Comprehensive X-ray spectral-timing analysis of GRS 1915+105 based on <i>Insight–HXMT</i> observations

ABSTRACT GRS 1915+105 has been well studied since its discovery and is well known for its complex light-curve variability. Using the full currently available Hard X-ray Modulation Telescope (Insight–HXMT) data set from 2017 July to 2023 June, we make a comprehensive spectral-timing analysis of this source and report four main findings. First, we uncover a quasi-periodic oscillation (QPO) frequency-rising branch between MJD 58206 and 58230, where the centroid frequency increases from $\sim$2 to $\sim$6 Hz, consistent with a spectral state transition from the hard to the intermediate state. This rising branch completes the full QPO frequency evolution cycle when combined with the subsequent frequency decay phase, which had been missed in prior Neutron Star Interior Composition Explorer (NICER) and Insight–HXMT studies. Second, we identify a previously unreported Flare 3 during the obscured state, which shows distinct spectral and timing properties compared with the earlier flares. Third, we detect subhertz QPOs (&amp;lt;1 Hz) in all three flares, specifically at $\sim$0.01 Hz in Flare 1 and at $\sim$0.2 Hz in both Flare 2 and Flare 3. In particular, the weak $\sim$0.2-Hz signals observed in Flare 3 indicate ongoing coronal activity despite strong obscuration. Finally, a comparison between QPOs above and below 1 Hz suggests distinct origins, with the former likely arising from Lense–Thirring precession of the inner hot flow and the latter from magnetic perturbations driving a failed disc wind. These findings offer new insights into the unique accretion geometry and variability behaviours of GRS 1915+105.