Wideband study of the brightest black hole X-ray binary 4U 1543-47 in the 2021 outburst: signature of disc-wind regulated accretion

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 ($\lt 3{{\ \rm per\ cent}}$) of the inverse-Comptonized photons. The spectral modelling reveals a steep photon index (Γ ~ 2-2.6) and relatively high inner disc temperature (T_in ~ 0.9-1.27 keV). The line-of-sight column density varies between (0.45-0.54) × 10²² cm^-2. Reflection modelling using the RELXILL model suggests that 4U 1543-47 is a low-inclination system (θ ~ 32°-40°). The accretion disc is highly ionized (log ξ > 3) and has super solar abundance (3.6-10 A_Fe,⊙) over the entire period of study. We detected a prominent dynamic absorption feature between ~8 and 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 disc winds can produce the observed absorption feature. The phenomenological spectral modelling also shows the presence of a neutral absorption feature ~7.1-7.4 keV, and both ionized and neutral absorption components follow each other with a delay of a typical viscous time-scale of 10-15 d.