Abstract: We explore the thermal state of the neutron star in the Cassiopeia A supernova remnant using the recent result of Ho & Heinke that the thermal radiation of this star is well described by a carbon atmosphere model and the emission comes from the entire stellar surface. Starting from neutron star cooling theory, we formulate a robust method to extract neutrino cooling rates of thermally relaxed stars at the neutrino cooling stage from observations of thermal surface radiation. We show how to compare these rates with the rates of standard candles - stars with non-superfluid nucleon cores cooling slowly via the modified Urca process. We find that the internal temperature of standard candles is a well-defined function of the stellar compactness parameter x = r(g)/R, irrespective of the equation of state of neutron star matter (R and r(g) are circumferential and gravitational radii, respectively). We demonstrate that the data on the Cassiopeia A neutron star can be explained in terms of three parameters:f(l), the neutrino cooling efficiency with respect to the standard candle; the compactness x; and the amount of light elements in the heat-blanketing envelope. For an ordinary (iron) heat-blanketing envelope or a low-mass (less than or similar to 10(-13) M(circle dot)) carbon envelope, we find the efficiency f(l) similar to 1 (standard cooling) for x less than or similar to 0.5 and f(l) similar to 0.02 (slower cooling) for a maximum compactness x approximate to 0.7. A heat blanket containing the maximum mass (similar to 10(-8) M(circle dot)) of light elements increases f(l) by a factor of 50. We also examine the (unlikely) possibility that the star is still thermally non-relaxed.
Abstract: We briefly review analytic approximations of thermodynamic functions of fully ionized nonideal electron-ion plasmas, applicable in a wide range of plasma parameters, including the domains of nondegenerate and degenerate, nonrelativistic and relativistic electrons, weakly and strongly coupled Coulomb liquids, classical and quantum Coulomb crystals. We present improvements to previously published approximations. Our code for calculation of thermodynamic functions based on the reviewed approximations is made publicly available. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: We present a new computer code for modeling magnetized neutron star atmospheres in a wide range of magnetic fields (10(12)-10(15) G) and effective temperatures (3 x 10(5)-10(7) K). The atmosphere is assumed to consist either of fully ionized electron-ion plasmas or of partially ionized hydrogen. Vacuum resonance and partial mode conversion are taken into account. Any inclination of the magnetic field relative to the stellar surface is allowed. We use modern opacities of fully or partially ionized plasmas in strong magnetic fields and solve the coupled radiative transfer equations for the normal electromagnetic modes in the plasma. Using this code, we study the possibilities to explain the soft X-ray spectra of isolated neutron stars by different atmosphere models. In particular, the outgoing spectrum using the "sandwich" model (thin atmosphere with a hydrogen layer above a helium layer) is constructed. Thin partially ionized hydrogen atmospheres with vacuum polarization are shown to be able to improve our understanding of the observed spectrum of the nearby isolated neutron star RBS 1223 (RX J1308.8+2127). (C) 2009 COSPAR. Published by Elsevier Ltd. All rights reserved.
Abstract: Topical problems in the physics of and basic facts about neutron stars are reviewed. The observational manifestations of neutron stars, their core and envelope structure, magnetic fields, thermal evolution, and masses and radii are briefly discussed, along with the underlying microphysics.
Abstract: Some X-ray dim isolated neutron stars (XDINS) and central compact objects in supernova remnants (CCO) contain absorption features in their thermal soft X-ray spectra. It has been hypothesized that this absorption may relate to periodic peaks in free-free absorption opacities, caused by either Landau quantization of electron motion in magnetic fields B less than or similar to 10(11) G or analogous quantization of ion motion in magnetic fields B > 10(13) G. Here, I review the physics behind cyclotron quantum harmonics in free-free photoabsorption, discuss different approximations for their calculation, and explain why the ion cyclotron harmonics (beyond the fundamental) cannot be observed.
