On Spectra of X-Ray Bursters in the Expansion Phase

Abstract

Two alternative approaches to the theoretical description of spectra of X-ray bursters showing radial expansion near the light curve maximum are presented. The first one, using the LTE assumption, can be used to explain the noticeable soft excess which is present in corresponding Exosat observations. The second one is based on numerical calculations of radiation transfer and provides not only the qualitative scenario of the drastic softening of spectra as the expansion occurs, but also the ratio of maximum values of Planckian fitting temperatures which seems to be quite a good estimate for the value observed in 4U/MXB 1820-30. The double-peaked structure of bursts with expansion is also a natural consequence of the scenario presented.