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

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.

💡 Research Summary

This paper investigates the origin of the large‑amplitude, quasi‑periodic X‑ray flux variations observed in the κ and ω variability classes of the Galactic microquasar GRS 1915+105. Using simultaneous observations from AstroSat’s Soft X‑ray Telescope (SXT; 0.8–7 keV) and Large Area X‑ray Proportional Counter (LAXPC; 3–20 keV), the authors perform flux‑resolved broadband spectral modeling together with covariance spectral analysis in the 0.015–5 Hz frequency range.

The light curves display strong, quasi‑periodic oscillations with rapid transitions between bright bursts (high‑flux intervals) and deep dips (low‑flux intervals) on timescales of tens of seconds. High‑flux intervals are characterized by a hot, optically thick accretion disc (diskbb) with inner‑disc temperatures of ≈2.5–3 keV and a steep Comptonized component (THCOMP) with photon indices Γ≈2.8–3.0. In contrast, low‑flux intervals show a systematic 1–2 keV drop in disc temperature (kT_in≈1.5–2 keV), reduced disc normalization (implying a partially recessed or cooled disc), and a harder coronal spectrum (Γ≈1.7–2.0, higher electron temperature). The total 0.8–20 keV flux can decrease by up to a factor of five between the two states, driven by the combined effect of disc cooling/recession and coronal hardening.

To pinpoint the spectral component responsible for the rapid coherent variability, the authors compute covariance spectra across the 0.015–5 Hz band. The covariance spectra are dominated by the disc component, showing strong energy dependence especially below ~5 keV, while the Comptonized component contributes negligibly. This indicates that the observed rapid variability is almost entirely disc‑driven, with the corona responding more slowly and incoherently.

These findings support a radiation‑pressure‑driven limit‑cycle scenario originally proposed by Belloni et al. (1997). In this picture, the inner disc becomes unstable, evacuates (or cools) leading to a drop in soft‑photon supply and a hardening of the corona; subsequently the disc refills, restoring the soft flux and producing the bright bursts. Both κ and ω classes exhibit this cycle, though the ω class shows longer high‑flux plateaus, suggesting a more prolonged refilling phase.

The paper also revisits earlier RXTE/PCA and NICER results that attributed “peak‑minus‑dip” spectra to a simple disc blackbody, confirming that disc variations dominate the flux changes and that coronal parameter changes alone cannot reproduce the observed spectra. By combining broadband spectral fitting with covariance analysis, the study provides the first direct evidence that disc structural changes, driven by radiation pressure, are the primary engine of the large‑amplitude oscillations in GRS 1915+105, while the corona plays a secondary, slower role. This work thus offers a comprehensive, physically motivated explanation for the κ and ω variability classes and sets a benchmark for future investigations of disc‑corona coupling and high‑frequency QPOs in this iconic source.