arXiv subjects

Athulya M. P.

Publications and source records attributed to Athulya M. P..

2 recordsLinked to original sources

arXiv2023

Multi-mission view of low-luminosity 'obscured' phase of GRS 1915+105

GRS 1915+105 is observed in an 'obscured' phase since May 2019, exhibiting steady and low X-ray luminosities, while being intervened by sporadic re-brightenings. In this work, we perform a comprehensive and wide-band analysis of the spectral and timing properties of the source during the period $2019-2021$ using AstroSat (SXT: $0.5-8$ keV; LAXPC: $3-60$ keV), NICER ($0.5-12$ keV), and NuSTAR ($3-60$ keV) observations. Spectral analysis reveals the presence of a highly variable obscurer (N$_{H_{1}}\sim~10^{22} - 10^{24}$ atoms cm$^{-2}$) throughout the observation period. Source is detected in the Low/Hard state for most of the time, with the spectra being described by a Comptonised component ($Γ\sim 1.16 - 1.79$, kT$_{e}\sim 2-31$ keV). The source spectra steepen ($Γ\sim2.5$) indicating softening of the spectrum during the rise of the re-brightenings. Various emission and absorption lines corresponding to the neutral Fe-K$α$, Fe-XXV K$α$, Fe-XXVI K$α$, and the Ni-XXVIII K$α$ were detected with equivalent widths varying between 70 eV $-$ 3.5 keV. The column density of the absorbing plasma varied between $10^{16} - 10^{18}$ atoms cm$^{-2}$ at a distance $\leq2\times$10$^{10}$ cm. Interestingly, the source is also seen exhibiting various variability classes ($ρ, λ, δ, χ$) at relatively low luminosities ($\sim$0.01L$_{Edd}$) during the re-brightening phases. Different variability classes show signature of QPOs ($ν_{QPO}$: 20--180 mHz, rms$_{QPO}$: 7.5% - 16%). The source showed a maximum bolometric luminosity {(L$_{bol}$)} of $\sim$0.01L$_{Edd}$ (Re-brightening phases) and a minimum L$_{bol}$ of 0.004L$_{Edd}$ (Quiet phase) during the period. We discuss the possible disc dynamics around the black hole during this low-luminosity `obscured' phase.

astro-ph.HE↗

arXiv2021

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

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 ($δ,χ,ρ,κ,ω$ and $γ$). We also report possible transitions from one class to another ($χ\rightarrowρ,ρ\rightarrowκ$ via an 'unknown' class and $ω\rightarrowγ\rightarrowω+γ$) 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 ($Γ$) from $1.83$ to $3.8$, disc temperature ($kT_{in}$) from $1.33$ to $2.67$ keV, and Quasi-periodic Oscillation (QPO) frequency ($ν$) from $4$ to $5.64$ Hz during the rise, while the parameters decrease to $Γ$ ~$1.18$, $kT_{in}$ ~$1.18$ keV, and $ν$ ~$1.38$ 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.

astro-ph.HE↗