Equation of State built upon Covariant Density Functionals

A modified version of the density dependent covariant density functional model proposed in [T. Malik, M. Ferreira, B. K. Agrawal and C. Providência, ApJ 930, 17 (2022)] is employed in a Bayesian analysis to determine the equation of state (EOS) of dense matter with nucleonic degrees of freedom. Various constraints from nuclear physics and microscopic calculations of pure neutron matter (PNM) along with a lower bound on the maximum mass of neutron stars (NSs) are imposed on the EOS models to investigate the effectiveness of progressive incorporation of the constraints, their compatibility as well as correlations among parameters of nuclear matter and properties of NSs. Our results include the different roles played by pressure and energy per particle of PNM in constraining the isovector behavior of nuclear matter; tension with the values of Dirac effective mass extracted from spin-orbit splitting; correlations between the radius of the canonical mass NS and second and third order coefficients in the Taylor expansion of energy per particle as a function of density; correlation between the central pressure of the maximum mass configuration and Dirac effective mass of the nucleon at saturation. For some of our models the tail of the NS maximum mass reaches $2.7~\mathrm{M}_{\odot}$, which means that the secondary object in GW190814 could have been a NS.

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arXiv2022

Standard cooling of rapidly rotating isolated neutron stars in 2D

We study the thermal evolution of axisymmetric rotating neutron stars in full general relativity. To this aim we develop "NSCool 2D Rot", a major upgrade of the 1D neutron stars thermal evolution code "NSCool" by D. Page. As a first application of our new code we address the standard cooling of isolated neutron stars with rotation frequencies up to the mass shedding limit. We investigate the effects of the equation of state (EOS) by considering different combinations of core and crust EOSs. The results indicate complex time-dependent evolution of temperature distribution throughout the whole volume of the star and, in particular, in the crust. We show that most of that complexity can be attributed to the formation of a "heat blob" in the crust and to the latitude dependence of the heat diffusion timescale through the crust.

astro-ph.HE↗

arXiv2022

Probing Strong Field $f(R)$ Gravity and Ultra-Dense Matter with the Structure and Thermal Evolution of Neutron Stars

Thermal evolution of neutron stars is studied in the $f(R)=R+αR^{2}$ theory of gravity. We first describe the equations of stellar structure and evolution for a spherically symmetric spacetime plus a perfect fluid at rest. We then present numerical results for the structure of neutron stars using four nucleonic dense matter equations of state and a series of gravity theories for $α$ ranging from zero, i.e., General Relativity, up to $α\approx 10^{16}$ cm$^2$. We emphasize properties of these neutron star models that are of relevance for their thermal evolution as the threshold masses for enhanced neutrino emission by the direct Urca process, the proper volume of the stellar cores where this neutrino emission is allowed, the crust thickness, and the surface gravitational acceleration that directly impact the observable effective temperature. Finally, we numerically solve the equations of thermal evolution and explicitly analyze the effects of altering gravity. We find that uncertainties in the dense matter microphysics, as the core chemical composition and superfluidity/superconductivity properties, as well as the astrophysical uncertainties on the chemical composition of the surface layers, have a much stronger impact than possible modifications of gravity within the studied family of $f(R)$ theories. We conclude that within this family of gravity theories, conclusions from previous studies of neutron star thermal evolution are not significantly altered by alteration of gravity. Conversely, this implies that neutron star cooling modeling may not be a useful tool to constrain deviations of gravity from Einstein theory unless these are much more radical than in the $f(R)=R+αR^{2}$ framework.

gr-qc↗

arXiv2022

A "Hyperburst" in the MAXI J0556-332 Neutron Star: Evidence for a New Type of Thermonuclear Explosion

