The X-ray spectral properties of accretion discs in X-ray binaries

Abstract

view Abstract Citations (271) References (73) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The X-Ray Spectral Properties of Accretion Disks in X-Ray Binaries White, N. E. ; Stella, L. ; Parmar, A. N. 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 could be modeled equally well by either a modified blackbody disk spectrum, or by a Comptonized spectrum. The fits to the spectra of the high-luminosity LMXRB systems require an additional blackbody component with a luminosity 16% to 34% that from the Comptonized component. Blackbody emission was detected only in one out of five X-ray burst sources, with an upper limit in one case as low as 5%. No evidence was found for any dependence of the spectrum of the X-ray burst sources on system inclination. The blackbody component, when detected, may originate from an optically thick boundary layer between the accretion disk and the neutron star surface. However, the luminosity relative to the Comptonized component (assuming that the latter comes from the accretion disk) is at least a factor of ~10 less than expected. This suggests either that the neutron star in each system rotates close to equilibrium with the Keplerian disk, or that most of the boundary layer emission is not well represented by a blackbody spectrum. Publication: The Astrophysical Journal Pub Date: January 1988 DOI: 10.1086/165901 Bibcode: 1988ApJ...324..363W Keywords: Accretion Disks; Neutron Stars; Spaceborne Astronomy; Stellar Spectrophotometry; X Ray Binaries; X Ray Spectra; Black Body Radiation; Black Holes (Astronomy); Compton Effect; Emission Spectra; Molecular Clouds; Stellar Models; Astrophysics; BLACK HOLES; STARS: ACCRETION; STARS: NEUTRON; X-RAYS: BINARIES; X-RAYS: SPECTRA full text sources ADS | data products SIMBAD (17)