<i>ASCA</i>Observations of the Absorption Line Features from the Superluminal Jet Source GRS 1915+105

Abstract

GRS1915+105 was observed with ASCA six times from 1994 to 1999. The source was so bright that most SIS data suffered from event pileup. We have developed a new technique to circumvent the pileup effect and carried out a high-resolution spectroscopy. In the energy spectra of 1994 and 1995, resonant absorption lines of Ca XX Ka, Fe XXV Ka, Fe XXVI Ka, as well as blends of the absorption lines of Ni XXVII Ka + Fe XXV Kb and Ni XXVIII Ka + Fe XXVI Kb, were observed. Such absorption lines have not been found in other objects, except for iron absorptions lines from GRO J1655-40, another super-luminal jet source (Ueda et al. 1998). We carried out a ``curve of growth'' analysis for the absorption lines, and estimated column densities of the absorbing ions. We found that a plasma of moderate temperature (0.1-10 keV) and cosmic abundance cannot account for the observed large equivalent widths. The hydrogen column density of such plasma would be so high that the optical depth of Thomson scattering would be too thick (N_H &gt;10^24 cm^-2). We require either a very high kinetic temperature of the ions (&gt; 100 keV) or extreme over-abundances (&gt; 100 Z). In the former case, the ion column densities have reasonable values (&lt; 10^18 cm^-2). We modeled the absorber as a photo-ionized disk which envelops the central X-ray source. Using a photo-ionization calculation code, we constrain physical parameters of the plasma disk, such as the ionization parameter, radius, and density. Estimated parameters were found to be consistent with those of a radiation-driven disk wind. These absorption-line features may be peculiar to super-luminal jet sources and related to the jet formation mechanism. Alternatively, they may be common characteristics of super-critical edge-on systems.