Deciphering radiation pressure instability signatures in GRS 1915+105 and its implications on perceiving other black hole X-ray transients

Abstract

We analysed NuSTAR data of GRS 1915+105 (2012–2024) to trace its outburst evolution and clues to the physical processes involved through different phases of intensity, especially those related to the radiation pressure instability. We examined its 1-second binned light curves and observation-long spectra to investigate the short- and long-term variability of GRS 1915+105, as an extension of the previous studies, which did not cover the fainter period that started in mid-2018. We assigned a new spectral group of variability, ‘states group 2’ (SG2), which consists of known classes, γ and ω, variation of φ (φ1), and an unclassified feature ‘uc’. SG2 is evident when GRS 1915+105 entered its most luminous state, when its innermost part of the accretion disc transformed to a slim disc state. Other previously known peculiar classes, κ and ρ, can be considered as the transition class between standard and slim disc, which act as signatures of the starting phase of radiation pressure instability. Only the basic variability classes (χ and φ2), which represent the standard disc, were commonly found in other BHXTs (e.g. GX 339–4, MAXI J1820+070, and 4U 1543–475). However, a class that represents the slim disc was found in 4U 1543–475, possibly due to both GRS 1915+105 and 4U 1543–475 having successfully reached a luminosity of ≥0.49 LEdd. Our analysis of the fainter period (mid-2018-2024) provides more evidence that GRS 1915+105 has not slipped into a quiescent phase.