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 possiblyproduced in an active corona atop an accretion disk. We suggest that theobserved weakness of X-ray reflection from the disk is due to bulk motion ofthe emitting hot plasma away from the reflector. A mildly relativistic motioncauses aberration reducing X-ray emission towards the disk. This in turnreduces the reprocessed radiation from the disk and leads to a hard spectrum ofthe X-ray source. The resulting spectral index is Gamma=1.9B^{1/2} whereB=gamma(1+beta) is the aberration factor for a bulk velocity beta=v/c. Theobserved Gamma=1.6 and the amount of reflection, R=0.3, in Cyg X-1 in the hardstate can both be explained assuming a bulk velocity beta=0.3. We discuss onepossible scenario: the compact magnetic flares are dominated by e+- pairs whichare ejected away from the reflector by the pressure of the reflected radiation.We also discuss physical constraints on the disk-corona model and argue thatthe magnetic flares are related to magneto-rotational instabilities in theaccretion disk.Comment: The final version, accepted for publication in ApJ Letter