Spectral evolution of the continuum and disc line in dipping in GRO J1655-40

The discovery is reported of emission features in the X-ray spectrum of GRO J1655-40 obtained using Rossi-XTE on 1997, Feb 26. The features have been fitted firstly by two Gaussian lines, which in four spectra 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 disc line from material with velocity similar to0.33 c. The blue wing is apparently less bright than expected for a disc line subject to Doppler boosting, however, known absorption in the spectrum of GRO J1655-40 at energies between similar to7 and 8 keV can reduce the apparent brightness of the blue wing. The spectra have also been fitted well using the full relativistic disc line model of Laor, plus an absorption line. This gives a restframe energy between 6.4 and 6.8 keV indicating that the line is from highly ionized iron K-alpha. The Laor model also shows that the line originates at radii extending from similar to 10 Schwarzschild radii (r(S)) outwards. The line is direct evidence for the black hole nature of the compact object. The continuum is well described by dominant disc blackbody emission plus Comptonized emission. During dipping, spectral evolution is well modelled by allowing progressive covering of the disc blackbody and simple absorption of the Comptonized emission showing that the thermal emission is more extended. Acceptable fits are only obtained by including the disc line in the covering term, indicating that it originates in the same inner disc region as the thermal continuum. Dip ingress times and durations are used to provide the radius of the disc blackbody emitter as 170-370 rs, and the radius of the absorber. (C) 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.