NEAR-SIMULTANEOUS ROSAT AND GINGA OBSERVATIONS OF THE 1991 X-RAY TRANSIENT IN MUSCA

During its All-Sky-Survey ROSAT has observed the Musca 1991 X-ray transient on January 24/25, two weeks after outburst, for about 170s. The intensity was found to be nearly 6 Crab in the ROSAT band. A combined fit of ROSAT and Ginga data from 1991 January 25 with a multitemperature disk black-body model plus a power law component results in a maximal temperature of the disk of about kT = 0.96 keV at an absorbing column of N(H) = 2.2 x 10(21) cm-2. With the knowledge of the mass function f(M) = 3.1 M. from optical observations we derive a minimum distance to the black hole binary of at least 4-5 kpc. Including the accretion disk inclination angle of i = 26-degrees +/- 25-degrees determined from the shape of the positron annihilation line, and the quiescent optical brightness together with the most probable spectral type of the companion we derive a black hole mass of M = (6 +/- 1.5) M. and a distance of about (11 +/- 3) kpc. Additional ROSAT observations in August 1991 and March 1992 suggest that the exponential intensity decay until 240 days after the outburst is followed by a steeper decline between days 240 and 410.