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

TL;DR

This study analyzes LFQPO time-lags in GRS 1915+105 using AstroSat data, revealing correlations between QPO properties, accretion flow changes, and disk-corona interactions in different variability classes.

Contribution

It provides new insights into the LFQPO time-lag behavior and accretion dynamics of GRS 1915+105 based on extensive AstroSat observations, expanding understanding beyond previous RXTE findings.

Findings

LFQPO frequencies detected across multiple classes.

Class transitions occur on timescales of hours with rapid accretion changes.

Soft-lag observed without sign reversal at 2 Hz, explained by a dynamical accretion disk model.

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 \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.