Optical and X-ray properties of cooling neutron stars

Blackbody fits to the soft X-ray spectra of cooling neutron stars will, in general, overestimate the surface temperature of the source. We examine blackbody fits to the spectra predicted by light-element neutron star model atmospheres folded with the ACIS response matrices and quantify the inferred temperature error for a range of magnetic field strengths representative of the broad populations of isolated cooling neutron stars. Results for the Position Sensitive Proportional Counter are derived for comparative purposes. In each example, the blackbody temperature systematically overestimates the underlying flux temperature of the model and is an unreliable gauge by which to estimate the surface properties of neutron stars. We demonstrate a linear relationship between flux temperature and peak energy, similar to Wien displacement, as a function of magnetic field strength for the model atmospheres. We calculate the excess optical magnitude predicted from the model atmospheres with respect to the Rayleigh-Jeans portion of the fitted blackbody. We detail the nature and magnitude of these discrepancies and identify their origin in the opacity of the atmospheric plasma, the magnitude of interstellar absorption, and the instrumental response function.