The Survey of Low‐Mass X‐Ray Binaries with the Einstein Observatory Solid‐State Spectrometer and Monitor Proportional Counter

Abstract

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 theirintensity, were fitted with a set of simple and complex spectralmodels. For all the sources, including Eddington-limited bulge sources,bursters, dippers, the soft spectrum black hole candidates, and a fewtransients in decline, the spectra could be fitted acceptably withcombinations of thermal bremsstrahlung and blackbody models or aComptonized spectrum and a blackbody. The results rule out opticallythick disk models for the bright (Z) sources and power-law models forthe bursters. The SSS can confirm only the strongest of previouslyreported low-energy emission lines due to O VIII or Fe L transitions.Uncertainties in column densities are modest, and together with considerationof a disk distribution of gas and dust in the galaxy, the columndensities provide distance estimates. These are the only measures yetavailable for eight sources. Several physical interpretations interms of an optically thick component located on or very near theneutron star, an optically thin region farther in radius from theneutron 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-colordiagrams are more complex than previously reported, although relatingluminosity changes to the mass accretion rate is still tenable.