Plasma ejection from magnetic flares and the X-ray spectrum of Cygnus X-1

The hard X-rays in Cyg X-1 and similar black hole sources are possibly produced in an active corona atop an accretion disk. We suggest that the observed weakness of X-ray reflection from the disk is due to bulk motion of the emitting hot plasma away from the reflector A mildly relativistic motion causes aberration, reducing Xray emission toward the disk. This in turn reduces the reprocessed radiation from the disk and leads to a hard spectrum of the X-ray source. The resulting spectral index is Gamma approximate to 1.9/root B, where B = gamma(l + beta) is the aberration factor for a bulk velocity beta = v/c. The observed Gamma approximate to 1.6 and the amount of reflection (R approximate to 0.3) in Cyg X-1 in the hard state can both be explained assuming a bulk velocity beta - 0.3. We discuss one possible scenario: the compact magnetic flares are dominated by e(+/-) pairs, which are ejected away from the reflector by the pressure of the reflected radiation. We also discuss physical constraints on the disk-corona model and argue that the magnetic flares are related to magnetorotational instabilities inr;he accretion disk.