Do the Spectra of Soft X-Ray Transients Reveal Bulk-Motion Inflow Phenomenon?

We present our analysis of the high-energy radiation from black hole (BH) transients using archival data obtained primarily with RXTE and a comprehensive test of the bulk-motion Comptonization (BMC) model for the high-soft state continuum. The emergent spectra of over 30 separate measurements of the GRO J1655-40, GRS 1915+105, GRS 1739-278, 4U 1630-47 XTE J1755-32, and EXO 1846-031 X-ray sources are successfully fitted by the BMC model, which has been derived from basic physical principles in previous work. This in turn provides direct physical insight into the innermost observable regions, where matter impinging on the event horizon can effectively be directly viewed. The BMC model is characterized by three parameters: the disk color temperature, a geometric factor related to the illumination of the BH site by the disk, and a spectral index related to the efficiency of the bulk-motion upscattering. For the case of GRO J1655-40, where there are distance and mass determinations, a self-consistency check of the BMC model has been made, in particular of the assumption regarding the dominance of gravitational forces over the pressure forces within the inner few Schwarzschild radii. We have also examined the time behavior of these parameters, which can provide information on the source structure. Using our inferred model parameters: color temperature, spectral index, and an absolute normalization, we present new, independently derived constraints on the BH mass, mass accretion rate, and the distance for the aforementioned sources. Also notable is the relationship between the color temperature and flux, which for GRO J1655-40 is entirely distinct from a simple T^4 dependence and strikingly consistent with the disk model we have invoked: a standard Shakura-Sunyaev disk with the modification to the electron scattering. This provides insight into the origin of the seed soft photons and allows us to impose an important estimation of the hardening parameter, T_h, which is the ratio of the color temperature to the effective temperature: we find T_h~=2.6, higher than previous estimates used in the literature