Reflection spectra from an accretion disc illuminated by a neutron star X-ray burst

Abstract

Recent time-resolved X-ray spectra of a neutron star undergoing a superburst revealed an Fe Kα line and edge consistent with reprocessing from the surrounding accretion disc. Here, we present models of X-ray reflection from a constant-density slab illuminated by a blackbody, the spectrum emitted by a neutron star burst. The calculations predict a prominent Fe Kα line and a rich soft X-ray line spectrum which is superimposed on a strong free–free continuum. The lines slowly vanish as the ionization parameter of the slab is increased, but the free–free continuum remains dominant at energies less than 1 keV. The reflection spectrum has a quasi-blackbody shape only at energies greater than 3 keV. If the incident blackbody is added to the reflection spectrum, the Fe Kα equivalent width varies between 100 and 300 eV depending on the ionization parameter and the temperature, kT, of the blackbody. The equivalent width is correlated with kT, and therefore we predict a strong Fe Kα line when an X-ray burst is at its brightest (if iron is not too ionized or the reflection amplitude too small). Extending the study of reflection features in the spectra of superbursts to lower energies would provide further constraints on the accretion flow. If the Fe Kα line or other features are relativistically broadened, then they can determine the system inclination angle (which leads to the neutron star mass), and, if the mass is known, a lower limit to the mass-to-radius ratio of the star.