ASCA observation of the X-ray transient GRS 1009-45

We report on the result of the X-ray observation of X-ray Nova Velorum 1993 (GRS 1009-45), made with ASCA on November 10-11, 1993. 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. Spectral analysis of the soft component allows estimation of the mass of the central object. For the optically-estimated distance (> 1 kpc) and inclination (> 37°), the mass is estimated to be > 3.1 M_⊙. These results strongly support that the compact object is a black hole.