Spectra of x-ray bursting neutron stars

We present self-consistent atmospheric models for X-ray bursting neutron stars which include the effects of inelastic Compton scattering and free-free and bound-free emission and absorption. We show that the calculated spectra are harder than blackbodies of the same effective temperature. This spectra hardening effect is attributable to the reduction of the radiation source function by electron scattering, and the strong frequency dependence of the free-free opacity. Using the calculated spectra to interpret the observations results in lower bolometric surface fluxes than has been inferred from the common assumption of blackbody emission at the effective temperature. The apparent super-Eddington fluxes inferred from observed spectral temperatures can thus be misleading unless accurate distance determinations are available. The expected strength of iron absorption features and their use as diagnostics for the gravitational redshift at the neutron star surface are also discussed