Time-Lag properties associated with LFQPO in X-ray variability classes of GRS 1915+105: Findings from <i>AstroSat</i>

Abstract We present a comprehensive analysis of Low Frequency Quasi-periodic Oscillation (LFQPO) associated time-lags in the persistently variable black hole binary GRS 1915+105 using 441 ks of AstroSat observations from March 2016 to March 2019. LFQPO frequency (1.38 − 7.38 Hz) are detected across the θ, β, ρ, and χ classes, with the χ class further subdivided into χ1, χ2, χ3, and χ4 based on spectro-temporal characteristics. Class transitions occur on timescales of a few hours, appearing either as a simultaneous increase in X-ray count rate and QPO frequency, or vice versa, indicating rapid changes in the accretion flow geometry. The rmsQPO increases with QPO frequency up to ~3.4 Hz and declines at higher frequencies, a trend similar to RXTE observations, where peak occurred at ~2 Hz. Spectro-temporal correlations reveal that increasing FComp drives higher rmsQPO and decreases the soft-lag magnitude, while νQPO and Γ also decline, suggesting that the observed time lag may result from the combined effects of multiple physical mechanisms. The consistent increase of rmsQPO with FComp provides clear evidence that modulated Comptonized photons enhance the rms power (rmsQPO). Moreover, the soft-lag (1.59 − 13.49 ms) observed across all QPO frequencies, without the sign reversal at ~ 2 Hz observed in RXTE observations, is interpreted within the framework of a dynamical accretion disk model around the black hole.