Application of a relativistic accretion disc model to X-ray spectra of LMC X-1 and GRO J1655-40

• Published in: Monthly notices of the Royal Astronomical Society Vol. 325; no. 3; pp. 1253 - 1265
• Main Authors: Gierliński, Marek, Maciołek-Niedźwiecki, Andrzej, Ebisawa, Ken
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 11-08-2001
• Subjects: accretion; accretion discs; accretion, accretion discs; relativity; stars: individual: GRO J1655-40; stars: individual: LMC X-1; X-rays: stars
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1046/j.1365-8711.2001.04540.x

We present a general relativistic accretion disc model and its application to the soft-state X-ray spectra of black hole binaries. The model assumes a flat, optically thick disc around a rotating Kerr black hole. The disc locally radiates away the dissipated energy as a blackbody. Special and general relativistic effects influencing photons emitted by the disc are taken into account. The emerging spectrum, as seen by a distant observer, is parametrized by the black hole mass and spin, the accretion rate, the disc inclination angle and the inner disc radius. We fit the ASCA soft-state X-ray spectra of LMC X-1 and GRO J1655-40 by this model. We find that, having additional limits on the black hole mass and inclination angle from optical/UV observations, we can constrain the black hole spin from X-ray data. In LMC X-1 the constraint is weak, and we can only rule out the maximally rotating black hole. In GRO J1655-40 we can limit the spin much better, and we find 0.68 ⩽ a ⩽ 0.88. Accretion discs in both sources are radiation-pressure dominated. We do not find Compton reflection features in the spectra of any of these objects.