GRS 1915+105 IN “SOFT STATE”: NATURE OF ACCRETION DISK WIND AND ORIGIN OF X-RAY EMISSION

Abstract

We present the results from simultaneous Chandra HETGS and Rossi X-ray Timing Explorer (RXTE) observations of the microquasar GRS 1915+105 in its quasi-stable "soft state" (or State A) performed on 2007 August 14, several days after the state transition from "hard state" (State C). The X-ray flux increased with spectral hardening around the middle of the Chandra observation, after which the 67 Hz quasi-periodic oscillation (QPO) became significant. The HETGS spectra reveal at least 32 narrow absorption lines from highly ionized ions including Ne, Mg, Si, S, Ar, Ca, Cr, Mn, Fe, whose features are the deepest among those ever observed with Chandra from this source. By fitting to the absorption-line profiles by Voigt functions, we find that the absorber has outflow velocities of ≈150 and ≈500 km s–1 with a line-of-sight velocity dispersion of ≈70 and ≈200 km s[superscript –1] for the Si XIV and Fe XXVI ions, respectively. The larger velocity and its dispersion in heavier ions indicate that the wind has a nonuniform dynamical structure along the line of sight. The location of the absorber is estimated at ~(1-3) × 10[superscript 5] r [subscript g] (where r[subscript g] is the gravitational radius) from the source, consistent with thermally and/or radiation-driven winds. By taking into account narrow spectral features detected with Chandra, the continuum spectra obtained with RXTE in the 3-25 keV band can be well described with a thermal Comptonization with an electron temperature of ≈4 keV and an optical depth of ≈5 from seed photons from the standard disk extending down to (4-7)r [subscript g]. In this interpretation, most of the radiation energy is produced in the Comptonization corona, which completely covers the inner part of the disk. A broad (1σ width of ≈0.2 keV) iron-K emission line and a smeared edge feature are detected, which can be explained by reflection from the accretion disk at radii larger than 400r [subscript g] when an emissivity power law index of –3 is assumed.