Self-consistent Comptonized models for the spectra of low-mass X-ray binaries

We have constructed Comptonized spectral models and fitted them to EXOSAT spectra of eleven bright low-mass X-ray binaries. The models are based on the premise that these systems contain circumstellar material which modifies the X-ray spectrum passing through it. We idealize this situation as a point source of X-rays, the neutron star, embedded in an optically thick scattering cloud. The temperature and ionization structure of the cloud are determined in a self-consistent manner. The emerging spectrum is a result of the interaction between the radiation field and the material in the cloud. By interpolating on a four-dimensional grid of pre-computed models, we are able to perform spectral fits to observed data in the usual way. Our fits to the X-ray continuum are as good as those obtained using standard two-component models, and a number of interesting systematic trends are apparent in the fitted model parameters. (author)