High-resolution Chandra HETGS and Rossi X-Ray Timing Explorer observations of GRS 1915-105:: A hot disk atmosphere and cold gas enriched in iron and silicon

The time-averaged 30 ks Chandra AO1 High Energy Transmission Grating Spectrometer observation of the microquasar GRS 1915-105 in the low hard state reveals for the first time in this source neutral K absorption edges from iron, silicon, magnesium, and sulphur. Ionized resonance (r) absorption from H- and He-like species of Fe (XXV, XXVI) and possibly Ca XX are also seen as well as possible emission from neutral Fe Kalpha and ionized Fe XXV (forbidden or the resonance emission component of a P Cygni profile). We report the tentative detection of the first astrophysical signature of X-ray absorption fine structure in the photoelectric edge of Si (and possibly Fe and Mg) attributed to material in grains. The large column densities measured from the neutral edges reveal anomalous Si and Fe abundances and illustrate the importance of high-resolution X-ray measurements for assessing material that surrounds bright X-ray sources, especially if depletion onto grains plays a prominent role. Scenarios for which the anomalous abundances can be attributed to surrounding cold material associated with GRS 1915-105 and/or for which the enrichment may signify either a highly unusual supernova/hypernova or external supernova activity local to the binary are discussed. We attribute the ionized features to a hot disk, disk wind, or corona environment. Based on H- and He-like Fe (XXV, XXVI), we estimate constraints on the ionization parameter (log xi greater than or similar to 4.15), temperature (T>2.4x10(6) K), and hydrogen equivalent number density (ngreater than or similar to10(12) cm(-3)) for this region. Variability studies with the simultaneous RXTE data show that the light-curve count rate tracks the changes in the disk blackbody as well as the power-law flux, with the most significant variations observed in the former. The Chandra data show spectral changes that also track the behavior of the light curve and may point to changes in both the ionizing flux and density of the absorber. A 3.69 Hz quasi-periodic oscillation and weak first harmonic are seen in the RXTE data.