Mass determination of black holes in LMC X-1 and Nova Muscae 1991 from their high-energy spectra

We offer a brief description of the bulk motion Comptonization (BMC) model for accretion onto black holes, illustrated by its application to observati onal data for LMC X-l and Nova Muscae 1991. We then extract some physical p arameters of these systems from observables within the context of the BMC m odel, drawing from results on GRO J1655-40, for which we presented extensiv e analysis previously. We derive estimates of the mass, (16 +/- 1) M. x [0. 5/ cos (i)](1/2), and mass accretion rate in the disk in Eddington units (( m) over dot (d) similar or equal to 2) for LMC X-l and (24 +/- 1) M. x [0.5 / cos(i)](1/2)d(5.5) and ((m) over dot (d) similar or equal to 3) for Nova Muscae 1991, where cos(i) and d(5.5) are the inclination angle cosine and d istance in 5.5 kpc units, respectively. Differences between these estimates and previous estimates based on dynamical studies are discussed. It is fur ther shown that the disk inner radius increases with the high-to-low-state transition in Nova Muscae 1991. Specifically, our analysis suggests that th e inner disk radius increases to r(in) similar or equal to 17 Scwarzschild radii as the transition to the low-hard state occurs.