The Transition Zone in Balmer-Dominated Shocks

We examine the structure of the post-shock region in supernova remnants (SNRs). The ``shock transition zone'' is set up by charge transfer and ionization events between atoms and ions, and has a width $\sim 10^{15}$ cm$^{-2}$ $n^{-1}_0$, where $n_0$ is the total pre-shock density (including both atoms and ions). For Balmer-dominated SNRs with shock velocity $v_s \gtrsim 1000$ km s$^{-1}$, the Rankine-Hugoniot conditions for ion velocity and temperature are obeyed instantly, leaving the full width at half-maximum (FWHM) of the broad H$\alpha$ line versus $v_s$ relation intact. However, the spatial variation in the post-shock densities is relevant to the problem of Ly$\alpha$ resonant scattering in young, core-collapse SNRs. Both two- (pre-shock atoms and ions) and three-component (pre-shock atoms, broad neutrals and ions) models are considered. We compute the spatial emissivities of the broad ($\xi_b$) and narrow ($\xi_n$) H$\alpha$ lines; a calculation of these emissivities in SN 1006 is in general agreement with the computed ones of Raymond et al. (2007). The (dimensionless) spatial shift, $\Theta_{\rm{shift}}$, between the centroids of $\xi_b$ and $\xi_n$ is unique for a given shock velocity and $f_{\rm{ion}}$, the pre-shock ion fraction. Measurements of $\Theta_{\rm{shift}}$ can be used to constrain $n_0$.Comment: 25 pages, 8 figures. Accepted by Astrophysical Journa

Polarized X-rays from Magnetized Neutron Stars

We review the polarization properties of X-ray emission from highly magnetized neutron stars, focusing on emission from the stellar surfaces. We discuss how x-ray polarization can be used to constrain neutron star magnetic field and emission geometry, and to probe strong-field quantum electrodynamics and possibly constrain the properties of axions.Comment: to appear in "X-ray Polarimetry: A New Window in Astrophysics", edited by R. Bellazzini, E. Costa, G. Matt and G. Tagliaferri (Cambridge University Press

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

Magnetic Hydrogen Atmosphere Models and the Neutron Star RX J1856.5-3754

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 \approx 1x10^20 cm^-2 and an emitting area with R^infty \approx 17 km (assuming a distance to RX J1856.5-3754 of 140 pc), temperature T^infty \approx 4.3x10^5 K, gravitational redshift z_g \sim 0.22, atmospheric hydrogen column y_H \approx 1 g cm^-2, and magnetic field B \approx (3-4)x10^12 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 neutron star surface as well as general relativistic effects, to determine pulsations; we find 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.Comment: 11 pages, 8 figures; MNRAS, accepte

Soft X-ray Polarization in Thermal Magnetar Emission

Emission spectra from magnetars in the soft X-ray band likely contain a thermal component emerging directly from the neutron star surface. However, the lack of observed absorption-like features in quiescent spectra makes it difficult to directly constrain physical properties of the atmosphere. We argue that future X-ray polarization measurements represent a promising technique for directly constraining the magnetar magnetic field strength and geometry. We construct models of the observed polarization signal from a finite surface hotspot, using the latest NS atmosphere models for magnetic fields B = 4 x 10^13--5 x 10^14 G. Our calculations are strongly dependent on the NS magnetic field strength and geometry, and are more weakly dependent on the NS equation of state and atmosphere composition. We discuss how the complementary dependencies of phase-resolved spectroscopy and polarimetry might resolve degeneracies that currently hamper the determination of magnetar physical parameters using thermal models.Comment: 23 pages, 7 figures; MNRAS accepte

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&

Effects of Neutral Particles on Modified Shocks at Supernova Remnants

H-alpha emission from supernova remnants (SNRs) implies the existence of neutral hydrogens in the ambient medium. In the precursor of an SNR shock modified by cosmic rays (CRs), upstream plasmas are pushed by the CR pressure, but neutral particles are not, so that the relative velocity appears and some neutral particles become pickup ions by the charge exchange process in the precursor. We investigate how the pickup ions generated in the precursor affect the shock structure and the particle acceleration. If the CR pressure is larger than 20% of the shock ram pressure, the compression of the subshock becomes smaller than that without pickup ions because of the pressure of the pickup ions. Moreover, even if the shock is modified by CRs, the total compression ratio can be smaller than 4. In addition, the pickup ions play an important role for the injection into the particle acceleration. If the shock is a quasi-perpendicular shock and if the the multiply reflected ion acceleration occurs, the CR spectrum can be harder than that of the test particle diffusive shock acceleration below GeV.Comment: 13 pages, 1 figures, accepted for publication in ApJ Letter

Thermal Radiation from Isolated Neutron Stars: Spectra and Polarizations

Recent observations of surface emission from isolated neutron stars (NSs) provide unique challenges to theoretical modeling of thermal radiative processes. We construct models of thermal emission from strongly magnetized NSs in which the outermost layer of the NS is in a condensed liquid or solid form, or is an ionized H or He atmosphere. We calculate the emission properties (spectrum and polarization) of NSs with condensed Fe and H surfaces using a generalized form of Kirchhoff's Law, in the regimes where condensation may be possible. For smooth condensed surfaces, the overall emission is reduced from blackbody by less than a factor of two. The spectrum exhibits modest deviation from blackbody across a wide energy range, and shows mild absorption features associated with the electron plasma and ion cyclotron frequencies in the condensed matter. The roughness of the solid Fe condensate decreases the reflectivity of the surface, making the emission spectrum even closer to blackbody. We provide an accurate treatment of vacuum polarization effects in magnetized NS atmosphere models. We treat the conversion of photon modes (due to ``vacuum resonance'' between plasma and vacuum polarizations), employing both the modal radiative transfer equations (coupled with an accurate mode conversion probability at the vacuum resonance) and the full radiative transfer equations for the photon Stokes parameters. We are able to quantitatively calculate the atmosphere structure, emission spectra, beam patterns, and polarizations for the range of magnetic field strengths $B=10^{12}-10^{15}$ G. In agreement with previous studies, we find that for NSs with magnetic field strengths B/2 \ga B_l\simeq 7\times 10^{13} G, vacuum polarization reduces the widths of spectral features and softens the hard tail of magnetized atmosphere models. For B\la B_l/2, vacuum polarization does not change the emission spectra, but can affect the polarization signals. We investigate the propagation of photon polarization in NS magnetospheres, and show that vacuum polarization induces a unique energy-dependent linear polarization signature, and can generate circular polarization in the magnetospheres of rapidly rotating NSs. We discuss the implications of our results for observations of thermally emitting isolated NSs and magnetars, and the prospects for future spectral and polarization studies