The discovery of low-level iron K line emission from Cyg X-1

We present the results of seven observations of the X-ray spectrum of Cyg X-1 made using the gas scintillation proportional counter on board the EXOSAT Observatory. The discovery of a broad emission feature at 6.2 keV is reported with an equivalent width of typically 120 eV. The line is resolved to have an FWHM of ~1.2 keV. There is no evidence for a modulation of the line parameters with the 5.6-day binary period. The properties of the line are not consistent with it originating from the fluorescence of the photosphere of the companion star. We identify the line as a Compton-redshifted iron Kline and suggest that it comes from recombination in an optically thick ($$\tau\gt5$$) photoionized corona above and below the accretion disc.