Multimission view of the low-luminosity ‘obscured’ phase of GRS 1915+105

Abstract

ABSTRACT GRS 1915+105 has been observed in an ‘obscured’ phase since 2019 May, exhibiting steady and low X-ray luminosities with sporadic re-brightenings. In this paper, we perform a comprehensive and wide-band analysis of the spectral and timing properties of the source during the period 2019–2021 using observations from AstroSat (SXT: 0.5–8 keV; LAXPC: 3–60 keV), the Neutron star Interior Composition Explorer (NICER) (0.5–12 keV), and the Nuclear Spectroscopic Telescope Array (NuSTAR) (3–60 keV). Spectral analysis reveals the presence of a highly variable obscurer ($N_{\rm H_{\rm 1}}\sim$ 1022–1024 atoms cm−2) throughout the observation period. The source is detected in the low/hard state for most of the time, with the spectra being described by a Comptonized component (Γ ∼ 1.16–1.79, kTe ∼ 2–31 keV). The source spectra steepen (Γ ∼ 2.5), indicating a softening of the spectrum, during the rise of the re-brightenings. Various emission and absorption lines corresponding to the neutral Fe Kα, Fe xxv Kα, Fe xxvi Kα, and the Ni xxviii Kα were detected, with the equivalent widths varying in the range 70 eV–3.5 keV. The column density of the absorbing plasma varied in the range 1016–1018 atoms cm−2 at a distance ≤2 × 1010 cm. Interestingly, the source is also seen to exhibit various variability classes (ρ, λ, δ, χ) at relatively low luminosities (∼0.01 LEdd) during the re-brightening phases. Different variability classes show the signature of quasi-periodic oscillations (νQPO: 20–180 mHz, rmsQPO: 7.5 per cent–16 per cent). The source showed a maximum bolometric luminosity (Lbol) of ∼0.01 LEdd (re-brightening phases) and a minimum Lbol of 0.004 LEdd (quiet phase) during the period. We discuss the possible disc dynamics around the black hole during this low-luminosity ‘obscured’ phase.