Abstract: Context. In the X-ray spectra of most X-ray dim isolated neutron stars (XDINSs), absorption features with equivalent widths (EWs) of 50-200 eV are observed. These features are usually connected with the proton cyclotron line, but their nature is not yet well known. Aims. We theoretically investigate different models to explain these absorption features and compare their properties with observations to obtain a clearer understanding of the radiation properties of magnetized neutron star surfaces. Based on these models, we create a fast and flexible code to fit observed spectra of isolated neutron stars. Methods. We consider various theoretical models of the magnetized neutron star surface, including naked condensed iron surfaces and partially ionized hydrogen model atmospheres, with semi-infinite and thin atmospheres above a condensed surface. Spectra of condensed iron surfaces are represented by a simple analytical approximation. The condensed surface radiation properties are considered as the inner atmosphere boundary condition for the thin atmosphere. The properties of the absorption features (especially equivalent widths) and the angular distributions of the emergent radiation are described for all models. A code for computing light curves and integral emergent spectra of magnetized neutron stars is developed. We assume a dipole surface magnetic field distribution with a possible toroidal component and corresponding temperature distribution. A model with two uniform hot spots at the magnetic poles may also be employed. Results. Light curves and spectra of highly magnetized neutron stars with parameters typical of XDINSs are computed using different surface temperature distributions and various local surface models. Spectra of magnetized model atmospheres are approximated by diluted black-body spectra with one or two Gaussian lines having parameters, which allow us to describe the model absorption features. The EWs of the absorption features in the integral spectra cannot significantly exceed 100 eV, if a local surface model assumes either a semi-infinite magnetic atmosphere or a naked condensed surface. A thin atmosphere above a condensed surface can have an absorption feature whose EW exceeds 200 eV in the integrated spectrum. If the toroidal component of the magnetic field on the neutron star atmosphere is 3-7 times higher than the poloidal component, the absorption feature in the integral spectrum is too wide and shallow to be detectable. Conclusions. To explain the prominent absorption features in the soft X-ray spectra of XDINSs, we infer that a thin atmosphere above the condensed surface must be present, whereas a strong toroidal magnetic field component on the XDINS surfaces can be excluded.
Abstract: Zaghloul [Phys. Plasmas 17, 062701 (2010)] reconsiders the occupation probability formalism in plasma thermodynamics and claims inconsistencies in previous models. I show that the origin of this incorrect claim is an omission of the configurational factor from the partition function. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3531706]
Abstract: We develop analytic approximations of thermodynamic functions of fully ionized nonideal electron-ion plasma mixtures. In the regime of strong Coulomb coupling, we use our previously developed analytic approximations for the free energy of one-component plasmas with rigid and polarizable electron background and apply the linear mixing rule (LMR). Other thermodynamic functions are obtained through analytic derivation of this free energy. In order to obtain an analytic approximation for the intermediate coupling and transition to the Debye-Huckel limit, we perform hypernetted-chain calculations of the free energy, internal energy, and pressure for mixtures of different ion species and introduce a correction to the LMR, which allows a smooth transition from strong to weak Coulomb coupling, in agreement with the numerical results.
Abstract: We study the thermal structure and evolution of magnetars as cooling neutron stars with a phenomenological heat source in an internal layer. We focus on the effect of magnetized (B >= 10(14) G) non-accreted and accreted outermost envelopes composed of different elements, from iron to hydrogen or helium. We discuss a combined effect of thermal conduction and neutrino emission in the outer neutron star crust and calculate the cooling of magnetars with a dipole magnetic field for various locations of the heat layer, heat rates and magnetic field strengths. Combined effects of strong magnetic fields and light-element composition simplify the interpretation of magnetars in our model: these effects allow one to interpret observations assuming less extreme (therefore, more realistic) heating. Massive magnetars, with fast neutrino cooling in their cores, can have higher thermal surface luminosity.
Abstract: Recently developed analytic approximation for the equation of state of fully ionized nonideal electron-ion plasma mixtures [A. Y. Potekhin, G. Chabrier, and F. J. Rogers, Phys. Rev. E 79, 016411 (2009)], which covers the transition between the weak and strong Coulomb coupling regimes and reproduces numerical results obtained in the hypernetted-chain (HNC) approximation, is modified in order to fit the small deviations from the linear mixing in the strong-coupling regime, revealed by recent Monte Carlo simulations. In addition, a mixing rule is proposed for the regime of weak coupling, which generalizes post-Debye density corrections to the case of mixtures and numerically agrees with the HNC approximation in that regime.
