Discovery of a red and blue shifted iron disk line in the galactic jet source GROJ 1655 - 40

We report the discovery of emission features in the X-ray spectrum of GRO J1655-40 obtained with RXTE during the observation of 1997, Feb 26. We have fitted the features firstly by two Gaussian lines which in four spectra analysed have average energies of 5.85 ± 0.08 keV and 7.32 ± 0.13 keV, strongly suggestive that these are the red and blue shifted wings of an iron disk line. These energies imply a velocity of ∼0.33 c. The blue wing is less bright than in the calculated profiles of disk lines near a black hole subject to Doppler boosting, however known Fe absorption lines in GRO J1655-40 at energies between ∼7 and 8 keV can reduce the apparent brightness of the blue wing. Secondly, we have fitted the spectra using the disk line model of Laor based on a full relativistic treatment plus an absorption line, and show that good fits are obtained. This gives a restframe energy of the disk line between 6.4 and 6.8 keV indicating that the line is iron K α emission probably of significantly ionized material. The Laor model shows that the line originates in a region of the accretion disk extending from ∼10 Schwarzschild radii from the black hole outwards. The line is direct evidence for the black hole nature of the compact object and is the first discovery of a highly red and blue shifted iron disk line in a Galactic source.