First Detection of Soft-Lag in GRS 1915+105 at HFQPO during Soft State: A Multi-Mission Approach

The Galactic X-ray black hole candidate GRS 1915+105 exhibits high-frequency quasi-periodic oscillations (HFQPOs) at ~ 67 Hz only during the radio-quiet ‘softer’ variability classes. We have studied the long-term X-ray variability from 1996 to 2017 using observations from RXTE, AstroSat, NuSTAR and NICER. AstroSat observations reveal the first detection of a soft-lag for higher energy photons (6-25 keV) relative to soft photons (3-6 keV), associated with the ~ 67 Hz HFQPO, in the range from 0.40 to 1.68 milliseconds. We find a consistent lag-energy correlation, where the soft-lag increases with energy, peaking at ~18 keV across all variability classes (δ, ω, κ, and γ). An extensive study of the γ variability class reveals an evolution of HFQPOs from ~ 69 Hz to 66 Hz and then up to 71 Hz. We observed a transition from hard to soft lag using the RXTE and AstroSat observations. Broadband spectral analysis indicates a higher bolometric flux with a lower electron temperature (kTe) of the corona during observations without HFQPO. We confirmed from the NICER/NuSTAR observations that the observed soft-lag could be due to the presence of a reflection mechanism in accretion dynamics. Spectral and temporal properties of accretion flows and jets around compact objects and the