Abstract: Context. Observed X-ray spectra of some isolated magnetized neutron stars display absorption features, sometimes interpreted as ion cyclotron lines. Modeling the observed spectra is necessary to check this hypothesis and to evaluate neutron star parameters. Aims. We develop a computer code for modeling magnetized neutron star atmospheres in a wide range of magnetic fields (10(12)-10(15) G) and effective temperatures (3 x 10(5)-10(7) K). Using this code, we study the possibilities to explain the soft X-ray spectra of isolated neutron stars by different atmosphere models. Methods. The atmosphere is assumed to consist either of fully ionized electron-ion plasmas or of partially ionized hydrogen. Vacuum resonance and partial mode conversion are taken into account. Any inclination of the magnetic field relative to the stellar surface is allowed. We use modern opacities of fully or partially ionized plasmas in strong magnetic fields and solve the coupled radiative transfer equations for the normal electromagnetic modes in the plasma. Results. Spectra of outgoing radiation are calculated for various atmosphere models: fully ionized semi-infinite atmosphere, thin atmosphere, partially ionized hydrogen atmosphere, or novel "sandwich" atmosphere ( thin atmosphere with a hydrogen layer above a helium layer). Possibilities of applications of these results are discussed. In particular, the outgoing spectrum using the "sandwich" model is constructed. Thin partially ionized hydrogen atmospheres with vacuum polarization are shown to be able to improve the fit to the observed spectrum of the nearby isolated neutron star RBS 1223 (RX J1308.8+2127).
Abstract: Recent observations of thermally emitting isolated neutron stars revealed spectral features that could be interpreted as radiative transitions of He in a magnetized neutron star atmosphere. We present Hartree-Fock calculations of the polarization-dependent photoionization cross-sections of the He atom in strong magnetic fields ranging from 10(12) to 10(14) G. Convenient fitting formulae for the cross-sections are given along with the related oscillator strengths for various bound-bound transitions. The effects of finite nucleus mass on the radiative absorption cross-sections are examined using perturbation theory.
Abstract: We construct partially ionized hydrogen atmosphere models for magnetized neutron stars in radiative equilibrium with fixed surface fields between B = 10(12) and 2 x 10(13) G and effective temperatures log T(eff) = 5.5-6.8, as well as with surface B and T(eff) distributions around these values. The models are based on the latest equation of state and opacity results for magnetized partially ionized hydrogen plasmas. The atmospheres directly determine the characteristics of thermal emission from the surface of neutron stars. We also incorporate these model spectra into XSPEC, under the model name NSMAX, thus allowing them to be used by the community to fit X-ray observations of neutron stars.
Abstract: Strong (B >> 10(9) G) and superstrong (B greater than or similar to 10(14) G) magnetic fields profoundly affect many thermodynamic and kinetic characteristics of dense plasmas in neutron star envelopes. In particular, they produce strongly anisotropic thermal conductivity in the neutron star crust and modify the equation of state and radiative opacities in the atmosphere, which are major ingredients of the cooling theory and spectral atmosphere models. As a result, both the radiation spectrum and the thermal luminosity of a neutron star can be affected by the magnetic field. We briefly review these effects and demonstrate the influence of magnetic field strength on the thermal structure of an isolated neutron star, putting emphasis on the differences brought about by the superstrong fields and high temperatures of magnetars. For the latter objects, it is important to take proper account of a combined effect of the magnetic field on thermal conduction and neutrino emission at densities rho greater than or similar to 10(10) g cm(-3). We show that the neutrino emission puts a B-dependent upper limit on the effective surface temperature of a cooling neutron star.
Abstract: We model thermal evolution of magnetars with a phenomenological heat source in a spherical internal layer and compare the results with observations of persistent thermal radiation from magnetars. We show that the heat source should be located in the outer magnetar's crust, at densities rho empty set 5 x 10(11) g cm(-3), and the heating rate should be similar to 10(20) erg cm(-3) s(-1) . Heating deeper layers is extremely inefficient because the thermal energy is mainly radiated away by neutrinos and does not warm up the surface to the magnetar's level. This deep heating requires too much energy; it is inconsistent with the energy budget of neutron stars.
