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 towards 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/ √ B 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 magneto-rotational instabilities in the accretion disk. Subject headings: accretion, accretion disks – black hole physics – gamma-rays: theory – radiation mechanisms: thermal – stars: individual: Cyg X-1 – X-rays: general