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

Abstract

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.