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

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&

Thermal states of coldest and hottest neutron stars in soft X-ray transients

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 > 100) on the top (Haensel & Zdunik 1990, 2003, astro-ph/0305220). The relation between the internal and surface stellar temperatures is obtained and fitted. 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, astro-ph/0206376) 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.Comment: 12 pages, 5 figures, 4 tables, LaTeX2e with aa.cls v.5.3 (included). Accepted by A&

Thermalisation time and specific heat of neutron stars crust

We discuss the thermalisation process of the neutron stars crust described by solving the heat transport equation with a microscopic input for the specific heat of baryonic matter. The heat equation is solved with initial conditions specific to a rapid cooling of the core. To calculate the specific heat of inner crust baryonic matter, i.e., nuclear clusters and unbound neutrons, we use the quasiparticle spectrum provided by the Hartree-Fock-Bogoliubov approach at finite temperature. In this framework we analyse the dependence of the crust thermalisation on pairing properties and on cluster structure of inner crust matter. It is shown that the pairing correlations reduce the crust thermalisation time by a very large fraction. The calculations show also that the nuclear clusters have a non-negligible influence on the time evolution of the surface temperature of the neutron star.Comment: 7 pages, 5 figures, submitted to Phys. Rev.

Models of magnetized neutron star atmospheres: Thin atmospheres and partially ionized hydrogen atmospheres with vacuum polarization

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. Alms. We develop a computer code for modeling magnetized neutron star atmospheres in a wide range of magnetic fields (10 12-1015 G) and effective temperatures (3 x 10 5-107 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). © ESO 2009

Models of magnetized neutron star atmospheres

We present a new computer code for modeling magnetized neutron star atmospheres in a wide range of magnetic fields (1012 s(-) 1015 G) and effective temperatures (3 × 105 s(-) 107 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). © 2009 COSPAR

Models of magnetized neutron star atmospheres: thin atmospheres and partially ionized hydrogen atmospheres with vacuum polarization

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.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 \times 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. 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. 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).Comment: Accepted for publications in Astronomy and Astrophysics, 9 pages, 12 figure