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

Abstract

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 are presented. 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 about 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 its originating from the fluorescence of the photosphere of the companion star. The line is identified as a Compton-redshifted iron K line and it is suggested that it comes from recombination in an optically thick (tau greater than 5) photoionized corona above and below the accretion disk.