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 state of iron suggests that the plasma is photoionized by X-ray irradiation. The observed equivalent width (about 60 eV for the helium-like Kα line) and the ionization parameter ξ (~103) constrain the size of the plasma to ~1010 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.