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 observational data for LMC X-1 and Nova Muscae 1991. We then extract some physical parameters of these systems from observables within the context of the BMC model, drawing from results on GRO J1655-40, for which we presented extensive analysis previously. We derive estimates of the mass, (16 ± 1) M☉ × [0.5/ cos(i)]1/2, and mass accretion rate in the disk in Eddington units (ṁd ≃ 2) for LMC X-1 and (24 ± 1) M☉ × [0.5/ cos(i)]1/2d5.5 and (ṁd ≃ 3) for Nova Muscae 1991, where cos(i) and d5.5 are the inclination angle cosine and distance in 5.5 kpc units, respectively. Differences between these estimates and previous estimates based on dynamical studies are discussed. It is further shown that the disk inner radius increases with the high-to-low-state transition in Nova Muscae 1991. Specifically, our analysis suggests that the inner disk radius increases to rin ≃ 17 Scwarzschild radii as the transition to the low-hard state occurs.