Unravelling the foretime of GRS 1915+105 using AstroSat observations: Wide-band spectral and temporal characteristics

Abstract

ABSTRACT We present a comprehensive study of GRS 1915+105 in wide-energy band (0.5–60 keV) using AstroSat – SXT and LAXPC observations during the period of 2016–2019. The MAXI X-ray light curve of the source shows rise and decay profiles similar to canonical outbursting black holes. However, the source does not follow the exemplary ‘q’-diagram in the hardness–intensity diagram. Model independent analysis of light curves suggest that GRS 1915+105 displays various types of variability classes (δ, χ, ρ, κ, ω, and γ). We also report possible transitions from one class to another (χ → ρ, ρ → κ via an ‘unknown’ class and ω → γ → ω + γ) within a few hours duration. Detailed ‘spectrotemporal’ analysis indicates a gradual increase in the photon index (Γ) from 1.83 to 3.8, disc temperature (kTin) from 1.33 to 2.67 keV, and quasi-periodic oscillation frequency (ν) from 4 to 5.64 Hz during the rise, while the parameters decrease to Γ ∼1.18, kTin ∼1.18 keV, and ν ∼1.38 Hz, respectively, in the decline phase. The source shows maximum bolometric luminosity (Lbol) during the peak at ∼36 per cent of Eddington luminosity (LEdd), and a minimum of ∼2.4 per cent of LEdd during the decay phase. Further evolution of the source towards an obscured low-luminosity (Lbol of ∼1 per cent LEdd) phase, with a decrease in the intrinsic bolometric luminosity of the source due to obscuration, has also been indicated from our analysis. The implication of our results are discussed in the context of accretion disc dynamics around the black hole.