Abstract: RX J1856.5-3754 is one of the brightest nearby isolated neutron stars (INSs), and considerable observational resources have been devoted to it. However, current models are unable to satisfactorily explain the data. We show that our latest models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a condensed surface can fit the entire spectrum, from X-rays to optical, of RX J1856.5-3754, within the uncertainties. In our simplest model, the best-fitting parameters are an interstellar column density N-H approximate to 1 x 10(20) cm(-2) and an emitting area with R-infinity approximate to 17 km (assuming a distance to RX J1856.5-3754 of 140 pc), temperature T-infinity approximate to 4.3 x 10(5) K, gravitational redshift z(g) similar to 0.22, atmospheric hydrogen column y(H) approximate to 1 g cm(-2), and magnetic field B approximate to (3-4) x 10(12) G; the values for the temperature and magnetic field indicate an effective average over the surface. We also calculate a more realistic model, which accounts for magnetic field and temperature variations over the NS surface as well as general relativistic effects, to determine pulsations; we find that there exist viewing geometries that produce pulsations near the currently observed limits. The origin of the thin atmospheres required to fit the data is an important question, and we briefly discuss mechanisms for producing these atmospheres. Our model thus represents the most self-consistent picture to date for explaining all the observations of RX J1856.5-3754.
Abstract: We simulate the cooling of the neutron star in the X-ray transient KS 1731-260 after the source returned to quiescence in 2001 from a long (>= 12.5 yr) outburst state. We show that the cooling can be explained assuming that the crust underwent deep heating during the outburst stage. In our best theoretical scenario the neutron star has no enhanced neutrino emission in the core, and its crust is thin, superfluid, and has the normal thermal conductivity. The thermal afterburst crust-core relaxation in the star may not be over.
Abstract: RX J1856.5-3754 is one of the brightest nearby isolated neutron stars, and considerable observational resources have been devoted to it. However, current models are unable to satisfactorily explain the data. We show that our latest models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a condensed surface can fit the entire spectrum, from X-rays to optical, of RX J1856.5-3754, within the uncertainties. In our simplest model, the best-fit parameters are an interstellar column density N-H approximate to 1 x 10(20) cm(-2) and an emitting area with R infinity approximate to 17 km (assuming a distance to RX J1856.5-3754 of 140 pc), temperature T infinity approximate to 4.3 x 10(5) K, gravitational redshift z(g) similar to 0.22, atmospheric hydrogen column y(H) approximate to 1 g cm(-2), and magnetic field B approximate to (3-4) x 10(12) G; the values for the temperature and magnetic field indicate an effective average over the surface.
Abstract: We calculate the transition rates between proton Landau levels due to non-radiative and radiative Coulomb collisions in an electron-proton plasma with strong magnetic field B. Both electron-proton collisions and proton-proton collisions are considered. The roles of the first-order cyclotron absorption and second-order free-free absorption and scattering in determining the line strength and shape as well as the continuum are analysed in detail. We solve the statistical balance equation for the populations of proton Landau levels. For temperatures similar to 10(6)-10(7) K, the deviations of the proton populations from local thermal equilibrium are appreciable at density rho less than or similar to 0. 1 B-14(3.5) g cm(-3), where B-14 = B/( 10(14) G). We present general formulae for the plasma emissivity and absorption coefficents under a wide range of physical conditions. Our results are useful for studying the possibility and the conditions of proton/ion cyclotron line formation in the near vicinity of highly magnetized neutron stars.
Abstract: 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: We study the equation of state, polarization and radiation properties for nonideal, strongly magnetized plasmas which compose outer envelopes of magnetic neutron stars. Detailed calculations are performed for partially ionized hydrogen atmospheres and for condensed hydrogen or iron surfaces of these stars.
Abstract: We briefly examine the properties of the dense plasmas characteristic of the interior of giant planets and of the atmospheres of neutron stars. Special attention is devoted to the equation of state of hydrogen and helium at high density and to the effect of magnetic fields on the properties of dense matter.
Abstract: We study thermal structure and evolution of magnetars as cooling neutron stars with a phenomenological heat source in a spherical internal layer. We explore the location of this layer as well as the heating rate that could explain high observable thermal luminosities of magnetars and would be consistent with the energy budget of neutron stars. We conclude that the heat source should be located in an outer magnetar's crust, at densities rho less than or similar to 5 x 10(11) g cm(-3), and should have the heat intensity of similar to 10(20) erg cm(-3) s(-1). Otherwise the heat energy is mainly emitted by neutrinos and cannot warm up the surface.
