X-ray study of accretion flow in the Galactic microquasar GRS 1915+105

Kazutaka Yamaoka$^1$, Yoshihiro Ueda$^1$, Hajime Inoue$^1$

$^1$ ISAS, Sagamihara, Kanagawa, Japan


We have studied X-ray properties of the Galactic microquasar GRS
1915+105 using 164 {\it RXTE} observations from January 1999 to May 2000
and 7 {\it ASCA} observations. The energy spectrum consists of two
components, {\it i.e.} a ``slow varying'' soft component and a ``fast
varying'' hard component. Analyzing the frequency-resolved spectra, we
found that the energy spectra of the hard component were well
explained by a broken power law having a photon index of 1.8 below the
break energy at 5--10 keV. We succeeded in modeling all the 
{\it RXTE}/{\it ASCA} spectra with the multi-color disk model 
plus the broken power
law. Consequently, we revealed that GRS 1915+105 has two distinct
spectral states. One is high temperature branch (HTB), which has an
innermost temperature $T_{\rm in}$ $\sim$ 2 keV and an innermost
radius $R_{\rm in}$ $\sim$ 20 km. The other is low temperature branch
(LTB), characterized by $T_{\rm in}$ $\sim$ 1.0 keV and $R_{\rm in}$
$\sim$ 50 km. The peculiar variability of this source seemed to be
 caused by a rapid transition between the HTB and the LTB.  We suggest
that the HTB and the LTB correspond to the optically thick ADAF (slim
disk) and the standard disk, respectively, as predicted in the
accretion-disk theory. The origin of the hard component is also
discussed.