Aims. We study the relative importance of several recent updates of microphysics input to the neutron star cooling theory and the effects brought about by superstrong magnetic fields of magnetars, ...including the effects of the Landau quantization in their crusts. Methods. We use a finite-difference code for simulation of neutron-star thermal evolution on timescales from hours to megayears with an updated microphysics input. The consideration of short timescales (≲1 yr) is made possible by a treatment of the heat-blanketing envelope without the quasistationary approximation inherent to its treatment in traditional neutron-star cooling codes. For the strongly magnetized neutron stars, we take into account the effects of Landau quantization on thermodynamic functions and thermal conductivities. We simulate cooling of ordinary neutron stars and magnetars with non-accreted and accreted crusts and compare the results with observations. Results. Suppression of radiative and conductive opacities in strongly quantizing magnetic fields and formation of a condensed radiating surface substantially enhance the photon luminosity at early ages, making the life of magnetars brighter but shorter. These effects together with the effect of strong proton superfluidity, which slows down the cooling of kiloyear-aged neutron stars, can explain thermal luminosities of about a half of magnetars without invoking heating mechanisms. Observed thermal luminosities of other magnetars are still higher than theoretical predictions, which implies heating, but the effects of quantizing magnetic fields and baryon superfluidity help to reduce the discrepancy.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Aims. We study the long-term thermal evolution of neutron stars in soft X-ray transients (SXTs), taking the deep crustal heating into account consistently with the changes of the composition of the ...crust. We collect observational estimates of average accretion rates and thermal luminosities of such neutron stars and compare the theory with observations. Methods. We performed simulations of thermal evolution of accreting neutron stars, considering the gradual replacement of the original nonaccreted crust by the reprocessed accreted matter, the neutrino and photon energy losses, and the deep crustal heating due to nuclear reactions in the accreted crust. We also tested and compared results for different modern theoretical models. We updated a compilation of the observational estimates of the thermal luminosities in quiescence and average accretion rates in the SXTs and compared the observational estimates with the theoretical results. Results. The long-term thermal evolution of transiently accreting neutron stars is nonmonotonic. The quasi-equilibrium temperature in quiescence reaches a minimum and then increases toward the final steady state. The quasi-equilibrium thermal luminosity of a neutron star in an SXT can be substantially lower at the minimum than in the final state. This enlarges the range of possibilities for theoretical interpretation of observations of such neutron stars. The updates of the theory and observations leave the previous conclusions unchanged, namely that the direct Urca process operates in relatively cold neutron stars and that an accreted heat-blanketing envelope is likely present in relatively hot neutron stars in the SXTs in quiescence. The results of the comparison of theory with observations favor suppression of the triplet pairing type of nucleon superfluidity in the neutron-star matter.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
The early 21st century is witnessing a breakthrough in the study of the thermal radiation of neutron stars. Observations with modern space telescopes have provided a wealth of valuable information, ...which, when properly interpreted, can elucidate the physics of superdense matter in the interior of these stars. This interpretation is underlain by the theory of formation of the neutron star thermal spectra, which is in turn based on plasma physics and on the understanding of radiative processes in stellar photospheres. In this paper, the current status of the theory is reviewed, with particular emphasis on neutron stars with strong magnetic fields. In addition to the conventional deep (semi-infinite) atmospheres, radiative condensed surfaces of neutron stars and 'thin' (finite) atmospheres are considered.
Context.
The thermal evolution of neutron stars in soft X-ray transients (SXTs) is sensitive to the equation of state, nucleon superfluidity, and the composition and structure of the crust. Carrying ...out comparisons of the observations of their crust cooling with simulations offers a powerful tool for verifying theoretical models of dense matter.
Aims.
We study the effect of physics input on the thermal evolution of neutron stars in SXTs. In particular, we consider different modern models of the sources of deep crustal heating during accretion episodes and the effects brought on by impurities embedded in the crust during its formation.
Methods.
We simulated the thermal structure and evolution of episodically accreting neutron stars under different assumptions regarding the crust composition and on the distribution of heat sources and impurities. For the non-accreted crust, we considered the nuclear charge fluctuations that arise at crust formation. For the accreted crust, we compared different theoretical models of composition and internal heating. We also compared the results of numerical simulations to observations of the crust cooling in SXT MXB 1659−29.
Results.
The non-accreted part of the inner crust of a neutron star can have a layered structure, with almost pure crystalline layers interchanged with layers composed of mixtures of different nuclei. The latter layers have relatively low thermal conductivities, which has an effect on the thermal evolution of the transients. The impurity distribution in the crust strongly depends on models of the dense matter and the crust formation scenario. The shallow heating that is needed to reach an agreement between the theory and the observations depends on characteristics of the crust and envelope.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Context. The modeling of planetary interiors requires accurate equations of state (EOSs) for the basic constituents with proven validity in the difficult pressure–temperature regime extending up to ...50 000 K and hundreds of megabars. While EOSs based on first-principles simulations are now available for the two most abundant elements, hydrogen and helium, the situation is less satisfactory for water where no wide-range EOS is available despite its requirement for interior modeling of planets ranging from super-Earths to planets several times the size of Jupiter. Aims. As a first step toward a multi-phase EOS for dense water, we develop a temperature-dependent EOS for dense water covering the liquid and plasma regimes and extending to the super-ionic and gas regimes. This equation of state covers the complete range of conditions encountered in planetary modeling. Methods. We use first-principles quantum molecular dynamics simulations and the Thomas-Fermi extension to reach the highest pressures encountered in giant planets several times the size of Jupiter. Using these results, as well as the data available at lower pressures, we obtain a parametrization of the Helmholtz free energy adjusted over this extended temperature and pressure domain. The parametrization ignores the entropy and density jumps at phase boundaries but we show that it is sufficiently accurate to model interior properties of most planets and exoplanets. Results. We produce an EOS given in analytical form that is readily usable in planetary modeling codes and dynamical simulations (a fortran implementation is provided). The EOS produced is valid for the entire density range relevant to planetary modeling, for densities where quantum effects for the ions can be neglected, and for temperatures below 50 000K. We use this EOS to calculate the mass-radius relationship of exoplanets up to 5000 MEarth, explore temperature effects in the wet Earth-like, ocean planets and pure water planets, and quantify the influence of the water EOS for the core on the gravitational moments of Jupiter.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Thermal luminosities of cooling neutron stars Potekhin, A Y; Zyuzin, D A; Yakovlev, D G ...