Abstract: The purpose of the present paper is to describe the effects of electron-electron collisions on proton electronic stopping in plasmas of any degeneracy. Plasma targets are considered fully ionized so electronic stopping is only due to the free electrons. The stopping due to free electrons is obtained from an exact quantum mechanical evaluation in the random phase approximation, which takes into account the degeneracy of the target plasma. The result is compared with common classical and degenerate approximations. Differences are around 30% in some cases which can produce bigger mistakes in further energy deposition and projectile range studies. We focus our analysis on plasmas in the limit of weakly coupled plasmas then electron-electron collisions have to be considered. Differences with the same results without taking into account collisions are more than 50%.
Abstract: We construct hydrogen atmosphere models for strongly magnetized neutron stars in thermodynamic equilibrium, taking into account partial ionization. The presence of bound states affects the equation of state, absorption coefficients, and polarizability tensor of a strongly magnetized plasma. Therefore the partial ionization influences the polarization vectors and opacities of normal electromagnetic waves, and thus the spectra of outgoing radiation. Here, we review a model suitable for the most typical neutron-star atmospheres and focus on the problems that remain to be solved for its extension to other atmospheric parameters. (c) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.
Abstract: The impact of nuclear physics theories on cooling of isolated neutron stars is analyzed. Physical properties of neutron star matter important for cooling are reviewed such as composition, the equation of state, superfluidity of various baryon species, neutrino emission mechanisms. Theoretical results are compared with observations of thermal radiation from neutron stars. Current constraints on theoretical models of dense matter, derived from such a comparison, are formulated.
Abstract: Recent observations show that the thermal X-ray spectra of many isolated neutron stars are featureless and in some cases ( e. g., RX J1856.5 - 3754) well fit by a blackbody. Such a perfect blackbody spectrum is puzzling since radiative transport through typical neutron star atmospheres causes noticeable deviation from blackbody. Previous studies have shown that in a strong magnetic field, the outermost layer of the neutron star may be in a condensed solid or liquid form because of the greatly enhanced cohesive energy of the condensed matter. The critical temperature of condensation increases with the magnetic field strength and can be as high as 106 K ( for Fe surface at B similar to 10(13) Gor H surface at B similar to a few x 10(14) G). Thus the thermal radiation can directly emerge from the degenerate metallic condensed surface without going through a gaseous atmosphere. Here we calculate the emission properties ( spectrum and polarization) of the condensed Fe and H surfaces of magnetic neutron stars in the regimes in which such condensation may be possible. For a smooth condensed surface, the overall emission is reduced from the blackbody by less than a factor of 2. The spectrum exhibits modest deviation from blackbody across a wide energy range and shows mild absorption features associated with the ion cyclotron frequency and the electron plasma frequency in the condensed matter. The roughness of the solid condensate ( in the Fe case) tends to decrease the reflectivity of the surface and make the emission spectrum even closer to blackbody. We discuss the implications of our results for observations of dim, isolated neutron stars and magnetars.
Abstract: We study the relation between the mean effective surface temperature (T) over bar (s) and the internal temperature T-b for magnetic neutron stars, assuming that the magnetic field near the surface has a small-scale structure. The heavy-element (iron) and light-element (accreted) heat-blanketing envelopes are considered, and the results are compared with the case of a dipole magnetic field. We argue that the difference in the T-b((T) over bar (s))-relation for different magnetic configurations is always much smaller than the possible difference caused by variations of the chemical composition in the envelope.
Abstract: Equation of state for partially ionized carbon at temperatures T greater than or similar to 10(5) K is calculated in a wide range of densities, using the method of free energy minimization in the framework of the chemical picture of plasmas. The free energy model includes the internal partition functions of bound species. The latter are calculated by a self-consistent treatment of each ionization stage in the plasma environment taking into account pressure ionization. The long-range Coulomb interactions between ions and screening of the ions by free electrons are included using our previously published analytical model.
