Spectra of X-ray bursts at near-Eddington luminosities

We have developed numerical and analytical models of photospheres of X-ray bursters for the case when the burster luminosity is very close to the Eddington limit, l= L/L_Ed = 0.9-0.999. It is shown that the burst spectrum at observable energies can be described by a diluted Wien spectrum with the spectral temperature T coinciding with the electron temperature in an isothermal outer layer formed by Compton heating in the neutron star photosphere. The dilution factor is proportional to (T_eff/ T) ⁴, where T_eff is the effective temperature. We obtained simple expressions for T/T_eff, and for the dilution factor, which are determined by the parameter (1- l)/ (3 + 5X), where X is the hydrogen abundance. Fitting spectra observed around peaks of strong bursts may put additional constraints on masses and radii of neutron stars, and give new information on chemical composition of their photospheres and on distances to the bursters.