Evidence for a Black Hole in the X-Ray Transient GRS 1009-45

We report on the result of an X-ray observation of the X-ray Nova Velorum 1993 (GRS 1009-45), made with ASCA on 1993 November 10--11. The energy spectrum was extremely soft and accompanied by a hard tail, characteristics of the Galactic black-hole binaries in the soft state. It is well represented by a two-component model consisting of a multicolor disk model or a general relativistic accretion disk model, both describing emission from an optically thick accretion disk, and a power-law component with a photon index of ~ 2.5. %The former component describes emission from an optically-thick accretion disk, with a color temperature of 0.49 keV at the inner disk boundary. A spectral analysis of the soft component %multicolor disk component allows an estimation of %the inner radius of the accretion disk which is thought to be proportional to the mass of the central object. For the optically-estimated distance (>1 kpc) and inclination (>37(deg) ), the mass is estimated to be >3.1 MO . These results strongly support that the compact object is a black hole.