Abstract: We construct hydrogen atmospheric models for magnetized neutron stars (NSs) in radiative equilibrium with surface fields B = 10(12)-5 x 10(14) G and effective temperatures T-eff similar to a few x 10(5)-10(6) K by solving the full radiative transfer equations for both polarization modes in the magnetized hydrogen plasma. The atmospheres directly determine the characteristics of thermal emission from isolated NSs. We study the effects of vacuum polarization and bound atoms on the atmosphere structure and spectra. For the lower magnetic field models (B similar to 10(12) G), the spectral features due to neutral atoms lie at extreme UV and very soft X-ray energies and therefore are not likely to be observed. However, the continuum flux is also different from the fully ionized case, especially at lower energies. For the higher magnetic field models, we find that vacuum polarization softens the high energy tail of the thermal spectrum. We show that this depression of continuum flux strongly suppresses not only the proton cyclotron line but also spectral features due to bound species; therefore, spectral lines or features in thermal radiation are more difficult to observe when the NS magnetic field is greater than or similar to 10(14) G. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.
Abstract: The equation of state and radiative opacities of partially ionized, strongly magnetized hydrogen plasmas, presented in a previous paper for the magnetic field strengths, 8 x 10(11) Gless than or similar toBless than or similar to3 x 10(13) G, are extended to the field strengths, 3 x 10(13) Gless than or similar toB less than or equal to 10(15) G, relevant for magnetars. The first- and second-order thermodynamic functions and radiative opacities are calculated and tabulated for 5 x 10(5) K less than or equal to T less than or equal to 4 x 10(7) K in a wide range of densities. We show that bound-free transitions give an important contribution to the opacities in the considered range of B in the outer neutron star atmosphere layers. Unlike the case of weaker fields, bound-bound transitions are unimportant.
Abstract: The cooling theory of isolated neutron stars is reviewed. The main cooling regulators are discussed, first of all, operation of direct Urca process (or similar processes in exotic phases of dense matter) and superfluidity in stellar interiors. The prospects to constrain gross parameters of supranuclear matter in neutron star interiors by confronting cooling theory with observations of isolated neutron stars are outlined. A related problem of thermal states of transiently accreting neutron stars with deep crustal heating of accreted matter is discussed in application to soft X-ray transients. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.
Abstract: We calculate the thermal Structure and quiescent thermal luminosity of accreting neutron stars (warmed by deep crustal heating in accreted matter) in soft X-ray transients (SXTs). We consider neutron stars with nucleon and hyperon cores and with accreted envelopes. It is assumed that an envelope has an outer helium layer (of variable depth) and deeper layers of heavier elements, either with iron or with much heavier nuclei (of atomic weight A greater than or similar to100) on the top (Haensel & Zdunik 1990, 2003). The relation between the internal and surface stellar temperatures is obtained and fitted by simple expressions. The quiescent luminosity of the hottest (low-mass) and coldest (high-mass) neutron stars is calculated, together with the ranges of its possible variations due to variable thickness of the helium layer. The results are compared with observations of SXTs. particularly, containing the coldest (SAX J1808.4-3658) and the hottest (Aql X-1) neutron stars. The observations of SAX J1808.4-3658 in a quiescent state on March 24, 2001 (Campana et al. 2002) can be explained only if this SXT contains a massive neutron star with a nucleon/hyperon core; a hyperon core with a not too low fraction of electrons is preferable. Future observations may discriminate between the various models of hyperon/nucleon dense matter. The thermal emission of SAX J1808.4-3658 is also sensitive to the models of plasma ionization in the outermost surface layers and can serve for testing such models.
Abstract: The polarizability tensor of a strongly magnetized plasma and the polarization vectors and opacities of normal electromagnetic waves are studied for conditions typical of neutron star atmospheres, taking account of partial ionization effects. Vacuum polarization is also included, and a new set of fitting formulae are used that are accurate for wide range of field strengths. The full account of the coupling of the quantum mechanical structure of the atoms to their center-of-mass motion across the magnetic field is shown to be crucial for the correct evaluation of the polarization properties and opacities of the plasma. The self-consistent treatment of the polarizability and absorption coefficients proves to be necessary if the ionization degree of the plasma is low, which can occur in the atmospheres of cool or ultramagnetized neutron stars. Atmosphere models and spectra are presented to illustrate the importance of such self-consistent treatment.
Abstract: Analytical representations are derived for two equations of state (EOSs) of neutron-star matter: FPS and SLy. Each of these EOSs is unified, that is, it describes the crust and the core of a neutron star using the same physical model. Two versions of the EOS parametrization are considered. In the first one, pressure and mass density are given as functions of the baryon density. In the second version, pressure, mass density, and baryon density are given as functions of the pseudo-enthalpy, which makes this representation particularly useful for 2-D calculations of stationary rotating configurations of neutron stars.
