X-ray fluorescence from the inner disc in Cygnus X-1.

The quasi-blackbody plus power-law spectra of many accreting black-hole sources suggests that relatively cold matter is surrounded by hard X-ray emitting plasma. Fluorescent iron lines are produced by X-irradiation of the cold gas. The shape and variability of these lines can be used to map the innermost regions around the black hole. In the case of a disc geometry for the cold gas, the effects of doppler-broadening and gravitational and transverse redshifts produce a characteristic line profile which depends upon inclination. We show here that the broad, iron emission line found in Cyg X-1 by Barr, White & Page is well modelled by fluorescent emission from the inner parts of an accretion disc inclined at ~3O degrees. The mass of the central object and properties of the accretion flow can be determined by future higher resolution studies of this and similar sources, including Active Galaxies.