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

TL;DR

This study uses AstroSat data from 2016-2019 to analyze the spectral and temporal behavior of GRS 1915+105, revealing variability classes, spectral evolution, and luminosity changes, enhancing understanding of accretion processes around black holes.

Contribution

It provides the first detailed broadband spectral and temporal analysis of GRS 1915+105 over multiple years, identifying class transitions and evolution of key parameters.

Findings

Identification of multiple variability classes and their transitions.

Observation of spectral parameter evolution during rise and decline phases.

Maximum luminosity reaching 36% of Eddington luminosity.

Abstract

We present a comprehensive study of GRS 1915+105 in wide energy band ($0.5-60$ keV) using AstroSat observations during the period of $2016-2019$. The MAXI X-ray lightcurve 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 (HID). Model independent analysis of lightcurves suggests that GRS 1915+105 displays various types of variability classes ($\delta,\chi,\rho,\kappa,\omega$ and $\gamma$). We also report possible transitions from one class to another ($\chi\rightarrow\rho,\rho\rightarrow\kappa$ via an 'unknown' class and $\omega\rightarrow\gamma\rightarrow\omega+\gamma$) within a few hours duration. Broadband energy spectra are well modeled with multi-coloured disc blackbody and Comptonised components. We explore the 'spectro-temporal' features of the source in the different variability classes, transitions between classes, and evolution during $2016-2019$. Detailed analysis indicates a gradual increase in the photon index ($\Gamma$) from $1.83$ to $3.8$, disc temperature ($kT_{in}$) from $1.33$ to $2.67$ keV, and Quasi-periodic Oscillation (QPO) frequency ($\nu$) from $4$ to $5.64$ Hz during the rise, while the parameters decrease to $\Gamma$ ~$1.18$, $kT_{in}$ ~$1.18$ keV, and $\nu$ ~$1.38$ Hz respectively in the decline phase. The source shows maximum bolometric luminosity (L$_{bol}$) during the peak at ~$36$% of Eddington luminosity (L$_{EDD}$), and a minimum of ~$2.4$% L$_{EDD}$ during the decay phase. Further evolution of the source towards an obscured low-luminosity (L$_{bol}$ of ~ 1% L$_{EDD}$) 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.