Accretion disk in Cyg X-1 in the soft state

Abstract

We analyze the simultaneous ASCA/RXTE observation of Cyg X-1 in the soft state on 1996 May 30. We apply a multi-color disk model with a torque-free boundary condition in the pseudo-Newtonian potential, and point out the main discrepancies between the commonly-used simplified `diskbb' model and our one. We estimate the black hole mass and the accretion rate, and show that during this observation the disk probably did not reach the last stable orbit and the transition from the hard to the soft state was not completed yet.

6 pages, 3 figures, to appear in the Proceedings of the Conference "High Energy Processes in Accreting Black Holes", Graftaavallen, Sweden, June 29--July 4, 1998