The survey of low-mass X-ray binaries with the Einstein Observatory solid-state spectrometer and monitor proportional counter

The HEAO 2 or Einstein solid-state spectrometer (SSS; 0.5-4.5 keV) and monitor proportional counter (MPC; 1.2-20.0 keV) carried out an extensive survey of 49 low-mass X-ray binaries (LMXBs). Simultaneous SSS plus MPC spectra, selected on the basis of their intensity, were fitted with a set of simple and complex spectral models. For all the sources, including Eddington-limited bulge sources, bursters, dippers, the soft spectrum black hole candidates, and a few transients in decline, the spectra could be fitted acceptably with combinations of thermal bremsstrahlung and blackbody models or a Comptonized spectrum and a blackbody. The results rule out optically thick disk models for the bright (Z) sources and power-law models for the bursters. The SSS can confirm only the strongest of previously reported low-energy emission lines due to O VIII or Fe L transitions. Uncertainties in column densities are modest, and together with consideration of a disk distribution of gas and dust in the galaxy, the column densities provide distance estimates. These are the only measures yet available for eight sources. Several physical interpretations in terms of an optically thick component located on or very near the neutron star, an optically thin region farther in radius from the neutron star, and contribution of the accretion disk are possible. Assumptions required of the pulsar in a cocoon model are problematical. Correlations of spectral parameters with intensity observed in color-color diagrams are more complex than previously reported, although relating luminosity changes to the mass accretion rate is still tenable.