Abstract: We study the thermal structure of neutron stars with magnetized envelopes composed of accreted material, using updated thermal conductivities of plasmas in quantizing magnetic fields, as well as the equation of state and radiative opacities for partially ionized hydrogen in strong magnetic fields. The relation between the internal and local surface temperatures is calculated and fitted by an analytic function of the internal temperature, magnetic field strength, angle between the field lines and the normal to the surface, surface gravity, and the mass of the accreted material. The luminosity of a neutron star with a dipole magnetic field is calculated for various values of the accreted mass, internal temperature, and magnetic field strength. Using these results, we simulate cooling of superfluid neutron stars with magnetized accreted envelopes. We consider slow and fast cooling regimes, paying special attention to very slow cooling of low-mass, superfluid neutron stars. In the latter case, the cooling is strongly affected by the combined effect of magnetized accreted envelopes and neutron superfluidity in the stellar crust. Our results are important for the interpretation of observations of the isolated neutron stars hottest for their age, such as RX J0822-43 and PSR B1055-52.
Abstract: We present an equation of state and radiative opacities for a strongly magnetized hydrogen plasma at magnetic fields B, temperatures T, and densities rho typical for atmospheres of isolated neutron stars. The first- and second-order thermodynamic functions, monochromatic radiative opacities, and Rosseland mean opacities are calculated and tabulated, taking account of partial ionization, for 8 x 10(11) G less than or equal to B less than or equal to 3 x 10(13) G, 2 x 10(5) K less than or equal to T less than or equal to 10(7) K, and a wide range of rho. We show that bound-bound and bound-free transitions give an important contribution to the opacities at T less than or similar to (1-5) x 10(6) K in the considered range of B in the outer neutron star atmosphere layers, which may substantially modify the X-ray spectrum of a typical magnetized neutron star. In addition, we reevaluate opacities due to free-free transitions, taking into account the motion of both interacting particles, electron and proton, in a strong magnetic field. Compared to the previous neutron star atmosphere models, the free-free absorption is strongly suppressed at photon frequencies below the proton cyclotron frequency. The latter result holds for any field strength, which prompts a revision of existing theoretical models of X-ray spectra of magnetar atmospheres.
Abstract: We construct partially ionized hydrogen atmosphere models for magnetized neutron stars in radiative equilibrium with surface fields B = 10(12)-5 x 10(14) G and effective temperatures T-eff similar to a few x 10(5)-10(6) K. These models are based on the latest equation of state and opacity results for magnetized, partially ionized hydrogen plasmas that take into account various magnetic and dense medium effects. The atmospheres directly determine the characteristics of thermal emission from isolated neutron stars. For the models with B = 10(12)-10(13) G, the spectral features due to neutral atoms lie at extreme UV and very soft X-ray energy bands and therefore are difficult to observe. However, the continuum flux is also different from the fully ionized case, especially at lower energies. For the superstrong field models (B greater than or similar to 10(14) G), we show that the vacuum polarization effect not only suppresses the proton cyclotron line as shown previously, but also suppresses spectral features due to bound species; therefore, spectral lines or features in thermal radiation are more difficult to observe when the neutron star magnetic field is greater than or similar to 10(14) G.
Abstract: The low-frequency electric microfield distribution in a Coulomb plasma is calculated for various plasma parameters, from weak to strong Coulomb coupling and from zero to strong electron screening. Two methods of numerical calculations are employed: the adjustable-parameter exponential approximation and the Monte Carlo simulation. The results are represented by analytic fitting formulas suitable for applications.
Abstract: We briefly examine the properties of dense plasmas characteristic of the atmospheres of neutron stars and of the interior of massive white dwarfs. These astrophysical bodies are natural laboratories for studying respectively the problem of pressure ionization of hydrogen in a strong magnetic field and the crystallization of the quantum one-component plasma at finite temperature.
Abstract: Matter of subnuclear density in the inner crusts of neutron stars consists of neutron-rich atomic nuclei immersed in strongly degenerate relativistic gas of electrons and strongly nonideal liquid of neutrons. Thermodynamic and kinetic properties of this matter are greatly affected by Coulomb and nuclear interactions and can be studied, in principle, from observations of thermal radiation of young (age less than or similar to 100 yr) neutron stars.
