A study of the continuum and iron K line emission from low-mass X-ray binaries

Abstract

The spectral properties of a sample of six low-mass X-ray binaries are studied using data obtained with the Gas Scintillation Proportional Counter (GSPC) on EXOSAT. The various continuum models proposed for these sources are investigated. Only two-component models provide an acceptable fit to the data. Two possible model combinations were found: in both cases one component was a blackbody with the other either a thermal bremsstrahlung or an unsaturated Comptonized spectrum of low-energy photons on high-energy electrons. The characteristic flaring of these sources is caused by increases in the luminosity of the blackbody component. The thermal bremsstrahlung plus blackbody model was ruled out on the grounds that the derived emission measure infers a dimension for the emission region greater than 10⁸cm. The Compton plus blackbody model can be interpreted in terms of disc accretion on to a neutron star with a low magnetic field. The Comptonized component is identified as coming from the inner disc region, while the blackbody is from the heated surface of a neutron star. An iron K emission line centred between 6.4 and 6.8 kV was detected in five out of the six sources with equivalent widths of 70–130 eV and a full-width half-maximum of 1 keV. These results are similar to those of Sco X-1 obtained by the EXOSAT GSPC (White, Peacock & Taylor).