Optical Spectroscopy of GRO J1655−40

We have obtained optical spectra of the soft X-ray transient GRO J1655-40during different X-ray spectral states (quiescence, high-soft and hardoutburst) between 1994 Aug and 1997 Jun. Characteristic features observedduring the 1996-97 high-soft state were: a) broad absorption lines at Halphaand Hbeta, probably formed in the inner disk; b) double-peaked HeII 4686emission lines, formed in a temperature-inversion layer on the disk surface,created by the soft X-ray irradiation; c) double-peaked Halpha emission, with astrength associated with the hard X-ray flux, suggesting that it was probablyemitted from deeper layers than He II 4686. The observed rotational velocitiesof all the double-peaked lines suggest that the disk was extended slightlybeyond its tidal radius. Three classes of lines were identified in the spectrataken in 1994 Aug-Sep, during a period of low X-ray activity between two strongX-ray flares: broad absorption, broad (flat-topped) emission and narrowemission. We have found that the narrow emission lines (single-peaked ordouble-peaked) cannot be explained by a thin-disk model. We propose that thesystem was in a transient state, in which the accretion disk might have had anextended optically thin cocoon and significant matter outflow. After the onsetof a hard X-ray flare the disk signatures disappeared, and strong single-peakedHalpha and Paschen emission was detected, suggesting that the cocoon becameopaque to optical radiation. High-ionisation lines disappeared or weakened. Twoweeks after the end of the flare, the cocoon appeared to be once againoptically thin.Comment: 22 text pages + 21 figures + 3 tables. Accepted by ApJ on 1999 Nov 1