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 (Γnth) of 1.83−2.89 along with an additional steep (ΓPL ∼ 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 ≲ Γnth ≲ 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.

Study of relativistic accretion flow around KTN black hole with shocks

“…The excess thermal energy across the shock front can also deflects a part of the accreting matter to produce jets/outflow along the direction of the black hole rotation axis [64-67, and references therein]. Further, time dependent numerical simulation studies reveal that PSC undergoes aperiodic modulation as a consequence of resonant oscillation [49,50] and therefore, the time varying undulation of PSC is possibly responsible for yielding the quasi-periodic oscillation (QPO) commonly seen in the power density spectra of Galactic black hole sources [61,[68][69][70][71][72][73]. Needless to mention that all these studies involving shock were carried out considering the standard black hole background.…”

Properties of relativistic hot accretion flow around a rotating black hole with radially varying viscosity

“…Nevertheless, timedependent shocked accretion solutions may exist for θ > θ max , which were examined by numerical simulation to study the oscillatory behaviour of shock solutions [59,66,70,71]. Interestingly, the solutions of this kind satisfactorily account for the quasiperiodic oscillations (QPOs) phenomenon that are commonly observed in BH-XRBs [43,44,72]. However, we indicate that the study of time-dependent shock solution is beyond the scope of the present framework and will be reported elsewhere.…”

QPOs in Compact Sources as a Nonlinear Hydrodynamical Resonance: Determining Spin of Compact Objects

“…An additional twin peak of ∼168 Hz and ∼113 Hz were reported further Belloni et al 2006). However, later on, a prominent QPO around 70 Hz was observed by various groups; see Belloni et al (2019) and Majumder et al (2022) for the observation of AstroSat. Moreover, a very sporadic appearance of peaks at 41 Hz and 34 Hz was also reported (Belloni et al 2006;Belloni & Altamirano 2013).…”