Wide-band view of high-frequency quasi-periodic oscillations of GRS 1915+105 in ‘softer’ variability classes observed with AstroSat

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.