Wideband Study of the Brightest Black Hole X-ray Binary 4U 1543-47 in the 2021 Outburst: Signature of Disk-Wind Regulated Accretion

Abstract: A comprehensive wideband spectral analysis of the brightest black hole X-ray binary 4U $1543-47$ during its 2021 outburst is carried out for the first time using NICER, NuSTAR, and AstroSat observations by phenomenological and reflection modelling. The source attains a super-Eddington peak luminosity and remains in the soft state, with a small fraction ($< 3\%$) of the inverse-Comptonized photons. The spectral modelling reveals a steep photon index ($\Gamma \sim 2-2.6$) and relatively high inner disk temperature ($T_{in}\sim 0.9-1.27$ keV). The line-of-sight column density varies between ($0.45-0.54$)$\times10^{22}$ cm$^{-2}$. Reflection modelling using the RELXILL model suggests that 4U $1543-47$ is a low-inclination system ($\theta \sim 32^\circ - 40^\circ$). The accretion disk is highly ionized (log $\xi$ > 3) and has super solar abundance (3.6$-$10 $A_{Fe,\odot}$) over the entire period of study. We detected a prominent dynamic absorption feature between $\sim 8-11$ keV in the spectra throughout the outburst. This detection is the first of its kind for X-ray binaries. We infer that the absorption of the primary X-ray photons by the highly ionized, fast-moving disk-winds can produce the observed absorption feature. The phenomenological spectral modelling also shows the presence of a neutral absorption feature $\sim 7.1 - 7.4$ keV, and both ionized and neutral absorption components follow each other with a delay of a typical viscous timescale of $10-15$ days.