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

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 $θ$, $β$, $ρ$, and $χ$ classes, with the $χ$ class further subdivided into $χ_1$, $χ_2$, $χ_3$, and $χ_4$ 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 $ν_{\rm QPO}$ and $Γ$ 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.

Figures (6)

• Figure 1: The MAXI/GSC (2$-$10 keV) and Swift/BAT ($15-50$ keV) lightcurves of GRS 1915+105 from January 2016 to April 2019 are plotted in the unit of Crab, shown in the upper and middle panels respectively. The 'Colour' in the lower panel is defined as the ratio of BAT flux to the MAXI flux. The coloured vertical lines represent the considered AstroSat observations of different variability classes mentioned in the legend in the upper panel. All observations can be classified as five different phases, shown in the legend in lower panel. See text for details.
• Figure 2: Lightcurve and CCD of variability classes ($\theta$, $\chi$, $\beta$, $\rho$) of the source GRS 1915+105 during LFQPO observations using AstroSat. The background subtracted and dead-time corrected 1s binned LAXPC lightcurves are plotted in the 3–60 keV energy range with CCD (top-right inset). The sub-classifications of $\chi$ class is shown in the top-right panel. See text for detail.
• Figure 3: The broadband PDS (3$-$60 keV) and time-lag spectra ($6-20$ keV w.r.t $3-6$ keV) corresponding to different variability classes of AstroSat observations. The MJD of each observation and the QPO frequency are mentioned in each panel. The dashed vertical black line corresponds to the QPO frequency. See text for details.
• Figure 4: Variation of percentage rms amplitude ($\text{rms}_\text{QPO}$) and time-lag as a function of the centroid frequency ($\nu_\text{QPO}$) of the LFQPO. The star and circle represent RXTE and AstroSat observations respectively. The filled colours correspond to different variability classes shown in the legend. See text for details.
• Figure 5: Energy dependent percentage rms amplitude ($\text{rms}_\text{QPO}$) and time-lag at LFQPO using AstroSat shown in the upper and lower panel respectively. Different colours correspond to different observations. The MJD, QPO frequency and variability class are shown in the legend. See text for details.