Monthly Notices of the Royal Astronomical Society,
08/2020, Volume:
496, Issue:
4
Journal Article
Peer reviewed
Open access
ABSTRACT
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.
Context. An equation of state (EoS) of dense nuclear matter is a prerequisite for studies of the structure and evolution of compact stars. A unified EoS should describe the crust and the core of a ...neutron star using the same physical model. The Brussels-Montreal group has recently derived a family of such EoSs based on the nuclear energy-density functional theory with generalized Skyrme effective forces that have been fitted with great precision to essentially all the available mass data. At the same time, these forces were constrained to reproduce microscopic calculations of homogeneous neutron matter based on realistic two- and three-nucleon forces. Aims. We represent basic physical characteristics of the latest Brussels-Montreal EoS models by analytical expressions to facilitate their inclusion in astrophysical simulations. Methods. We consider three EoS models, which significantly differ by stiffness: BSk19, BSk20, and BSk21. For each of them we constructed two versions of the EoS parametrization. In the first version, pressure P and gravitational mass density ρ are given as functions of the baryon number density nb. In the second version, P, ρ, and nb are given as functions of pseudo-enthalpy, which is useful for two-dimensional calculations of stationary rotating configurations of neutron stars. In addition to the EoS, we derived analytical expressions for several related quantities that are required in neutron-star simulations: number fractions of electrons and muons in the stellar core, nucleon numbers per nucleus in the inner crust, and equivalent radii and shape parameters of the nuclei in the inner crust. Results. We obtain analytical representations for the basic characteristics of the models of cold dense matter, which are most important for studies of neutron stars. We demonstrate the usability of our results by applying them to calculations of neutron-star mass-radius relations, maximum and minimum masses, thresholds of direct Urca processes, and the electron conductivity in the neutron-star crust.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
The theory of the nuclear energy-density functional is used to provide a unified and thermodynamically consistent treatment of all regions of cold non-accreting neutron stars. In orderto assess the ...impact of our lack of complete knowledge of the density dependence of thesymmetry energy on the constitution and the global structure of neutron stars, we employfour different functionals. All of them were precision fitted to essentially all the nuclear massdata with the Hartree–Fock–Bogoliubov method and two different neutron-matter equationsof state based on realistic nuclear forces. For each functional, we calculate the composition,the pressure–density relation, and the chemical potentials throughout the star. We show thatuncertainties in the symmetry energy can significantly affect the theoretical results for thecomposition and global structure of neutron stars. To facilitate astrophysical applications, weconstruct analytic fits to our numerical results.
We review the theory of electron-conduction opacity, a fundamental ingredient in the computation of low-mass stellar models; shortcomings and limitations of the existing calculations used in stellar ...evolution are discussed. We then present new determinations of the electron-conduction opacity in stellar conditions for an arbitrary chemical composition that improve over previous works and, most importantly, cover the whole parameter space relevant to stellar evolution models (i.e., both the regime of partial and high electron degeneracy). A detailed comparison with the currently used tabulations is also performed. The impact of our new opacities on the evolution of low-mass stars is assessed by computing stellar models along both the H- and He-burning evolutionary phases, as well as main sequence models of very low-mass stars and white dwarf cooling tracks.
ABSTRACT
Magnetars are believed to host the strongest magnetic fields in the present universe ($B\gtrsim 10^{14}$ G) and the study of their persistent emission in the X-ray band offers an ...unprecedented opportunity to gain insight into physical processes in the presence of ultra-strong magnetic fields. Up to now, most of our knowledge about magnetar sources came from spectral analysis, which allowed to test the resonant Compton scattering scenario and to probe the structure of the star magnetosphere. On the other hand, radiation emitted from magnetar surface is expected to be strongly polarized and its observed polarization pattern bears the imprint of both scatterings on to magnetospheric charges and quantum electro-dynamics (QED) effects as it propagates in the magnetized vacuum around the star. X-ray polarimeters scheduled to fly in the next years will finally allow to exploit the wealth of information stored in the polarization observables. Here we revisit the problem of assessing the spectro-polarimetric properties of magnetar persistent emission. At variance with previous investigations, proper account for more physical surface emission models is made by considering either a condensed surface or a magnetized atmosphere. Results are used to simulate polarimetric observations with the forthcoming Imaging X-ray Polarimetry Explorer. We find that X-ray polarimetry will allow to detect QED vacuum effects for all the emission models we considered and to discriminate among them.
PDF
