ASCA observations of the absorption line features from the superluminal jet source GRS 1915+105

We have carried out a precise energy spectral analysis of the superluminal jet source GRS 1915+/-105 observed with ASCA six times from 1994 to 1999. T he source was SO bright that most SIS data suffered from event pileup. We h ave developed a new technique to circumvent the pileup effect, which enable d us to study the spectrum in detail and at high resolution (Delta E/E appr oximate to 2%). In the energy spectra of 1994 and 1995, resonant absorption lines of Ca XX K alpha, Fe XXV K alpha, Fe XXVI K alpha, as well as blends of the absorption lines of Ni XXVII K alpha + Fe XXV K beta and Ni XXVIII K alpha + Fe XXVI K beta, were observed. Such absorption lines have not bee n found in other objects, except for iron absorption lines from GRO J1655-4 0, another superluminal jet source. We carried out a "curve of growth" anal ysis for the absorption lines and estimated column densities of the absorbi ng ions. We found that a plasma of moderate temperature (0.1-10 keV) and co smic abundance cannot account for the observed large equivalent widths. The hydrogen column density of such plasma would be so high that the optical d epth of Thomson scattering would be too thick (N-H greater than or similar to 10(24) cm(-2)). We require either a very high kinetic temperature of the ions (greater than or similar to 100 keV) or extreme overabundances (great er than or similar to 100 Z.). In the former case, the ion column densities have reasonable values of 10(17)-10(18) cm(-2). We modeled the absorber as a photoionized 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 par ameters were found to be consistent with those of a radiation-driven disk w ind. These absorption line features may be peculiar to superluminal jet sou rces and related to the jet formation mechanism. Alternatively, they may be common characteristics of supercritical edge-on systems.