GRS 1915+105 in “Soft State”: Nature of Accretion Disk Wind and Origin of X-Ray Emission

Kavli Affiliate: Ronald Remillard

| First 5 Authors: Yoshihiro Ueda, Kazutaka Yamaoka, Ronald Remillard, ,

| Summary:

We present the results from simultaneous Chandra HETGS and 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 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. 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^{-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 non-uniform dynamical structure along the
line-of-sight. The location of the absorber is estimated at ~(1-3)x10^5 r_g
(r_g is the gravitational radius) from the source, consistent with thermally
and/or radiation driven winds. 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_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 (1sigma 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 400 r_g.

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