Abstract:
We present a comprehensive temporal and spectral analysis of the ‘softer’ variability classes (i.e. θ, β, δ, ρ, κ, ω and γ ) of
the source GRS 1915+105 observed by AstroSat during the 2016−2021 campaign. Wide-band (3−60 keV) timing studies
reveal the detection of high-frequency quasi-periodic oscillations (HFQPOs) with frequencies of 68.14−72.32 Hz, significance
of 2.75−11σ and rms amplitude of 1.48–2.66 per cent in δ, κ, ω and γ variability classes. Energy-dependent power spectra
show that HFQPOs are detected only in the 6−25 keV energy band and rms amplitude is found to increase (1–8 per cent) with
energy. The dynamical power spectra of the κ and ω classes demonstrate that HFQPOs seem to be correlated with high count
rates. We observe that wide-band (0.7−50 keV) energy spectra can be described by the thermal Comptonization component
(nthComp) with a photon index (
nth) of 1.83−2.89 along with an additional steep (
PL ∼ 3) power-law component. The
electron temperature (kTe) of 1.82−3.66 keV and optical depth (τ ) of 2−14 indicate the presence of a cool and optically thick
corona. In addition, nthComp components, 1.97
nth 2.44 and 1.06 × 10−8 Fnth (erg cm−2 s−1) 4.46 × 10−8,
are found to dominate in the presence of HFQPOs. Overall, these findings infer that HFQPOs are possibly the result of the
modulation of the ‘Comptonizing corona’. Further, we find that the bolometric luminosity (0.3−100 keV) of the source lies
within the sub-Eddington (3–34 per cent LEdd) regime. Finally, we discuss and compare the obtained results in the context of
existing models on HFQPOs.
