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**Session 8 - Relativistic Astrophysics.**

*Display session, Wednesday, January 07*

*Exhibit Hall, *

## [8.06] The High-Energy Spectra of Accreting Black Holes: A Signature of Converging Bulk Motion?

*C. R. Shrader, L. Titarchuk (NASA/GSFC)*

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 such accretion is likely to be occurring for a
wide variety of environments, and is likely to lead to a characteristic
high--energy spectroscopic signature. We have computed the expected
spectral
energy distribution for the case of a soft (thermal) photon source,
presumably due to viscous dissipation from the inner region of an
accretion disk, parameterized in terms of the mass accretion rate and a
geometric factor. A test of the model is presented using observational
data
covering \simeq 2-200 keV for several recent galactic black hole
X--ray nova outbursts. The resulting model fits are encouraging,
supporting
our bulk--motion hypothesis. We additionally have attempted to
calculate
some physical parameters -- the black hole mass and its distance -- from

the normalization to our model fits. The distance thus derived for
GRO J1655--40, is consistent with independent measurements, but the
inferred compact object mass required a 20% adjustment
in the hardening factor in order to reach reasonable agreement
with the dynamically determined value. Reasonable distance and mass
determinations are also obtained for GRS 1915+105.

**Program
listing for Wednesday**