X-ray spectral formation in low mass X-ray binaries

Abstract

The spectral properties of the non-pulsing low mass X-ray binaries, LMXRB, are reviewed using recent EXOSAT observations. The spectra of the persistent emission from a sample of the lower luminosity, 10³⁷ erg/s systems (usually X-ray burst sources),are well modelled by a simple power law attenuated at high energies by an exponential cutoff. Their spectra do not show a strong dependence on inclination. The higher luminosity 10³⁸ erg/s systems require an additional blackbody component, whose luminosity ranges from 10 to 40% the total. Various models for the inner region of an accretion disk and its interaction with a neutron star are considered. Blackbody disk models are not appropriate in this case because of the effects of electron scattering. A modified blackbody disk model does not have the correct spectral shape to reproduce the spectra from the lower luminosity sources. It does give a reasonable fit to the spectra of the higher luminosity systems, however the inferred mass transfer rates exceed the Eddington limit by a factor of 3 or more, and are implausibly large. A model where the disk spectrum results from the Comptonization of soft photons on hot electrons gives a consistent fit to the spectra of both luminosity classes. In the higher luminosity systems a second blackbody component, probably from the boundary layer is also required. This blackbody component is absent in the lower luminosity systems, suggesting they contain neutron stars that have been spun up to sub-millisecond periods.