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 X-ray 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 Γ≈1.9/ , where B =γ(1+β) is the aberration factor for a bulk velocity β= v / c . The observed Γ≈1.6 and the amount of reflection ( R ≈0.3) in Cyg X-1 in the hard state can both be explained assuming a bulk velocity β~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 in the accretion disk.