ASCA Observation of the Superluminal Jet Source GRO J1655–40 in the 1997 Outburst

We report on the results of an ASCA observation of the Galactic jet source GRO J1655–40 performed from 1997 February 25 to February 28 covering a full orbital period (2.62 d). The averaged 2–10 keV flux was about 1.1 Crab. An absorption line feature centered at 6.8 keV was detected both in the GIS and SIS spectra. We interpret this as a blend of two resonance-absorption $\mathrm{K}\alpha$ lines from H-like and He-like iron ions. We can consistently explain both the ASCA spectra and the simultaneous RXTE/PCA spectrum by a combination of K-absorption lines and K-absorption edges of iron ions. The fact that the absorption line is stably present over the whole orbital phase implies that the distribution of the highly ionized plasma is not affected by the companion star, which is consistent with its presence around the black hole. A curve of growth analysis shows that the plasma contains velocity dispersion along the line-of-sight larger than $300~\mathrm{km}~\mathrm{s}^{-1}$ attributed to bulk motions. It is probably a part of a geometrically thick accretion flow in turbulent motions with velocities of $500$-$1600~\mathrm{km}~\mathrm{s}^{-1}$ at an estimated radius of $\sim10^{10}~\mathrm{cm}$.