The X-ray spectral properties of accretion discs in X-ray binaries
Abstract
Using EXOSAT observations, the spectral properties of the persistent emission from a number of X-ray burst sources, high-luminosity low-mass X-ray binaries (LMXRB), and galactic black hole candidates are compared with various models for X-ray emission from an accretion disk surrounding a compact object in a binary system. These models include the optically thick blackbody disk model considered by Mitsuda et al. (1984), two different disk models where the blackbody spectrum is modified by electron scattering (Shakura and Sunyaev 1973; Nannurelli, Ruffini, and Stella 1987), and a model (not yet specific to a disk geometry) where losses by the Comptonization of cool photons on hot electrons dominates the spectral formation (Sunyaev and Titarchuk 1980). We note that for accretion onto a neutron star or black hole the simple blackbody accretion disk model is not correct and that it must be modified to take into account the effects of electron scattering. Disks that emit locally as a modified blackbody require super-Eddington mass accretion rates to give high enough temperatures to account for the spectra observed from the LMXRB. Such high accretion rates are not self-consistent with the model. For the persistent emission from the lower luminosity X-ray burst sources, the modified blackbody disk models predict spectra that are much cooler than required. Only a Comptonization model provides a good fit to all the spectra considered. The spectrum of the black hole candidate LMC X-3…
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