Abstract: Outer envelopes of neutron stars consist mostly of fully ionized, strongly coupled Coulomb plasmas characterized by typical densities rho similar to 10(4)-10(11) g cm(-3) and temperatures T similar to 10(4)-10(9) K. Many neutron stars possess magnetic fields B similar to 10(11)-10(14) G. Recent theoretical advances allow one to calculate thermodynamic functions and electron transport coefficients for such plasmas with an accuracy required for theoretical interpretation of observations.
Abstract: We discuss experiments and observations aimed at testing the possible space-time variability of fundamental physical constants, predicted by the modern theory. Specifically, we consider two of the dimensionless physical parameters which are important for atomic and molecular physics: the fine-structure constant and the electron-to-proton mass ratio. We review the current status of such experiments and critically analyze recent claims of a detection of the variability of the fine-structure constant on the cosmological time scale. We stress that such a detection remains to be checked by future experiments and observations. The tightest of the firmly established upper limits read that the considered constants could not vary by more than 0.015% on the scale similar to 10(10) years.
Abstract: The quadrupole moment of a hydrogen atom in a magnetic field B for field strengths from 0 to 4.414 x 10(13) G is calculated by two different methods. The first method is variational, and based on a single trial function. The second method deals with a solution of the Schrodinger equation in the form of a linear combination of Landau orbitals.
Abstract: We discuss recent laboratory experiments and astronomical observations aimed at testing the possible space-time variability of fundamental physical constants, predicted by the modern theory Specifically, we consider two of the dimensionless physical parameters which are important for atomic and molecular physics: the fine-structure constant alpha and the electron-to-proton mass ratio mu. We review the current status of such experiments and critically analyze recent claims of a detection of the variability of the fine-structure constant on the cosmological time scale. We stress that such a detection remains to be checked by future experiments and observations. The tightest of the firmly established upper limits, derived from analyses of quasar spectra formed at the early cosmological epoch (similar to 10(10) yr ago), read: \(alpha) over dot/alpha \ < 1.1 x 10(-14) yr(-1), \(mu) over dot/mu \ < 1.5 x 10(-14) yr(-1). These limits may be used as an effective tool for selection of theoretical models which predict space-time variations of physical constants.
Abstract: Phonon frequency moments and thermodynamic functions (electrostatic and vibrational parts of the free energy, internal energy, and heat capacity) are calculated for bcc and fcc Coulomb crystals in the harmonic approximation with a fractional accuracy less than or similar to5 10(-5). Temperature dependence of thermodynamic functions is fitted by analytical formulas with an accuracy of a few parts in 10(5). The static-lattice (Madelung) part of the free energy is calculated with an accuracy of similar to 10(-12). The Madelung constant and frequency moments of hep crystals are also computed.
Abstract: The thermal structure of neutron stars with magnetized envelopes is studied using modern physics input. The relation between the internal (T-int) and local surface temperatures is calculated and fitted by analytic expressions for magnetic field strengths B from 0 to 10(16) G and arbitrary inclination of the field lines to the surface. The luminosity of a neutron star with dipole magnetic field is calculated and fitted as a function of B, T-int, stellar mass and radius. In addition, we simulate cooling of neutron stars with magnetized envelopes. In particular, we analyse ultramagnetized envelopes of magnetars and also the effects of the magnetic field of the Vela pulsar on the determination of critical temperatures of neutron and proton superfluids in its core.
Abstract: The internal properties of the neutron star crust can be probed by observing the epoch of thermal relaxation. After the supernova explosion, powerful neutrino emission quickly cools the stellar core, while the crust stays hot. The cooling wave then propagates through the crust, as a result of its finite thermal conductivity. When the cooling wave reaches the surface (age 10-100 yr), the effective temperature drops sharply from 250 eV to 30 or 100 eV, depending on the cooling model. The crust relaxation time is sensitive to the (poorly known) microscopic properties of matter of subnuclear density, such as the heat capacity, thermal conductivity, and superfluidity of free neutrons. We calculate the cooling models with the new values of the electron thermal conductivity in the inner crust, based on a realistic treatment of the shapes of atomic nuclei. Superfluid effects may shorten the relaxation time by a factor of 4. The comparison of theoretical cooling curves with observations provides a potentially powerful method of studying the properties of the neutron superfluid and highly unusual atomic nuclei in the inner crust.
