Magnetized Atmospheres around Neutron Stars Accreting at Low Rates

We present a detailed investigation of atmospheres around accreting neutron stars with high magnetic field (B ∼> 1012 G) and low luminosity (L ∼< 1033 erg/s). We compute the atmospheric structure, intensity and emergent spectrum for a plane–parallel, pure hydrogen medium by solving the transfer equations for the normal modes coupled to the hydrostatic and energy balance equations. The hard tail found in previous investigations for accreting, non–magnetic neutron stars with comparable luminosity is suppressed and the X–ray spectrum, although still harder than a blackbody at the star effective temperature, is nearly planckian in shape. Spectra from accreting atmospheres, both with high and low fields, are found to exhibit a significant excess at optical wavelengths above the Rayleigh–Jeans tail of the X–ray continuum. Subject headings: Accretion, accretion disks — radiative transfer — stars: magnetic fields — stars: neutron — X–rays: stars