First detection of soft-lag in GRS 1915 + 105 at HFQPO using AstroSat observations

The Galactic black hole GRS 1915 + 105 exhibits generic high-frequency quasi-periodic oscillations (HFQPOs) at ~ 67 Hz only during the radio-quiet 'softer' variability classes. We present the time-lag properties associated with HFQPOs in the wide energy band (3-60 keV) using all AstroSat observations. For the first time, we detect soft-lag of 6-25 keV band w.r.t. 3-6 keV band for all 'softer' variability classes (δ, ω, κ, and γ). Moreover, our findings reveal that soft-lag increases gradually with the energy of the photons. These features are entirely opposite to the previous report of hard-lag obtained with the RXTE observations. The energy-dependent time-lag study exhibits a maximum soft-lag of ~ 3 and ~ 2.5 ms for the δ and ω classes, respectively, whereas the κ and γ classes both exhibit a maximum soft-lag of ~2.1 ms. We find a coherent lag-energy correlation for all four variability classes, where the amplitude of soft-lag increases with energy and becomes maximum at ~18 keV. We interpret this observed soft-lag as the reflection of hard photons in the 'cooler' accretion disc. A generic lag-rms correlation implies that the soft-lag increases with the rms amplitude of the HFQPO. The wideband (0.7-50 keV) spectral study suggests a high value of the optical depth (τ ~ 6.90-12.55) of the Comptonized medium and the magnitude of the soft-lag increases linearly with the increase in optical depth (τ). We explain the observed time-lag features at the HFQPOs in the context of a possible accretion disc scenario.