Theoretical spectra of nonmagnetized low-mass X-ray binaries

Abstract

Theoretical X-ray spectra of low-mass X-ray binaries with negligible magnetic fields are presented. The geometry of the X-ray emitting region, the energetic efficiency of the accretion in the disk and in the boundary layer which leads to a relation between the disk and the boundary layer luminosities, and the irradiation of the disk by the boundary layer are studied. The model of the radiation spectrum emerging from the neutron star and the innermost part of the disk is presented. The relativistic and Doppler effects and their influence on the spectrum as a function of inclination angle are discussed. A simple method for comparing the spectrum model with observations by studying the hardness ratio is given, and the results for three X-ray sources in globular clusters observed by the Einstein satellite are presented. The range of applicability of the spectrum models is also discussed.