On the Origin of HFQPOs of GRS 1915+105 during ‘Soft’ States: A Multi-mission approach

The black hole X-ray binary GRS 1915+105, known for its persistent brightness, exhibits HFQPOs during its 'softer' variability classes (γ, δ, κ, ω, ρ, μ, ν). We analyzed the ‘soft’ state observations of the source from 1996 to 2017 utilizing RXTE, AstroSat and NuSTAR observations. AstroSat observations of γ, δ, κ, ω classes reveal the first detection of soft-lag (0.40-1.68 milliseconds) associated with the ~ 67 Hz HFQPO while broadband timing analysis correlates the soft-lag to the rms amplitude of the HFQPOs. A long-term wide-band spectro-temporal study during “canonical” soft state observations of the γ, δ and φ classes reveal that HFQPOs are more likely observed during higher Comptonized flux (~ 38%), and lower optical depth (τ ≤ 8.5 ) in contrast to observations where HFQPOs are absent. We also find that the time lag decreases as τ increases, indicating that a higher τ enhances Compton down-scattering, thereby decreasing the hard-lag. Interestingly, during RXTE observations, the hard-lag (∼ 7 ms) gradually decreases as optical depth increases, eventually becoming a soft-lag (∼ 1 ms) in AstroSat observations. These constraints on spectro-temporal parameters for observing HFQPOs support a ‘compact’ coronal oscillation mechanism as a possible orgin of HFQPOs, which we attempt to interpret within the framework of a possible accretion scenario.