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

TLDR

Accreting black‑hole sources show quasi‑blackbody plus power‑law spectra, implying cold matter surrounded by hard X‑ray plasma that produces fluorescent iron lines whose Doppler, gravitational, and transverse redshifts shape the line profile depending on disc inclination. The study aims to use the shape and variability of these iron lines to map the innermost regions around the black hole and to determine the central mass and accretion flow properties through future high‑resolution observations of similar sources, including active galaxies. The broad iron emission line observed in Cyg X‑1 is well modeled by fluorescent emission from the inner accretion disc inclined at ~30°, confirming the disc geometry.

Abstract

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 ∼ 30 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.