Optical spectroscopy of GRO J1655-40

We have obtained optical spectra of the soft X-ray transient GRO J1655-40 d uring different X-ray spectral states (quiescence, high-soft, and hard outb urst) between 1994 August and 1997 June. Characteristic features observed d uring the 1996-1997 high-soft state were: (1) broad absorption lines at H a lpha and H beta, probably formed in the inner disk; (2) double-peaked He II lambda 4686 emission lines, formed in a temperature-inversion layer on the disk surface, created by the soft X-ray irradiation; and (3) double-peaked H alpha emission, with a strength associated with the hard X-ray flux, sug gesting that it was probably emitted from deeper layers than He II lambda 4 686. The He II lambda 4686 line profile appeared approximately symmetric, a s we would expect from a disk surface with an axisymmetric emissivity funct ion. The Balmer emission, on the other hand, appeared to come only from a d ouble-armed region on the disk, possibly the locations of tidal density wav es or spiral shocks. The observed rotational velocities of an the double-pe aked lines suggest that the disk was extended slightly beyond its tidal rad ius. Three classes of lines were identified in the spectra taken in 1994 Au gust-September, during a period of low X-ray activity between two strong X- ray flares: broad absorption, broad (flat-topped) emission, and narrow emis sion. We have found that the narrow (single-peaked or double-peaked) emissi on lines cannot be explained by a conventional thin accretion disk model. W e propose that the system was in a transient state, in which the accretion disk might have had an extended optically thin cocoon and significant matte r outflow, which would also explain the systematic blueshift of the narrow emission lines and the flat-topped profiles of the broad emission lines. Af ter the onset of a hard X-ray flare the disk signatures disappeared, and st rong single-peaked H alpha and Paschen emission was detected, suggesting th at the cocoon became opaque to optical radiation. High-ionization lines dis appeared or weakened. Two weeks after the end of the flare, the cocoon appe ared to be once again optically thin.