The study of transiently accreting neutron stars provides a powerful means to elucidate the properties of neutron star crusts. We present extensive numerical simulations of the evolution of the neutron star in the transient low-mass X-ray binary MAXI J0556--332. We model nearly twenty observations obtained during the quiescence phases after four different outbursts of the source in the past decade, considering the heating of the star during accretion by the deep crustal heating mechanism complemented by some shallow heating source. We show that cooling data are consistent with a single source of shallow heating acting during the last three outbursts, while a very different and powerful energy source is required to explain the extremely high effective temperature of the neutron star, ~350 eV, when it exited the first observed outburst. We propose that a gigantic thermonuclear explosion, a "hyperburst" from unstable burning of neutron rich isotopes of oxygen or neon, occurred a few weeks before the end of the first outburst, releasing 10^44 ergs at densities of the order of 10^11 g/cm^3. This would be the first observation of a hyperburst and these would be extremely rare events as the build up of the exploding layer requires about a millennium of accretion history. Despite its large energy output, the hyperburst did not produce, due to its depth, any noticeable increase in luminosity during the accretion phase and is only identifiable by its imprint on the later cooling of the neutron star.

astro-ph.HE↗

arXiv2020

NS 1987A in SN 1987A

The possible detection of a compact object in the remnant of SN 1987A presents an unprecedented opportunity to follow its early evolution. The suspected detection stems from an excess of infrared emission from a dust blob near the compact object's predicted position. The infrared excess could be due to the decay of isotopes like 44Ti, accretion luminosity from a neutron star or black hole, magnetospheric emission or a wind originating from the spindown of a pulsar, or thermal emission from an embedded, cooling neutron star (NS 1987A). It is shown that the last possibility is the most plausible as the other explanations are disfavored by other observations and/or require fine-tuning of parameters. Not only are there indications the dust blob overlaps the predicted location of a kicked compact remnant, but its excess luminosity also matches the expected thermal power of a 30 year old neutron star. Furthermore, models of cooling neutron stars within the Minimal Cooling paradigm readily fit both NS 1987A and Cas A, the next-youngest known neutron star. If correct, a long heat transport timescale in the crust and a large effective stellar temperature are favored, implying relatively limited crustal n-1S0 superfluidity and an envelope with a thick layer of light elements, respectively. If the locations don't overlap, then pulsar spindown or accretion might be more likely, but the pulsar's period and magnetic field or the accretion rate must be rather finely tuned. In this case, NS 1987A may have enhanced cooling and/or a heavy-element envelope.

astro-ph.HE↗

arXiv2019

Thermal evolution of neo-neutron stars. I: envelopes, Eddington luminosity phase and implications for GW170817

A neo-neutron star is a hot neutron star that has just become transparent to neutrinos. In a core collapse supernova or accretion induced collapse of a white dwarf the neo-neutron star phase directly follows the proto-neutron star phase, about 30 to 60 seconds after the initial collapse. It will also be present in a binary neutron star merger in the case the "born-again" hot massive compact star does not immediately collapse into a black hole. Eddington or even super-Eddington luminosities are present for some time. A neo-neutron star produced in a core collapse supernova is not directly observable but the one produced by a binary merger, likely associated with an off-axis short gamma-ray burst, may be observable for some time as well as when produced in the accretion induced collapse of a white dwarf. We present a first step in the study of this neo-neutron star phase in a spherically symmetric configuration, thus neglecting fast rotation, and also neglecting the effect of strong magnetic fields. We put particular emphasis on determining how long the star can sustain a near-Eddington luminosity and also show the importance of positrons and contraction energy during neo-neutron star phase. We finally discuss the observational prospects for neutron star mergers triggered by LIGO and for accretion-induced collapse transients.

astro-ph.HE↗

arXiv2018

Constraints on Axion-like Particles and Nucleon Pairing in Dense Matter from the Hot Neutron Star in HESS J1731-347

If the thermal evolution of the hot young neutron star in the supernova remnant HESS J1731-347 is driven by neutrino emission, it provides a stringent constraint on the coupling of light (mass $\ll 10$ keV) axion-like particles to neutrons. Using Markov-Chain Monte Carlo we find that for the values of axion-neutron coupling $g_{ann}^2 > 7.7 \times 10^{-20}$ (90% c.l.) the axion cooling from the bremsstrahlung reaction $n+n\rightarrow n+n +a$ is too rapid to account for the high observed surface temperature. This implies that the Pecci-Quinn scale or axion decay constant $f_a > 6.7 \times 10^7$ GeV for KSVZ axions and $f_a > 1.7 \times 10^9$ GeV for DFSZ axions. The high temperature of this neutron star also allows us to tighten constraints on the size of the nucleon pairing gaps.

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