Abstract:
The Galactic microquasar GRS 1915+105 shows rich variability that is categorized into different classes. In this
paper, we report the temporal and spectral analysis of GRS 1915+105 to study the properties of the accretion flow
when the light curve shows θ class variability. For this purpose, we use the Large Area X-ray Proportional Counter
data from the Target of Opportunity observations of India’s first multiwavelength astronomy satellite AstroSat. The
θ class is marked by the recurrent appearance of U-shaped regions in the light curve, where the photon count rate
first decreases rapidly and then increases slowly. For our analysis, we use U-shaped regions of the first two orbits
(02345 and 02346) on 2016 March 4. In both cases, the dynamic power-density spectra (PDS) showed significant
power at around 4–5 Hz, suggesting the presence of a low-frequency quasi-periodic oscillation (QPO) around that
frequency interval. The QPO frequency is found to increase with time when the energy flux is also enhanced. From
the evolution of the spectra, we determine the evolution of the accretion flow parameters in these two observations.
Fitting the spectra with the transonic flow solution-based two-component advective flow (TCAF) model in the
4–25 keV energy band shows that the Keplerian disk accretion rate increases with the increase in radiation
intensity, while the location of the centrifugal pressure-driven shock front decreases. In both these data, a gradual
increment of power-law photon index with intensity is observed, suggesting the progressive softening of the
source.
