Bulk Motion in X-ray Flares near Accreting Black Holes

The weakness of X-ray reflection in Cyg X-1 is suggested to be due to vertical motion of the emitting plasma in the active corona of an accretion disk. A mildly relativistic bulk motion causes a reduction of X-ray emission towards the disk which reflects/reprocesses the incident X-rays. The observed slope of the X-ray spectrum and the amount of reflection can both be explained if the flaring plasma has a bulk velocity ∼ c/3. If the energy is released in compact magnetic flares localized atop the accretion disk, then the resulting bulk motion implies that the flares are accompanied by plasma ejection from the active regions. We discuss one possible scenario: the flares are dominated by e pairs which are accelerated away from the reflector by the pressure of the reflected radiation. In this case, the self-consistent bulk velocity is estimated to be mildly relativistic. Subject headings: accretion, accretion disks – black hole physics – gamma-rays: theory – plasmas – radiation mechanisms: thermal – X-rays: general