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 \textit{ AstroSat} observations from March 2016 to March 2019. LFQPO frequency ($1.38-7.38$ Hz) are detected across the $\theta$, $\beta$, $\rho$, and $\chi$ classes, with the $\chi$ class further subdivided into $\chi_1$, $\chi_2$, $\chi_3$, and $\chi_4$ 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 $\text{ rms}_{ \rm QPO}$ increases with QPO frequency up to $\sim 3.4$ Hz and declines at higher frequencies, a trend similar to \textit{ RXTE} observations, where peak occurred at $\sim 2$ Hz. Spectro-temporal correlations reveal that increasing $F_{ \rm Comp}$ drives higher $\text{ rms}_{ \rm QPO}$ and decreases the soft-lag magnitude, while $\nu_{ \rm QPO}$ and $\Gamma$ also decline, suggesting that the observed time lag may result from the combined effects of multiple physical mechanisms. The consistent increase of $\text{ rms}_{ \rm QPO}$ with $F_{ \rm Comp}$ provides clear evidence that modulated Comptonized photons enhance the rms power ($\text{ rms}_{ \rm QPO}$). Moreover, the soft-lag ($1.59-13.49$ ms) observed across all QPO frequencies, without the sign reversal at $\sim$ 2 Hz observed in \textit{ RXTE} observations, is interpreted within the framework of a dynamical accretion disk model around the black hole.
Categories: astro-ph.HE