Measuring the Mass Ratios of X-Ray Transients from the Rotational Broadening of the Secondary Star's Spectrum

Extract

We describe a new computer program for calculating the spectra of stars that fill their Roche lobes in close binary stars.

Soft X-ray Transients (SXTs) are interacting binary stars in which a compact star is stripping mass from a relatively-normal companion star (the secondary star). One step in the usual method for measuring the mass of the compact stars in these systems is to determine the mass ratio from the rotational broadening of the absorption lines in the spectrum of the secondary. With a few outstanding exceptions (e.g., Shahbaz 2003), the observations have typically been analyzed assuming that the observed spectrum can be modeled by convolving the spectrum of a non-rotating star with a line-broadening function appropriate for a rotating, spherical star. The results are often inadequate because 1) the profiles of the absorption lines vary strongly with orbital phase, and 2) the limb darkening differs from line to line so that no single broadening function can represent the behavior of all lines.