The High-Energy Spectra of Accreting Black Holes: Observational Evidence for Bulk-Motion Infall

We discuss the emergent spectra from accreting black holes, considering in particular the case where the accretion is characterized by relativistic bulk motion. We suggest that such accretion is likely to occur in a wide variety of black hole environments, where the strong gravitational field is expected to dominate the pressure forces, and that this is likely to lead to a characteristic high-energy spectroscopic signature: an extended power-law tail. It is in the high (soft) state that matter impinging upon the event horizon can be viewed directly, and the intrinsic power law is seen. Certain types of active galactic nuclei (AGNs) may represent the extragalactic analog of the high-soft state accretion, which would further support our ideas, demonstrating the stability of the ( alpha ~1.8 ) power law. This stability is a result of the asymptotic independence of the spectral index on the mass accretion rate and its weak dependence on plasma temperatures. We have computed the expected spectral energy distribution for an accreting black hole binary in terms of our three model parameters: the disk color temperature, a geometric factor related to the illumination of the black hole site by the disk, and a spectral index related to the efficiency of the bulk-motion up-scattering. We emphasize that this is a fully self-consistent approach, and it is not to be confused with the more common phenomenological methods employing additive power law and blackbody or multicolor disk. A test of the model is presented using observational data from the Compton Gamma Ray Observatory (CGRO) and the Rossi X-Ray Timing Explorer (RXTE), covering ~=2-200 keV for two recent galactic black hole X-ray nova outbursts. The resulting model fits are encouraging and, along with some observational trends cited from the literature, they support our bulk-motion hypothesis