Detection of Absorption‐Line Features in the X‐Ray Spectra of the Galactic Superluminal Source GRO J1655–40

Abstract

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. SigniÐ-cant absorption-line features were found at 6.63 ^ 0.07 keV when the X-ray intensity was low (0.27È0.57 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 conÐguration 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 Ka line) and the ionization parameter m (D103) constrain the size of the plasma to D1010 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 rela-tivistic jets.