Spectro-timing origin of large amplitude X-ray variability in GRS 1915+105 using AstroSat/LAXPC and SXT

TL;DR

This study investigates the origin of large-amplitude X-ray variability in GRS 1915+105, revealing that rapid oscillations are primarily driven by disc structural changes with a slower coronal response, using broadband spectral analysis from AstroSat data.

Contribution

It provides new insights into the spectral and timing behavior of GRS 1915+105, demonstrating that disc variations dominate the observed quasi-periodic oscillations, supporting a cyclic disc evacuation and refilling model.

Findings

Rapid oscillations are mainly caused by disc changes.

High-flux states show a hot, steep disc and softer emission.

Low-flux states involve a cooler disc and harder spectrum.

Abstract

The origin of the large-amplitude, quasi-periodic X-ray flux variations in several classes of the Galactic microquasar GRS~1915+105 remains unresolved. We address this issue through flux-resolved, broadband (0.8-20 keV) spectral modelling and simultaneous covariance spectral analysis during two $\kappa$ and two $\omega$ class observations using \textit{AstroSat}/SXT and LAXPC. The lightcurves show strong, quasi-periodic oscillations involving rapid transitions between bright bursts and deep dips on timescales of a few tens of seconds. Flux-resolved spectroscopy indicates that high-flux intervals in both classes are dominated by a hot, optically thick accretion disc with steep Comptonized emission, whereas low-flux intervals correspond to a cooler or partially recessed disc and a harder coronal continuum. These transitions involve a systematic 1-2 keV drop in disc temperature and a pronounced hardening of the Comptonized component, with flux reductions of up to a factor of five. Using covariance spectra across 0.015-5 Hz, we show that the rapid coherent variability arises almost entirely from the disc, which exhibits strong energy-dependent variations, while the Comptonized component contributes minimally. The combined results suggest that radiation-pressure-driven structural changes in the disc, with a slower coronal response, produce the observed oscillations, consistent with cyclic disc evacuation and refilling in the $\kappa$ and $\omega$ classes.