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

TL;DR

This study addresses the origin of large-amplitude X-ray variability in GRS 1915+105 by combining flux-resolved broadband spectroscopy and covariance analysis of two κ-class and two ω-class AstroSat observations. The analysis reveals that rapid, sub-second variability is dominated by the accretion disc, with a systematic 1–2 keV drop in $T_{ m in}$ and concurrent coronal hardening during low-flux episodes, consistent with radiation-pressure-driven limit-cycle oscillations in the inner disc. Covariance spectra show the disc drives the fast variability (0.01–5 Hz) while the corona contributes little on these timescales, providing direct spectro-timing support for disc-evacuation/refilling scenarios. Collectively, the results place the κ and ω classes within a quantitative Belloni-like framework, linking disc geometry changes to observed quasi-periodic bursts and offering a cohesive view of inner-disc instabilities in near-Eddington accretion onto a stellar-mass black hole.

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 $κ$ and two $ω$ 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 $κ$ and $ω$ classes.

Figures (9)

• Figure 1: Background-subtracted 3--30.0 keV AstroSat/LAXPC (all units combined) X-ray lightcurve of $\kappa$ class with 1.0 sec time resolution is shown in the top left panel, while the same for the $\omega$ class is shown in the top right panel. The color-color diagrams for the $\kappa$ and $\omega$ class observations are shown in the middle left and middle right panels respectively while the hardness intensity diagrams for the same are shown in the bottom left and bottom right panels, respectively. Selections of high and low flux intervals are shown in green triangles and red squares, respectively in all panels.
• Figure 2: Unfolded AstroSat SXT/LAXPC20 mean spectra of GRS 1915+105 in the 1.0–25.0 keV energy range. The upper-left panel shows the best-fit joint spectra for the high-flux state of the $\kappa$ class (Obs. $\kappa_2$), including the model components and data-to-model ratio. The upper-right, and lower-left panels display the joint spectral fits for Case-A, and Case-B, respectively, corresponding to the low-flux state of the $\kappa$ class (Obs. $\kappa_2$). The lower-right panel presents the best-fit joint spectra for the low-flux (Case-C) of the $\kappa$ class.
• Figure 3: Unfolded AstroSat SXT/LAXPC20 mean spectra of GRS 1915+105 in the 1.0–25.0 keV energy range. The upper-left panel shows the best-fit joint spectra for the high-flux state of the $\omega$ class (Obs. $\omega_1$), including the model components and data-to-model ratio. The upper-right and middle-left panels display the joint spectral fits for Case-A and Case-B, respectively, corresponding to the low-flux state of the $\omega$ class (Obs. $\omega_1$). The lower-right panel presents the best-fit joint spectra for the low-flux (Case-C) of the $\omega$ class.
• Figure 4: The left panel presents the best-fit continuum models for both the high and low-flux observations of the $\kappa$ class (Obs. $\kappa_2$), while the right panel shows the best-fit continuum models for both the high and low-flux observations of the $\omega$ class (Obs. $\omega_1$).
• Figure 5: Modelling of the covariance spectra for $\kappa$ class in the Fourier frequency range 0.01-5.0 Hz are shown with the model components and fit residual. The top-left panel presents the spectral fits using only a diskbb component for the high-flux state, while the top-right panel shows similar spectral fits for the low-flux state. The bottom-left panel displays the best-fit covariance spectra, modeled with a combination of diskbb and Comptonized components for the high-flux state, and the bottom-right panel shows the same model applied to the low-flux state.