  We present our analysis of the high-energy radiation from black hole (BH)
transients, using archival data obtained primarily with RXTE observatory, 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 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 upon 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 black
hole (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. Using model parameters we present new, independent constraints on
the black hole mass, mass accretion rate and the distance for the
aforementioned sources. 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 consistent with the disk model we have invoked - standard
Shakura-Sunyaev's disk. This allows us to impose an important estimation of the
hardness parameter, the ratio of the color temperature to the effective
temperature - we find T_h~2.6, higher than previous estimates used in the
literature.
