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

Year: 2026
Type: article
Abstract: 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 2016 March to 2019 March. LFQPO frequency ($1.38\text{-}7.38$ Hz) is 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 time-scales of a few hours, appearing either as a simultaneous increase in X-ray count rate and quasi-periodic oscillation (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 Rossi X-ray Timing Explorer (RXTE) observations, where the 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 declining, 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 ($1.59\text{-}13.49$ ms) observed across all QPO frequencies, without the sign reversal at ${\sim}2$ Hz observed in RXTE observations, is interpreted within the framework of a dynamical accretion disc model around the black hole.
DOI: 10.1093/mnras/stag556
Language: English
Open Access status: gold
Estimated APC paid: $3,046
Award: G362946183
OpenAlex ID: W7140211122
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