  In this study, we undertake a spectral-timing analysis of the black hole
X-ray binary source GRS 1915+105 using simultaneous observations carried out by
AstroSat (LAXPC and SXT) and NICER in 2017. The source showed two flux levels
(high and low), whose energy spectra can be described by the thermal
comptonization of disk photons. The spectral parameters obtained by the joint
fitting of SXT/LAXPC and NICER/LAXPC were consistent. The power density spectra
from LAXPC and NICER revealed a broad, prominent feature at approximately 2 Hz.
The energy dependence of the fractional R.M.S. and time lag of this feature
cannot be explained by only variations of coronal spectral parameters. Instead,
a model where the coronal heating rate varies first and induces a change in the
disk temperature and inner radius can explain the variation. Our results
underline the importance of simultaneous observations by AstroSat and NICER and
highlight the need for more sophisticated models to explain the
spectral-temporal behavior of black hole systems.
