Detection of absorption-line features in the X-ray spectra of the galactic superluminal source GRO J1655-40

We report the detection of iron K-absorption lines in the energy spectra of the superluminal jet source GRO J1655-40. We observed the source with ASCA on four occasions from 1994 through 1996. Significant absorption-line features were found at 6.63 +/- 0.07 keV when the X-ray intensity was low (0.27-0.57 crab) and at 6.95 +/- 0.10 keV when the intensity was high (2.2 crab). The former are interpreted as resonance absorption lines from helium-like iron and the latter are those from hydrogen-like iron, respectively. This gives direct evidence for the presence of highly ionized plasma in a nonspherical configuration in the vicinity of the black hole. The change of the ionization stale of iron suggests that the plasma is photoionized by X-ray irradiation. The observed equivalent width (about 60 eV for the helium-like K alpha line) and the ionization parameter xi (similar to 10(3)) constrain the size of the plasma to similar to 10(10) cm. The similarity of these properties to those of the other superluminal source GRS 1915+105 may imply some connection between the presence of such highly ionized plasma and the formation of relativistic jets.