Absorption Features in Spectra of Magnetized Neutron Stars

The X-ray spectra of some magnetized isolated neutron stars (NSs) show absorption features with equivalent widths (EWs) of 50 - 200 eV, whose nature is not yet well known. To explain the prominent absorption features in the soft X-ray spectra of the highly magnetized (B ~ 10^{14} G) X-ray dim isolated NSs (XDINSs), we theoretically investigate different NS local surface models, including naked condensed iron surfaces and partially ionized hydrogen model atmospheres, with semi-infinite and thin atmospheres above the condensed surface. We also developed a code for computing light curves and integral emergent spectra of magnetized neutron stars with various temperature and magnetic field distributions over the NS surface. We compare the general properties of the computed and observed light curves and integral spectra for XDINS RBS\,1223 and conclude that the observations can be explained by a thin hydrogen atmosphere above the condensed iron surface, while the presence of a strong toroidal magnetic field component on the XDINS surface is unlikely. We suggest that the harmonically spaced absorption features in the soft X-ray spectrum of the central compact object (CCO) 1E 1207.4-5209 (hereafter 1E 1207) correspond to peaks in the energy dependence of the free-free opacity in a quantizing magnetic field, known as quantum oscillations. To explore observable properties of these quantum oscillations, we calculate models of hydrogen NS atmospheres with B ~ 10^{10} - 10^{11} G (i.e., electron cyclotron energy E_{c,e} ~ 0.1 - 1 keV) and T_eff = 1 - 3 MK. Such conditions are thought to be typical for 1E 1207. We show that observable features at the electron cyclotron harmonics with EWs \approx 100 - 200 eV can arise due to these quantum oscillations.Comment: 4 pages, 3 figures, conference "Astrophysics of Neutron Stars - 2010" in honor of M. Ali Alpar, Izmir, Turke

Thomas-Fermi Calculations of Atoms and Matter in Magnetic Neutron Stars II: Finite Temperature Effects

We present numerical calculations of the equation of state for dense matter in high magnetic fields, using a temperature dependent Thomas-Fermi theory with a magnetic field that takes all Landau levels into account. Free energies for atoms and matter are also calculated as well as profiles of the electron density as a function of distance from the atomic nucleus for representative values of the magnetic field strength, total matter density, and temperature. The Landau shell structure, which is so prominent in cold dense matter in high magnetic fields, is still clearly present at finite temperature as long as it is less than approximately one tenth of the cyclotron energy. This structure is reflected in an oscillatory behaviour of the equation of state and other thermodynamic properties of dense matter and hence also in profiles of the density and pressure as functions of depth in the surface layers of magnetic neutron stars. These oscillations are completely smoothed out by thermal effects at temperatures of the order of the cyclotron energy or higher.Comment: 37 pages, 17 figures included, submitted to Ap

Updated Electron-Conduction Opacities: The Impact on Low-Mass Stellar Models

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.Comment: 11 pages, 6 figures, ApJ in pres

Radiative properties of highly magnetized isolated neutron star surfaces and approximate treatment of absorption features in their spectra

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. We theoretically investigate different models to explain absorption features and compare their properties with the observations. We consider various theoretical models for the magnetized neutron star surface: naked condensed iron surfaces and partially ionized hydrogen model atmospheres, including semi-infinite and thin atmospheres above a condensed surface. 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 can also be employed. Light curves and spectra of highly magnetized neutron stars with parameters typical for XDINSs are computed using different surface temperature distributions and various local surface models. Spectra of magnetized model atmospheres are approximated by diluted blackbody spectra with one or two Gaussian lines having parameters, which allow us to describe the model absorption features. To explain the prominent absorption features in the soft X-ray spectra of XDINSs a thin atmosphere above the condensed surface can be invoked, whereas a strong toroidal magnetic field component on the XDINS surfaces can be excluded.Comment: 54 pages, 17 figures, accepted for publication in A&

Variability in the Thermal Emission from Accreting Neutron Star Transients

The composition of the outer 100 m of a neutron star sets the heat flux that flows outwards from the core. For an accreting neutron star in an X-ray transient, the thermal quiescent flux depends sensitively on the amount of hydrogen and helium remaining on the surface after an accretion outburst and on the composition of the underlying ashes of previous H/He burning. Because H/He has a higher thermal conductivity, a larger mass of H/He implies a shallower thermal gradient through the low density envelope and hence a higher effective temperature for a given core temperature. The mass of residual H and He varies from outburst to outburst, so the thermal quiescent flux is variable even though the core temperature is constant for timescales < 10 000 yr. Heavy elements settle from a H/He envelope in a few hours; we therefore model the quiescent envelope as two distinct layers, H/He over heavier elements, and treat the mass of H/He as a free parameter. We find that the emergent thermal quiescent flux can vary by a factor of 2 to 3 between different quiescent epochs. The variation is more pronounced at lower interior temperatures, making systems with low quiescent luminosities and frequent outbursts, such as SAX J1808.4-3658, ideal candidates from which to observe this effect. We compute, for different ash compositions, the interior temperatures of Cen X-4, Aql X-1, and SAX J1808.4-3658. In the case of Aql X-1, the inferred high interior temperature suggests that neutrino cooling contributes to the neutron star's thermal balance.Comment: 14 pages, 5 figures, uses emulateapj5 and psnfss fonts. To be published in The Astrophysical Journa

Thermal structure and cooling of superfluid neutron stars with accreted magnetized envelopes

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 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 interpretation of observations of isolated neutron stars hottest for their age, such as RX J0822-43 and PSR B1055-52.Comment: 15 pages, 12 figures, 2 tables. Corrected title only (v2

Nonideal strongly magnetized plasmas of neutron stars and their electromagnetic radiation

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. This is a companion paper to astro-ph/0511803Comment: 7 pages, 3 figures. Invited topical talk at Strongly Coupled Coulomb Systems (Moscow, June 20-25, 2005); to appear in Journal of Physics

Theory of cooling neutron stars versus observations

We review current state of neutron star cooling theory and discuss the prospects to constrain the equation of state, neutrino emission and superfluid properties of neutron star cores by comparing the cooling theory with observations of thermal radiation from isolated neutron stars.Comment: 9 pages, 4 figures, 3 tables, to appear in the proceedings of "40 Years of Pulsars" held in Montreal, Canada, August 12-17, 2007, eds. C. Bassa, Z. Wang, A. Cumming, V. Kaspi, AIP, in press (v.2 - minor bibliography corrections

Perturbative approach to the hydrogen atom in strong magnetic field

The states of hydrogen atom with principal quantum number n <= 3 and zero magnetic quantum number in constant homogeneous magnetic field H are considered. The perturbation theory series is summed with the help of Borel transformation and conformal mapping of the Borel variable. Convergence of approximate energy eigenvalues and their agreement with corresponding existing results are observed for external fields up to n^3 H ~ 5. The possibility of restoring the asymptotic behaviour of energy levels using perturbation theory coefficients is also discussed.Comment: LaTeX, 8 pages with 5 eps figure