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

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 loc ally radiates away the dissipated energy as a blackbody. Special and genera l relativistic effects influencing photons emitted by the disc are taken in to account. The emerging spectrum, as seen by a distant observer, is parame trized by the black hole mass and spin, the accretion rate, the disc inclin ation angle and the inner disc radius. We fit the ASCA soft-state X-ray spe ctra of LMC X-1 and GRO J1655-40 by this model. We find that, having additi onal limits on the black hole mass and inclination angle from optical/UV ob servations, 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 b lack hole. In GRO J1655-40 we can limit the spin much better, and we find 0 .68 less than or equal to a less than or equal to 0.88. Accretion discs in both sources are radiation-pressure dominated. We do not find Compton refle ction features in the spectra of any of these objects.