Probing the soft state evolution of 4U 1543-47 during its 2021 outburst using AstroSat

TL;DR

This study analyzes the spectral evolution of the black hole binary 4U 1543-47 during its 2021 outburst using AstroSat, revealing a disk-dominated state with a truncated inner disk and spectral hardening variations.

Contribution

It provides a detailed spectral analysis of 4U 1543-47 during its outburst, highlighting the disk truncation and spectral hardening behavior in the high/soft state.

Findings

Disk-dominated spectra with high inner disk temperature (~0.84 keV).

Inner disk radius is significantly truncated at >10 Rg.

Spectral hardening varies between 1.5 and 1.9 to fit models.

Abstract

4U 1543-47 underwent its brightest outburst in 2021 after two decades of inactivity. During its decay phase, AstroSat conducted nine observations of the source spanning from July $1^{st}$ to September $26^{th}$, 2021. The first three observations were performed with an offset of 40 arcmin with AstroSat/LAXPC, while the remaining six were on-axis observations. In this report, we present a comprehensive spectral analysis of the source as it was in the High/Soft state during the entire observation period. The source exhibited a disk-dominated spectra with a weak high-energy tail (power-law index $\geq2.5$) and a high inner disk temperature ($\sim$0.84 keV). Modelling the disk continuum with non-relativistic and relativistic models, we find inner radius to be significantly truncated at $>$$10~R_g$. Alternatively, to model the spectral evolution with the assumption that the inner disk is at the ISCO, it is necessary to introduce variation in the spectral hardening in the range $\sim$1.5-1.9.