4U 1543-47: The Brightest Black Hole X-ray Binary Observed by NICER and NuSTAR Yet

We report on simultaneous NICER/NuSTAR observations of the black hole X-ray binary 4U 1543-47. The source, a known black hole transient, went into outburst in June 2021, and rapidly transitioned into a bright soft spectral state with a hot accretion disk. 4U 1543-47 reached a flux of 9 Crab, approaching its Eddington luminosity, also making it the brightest black hole transient for over a decade. The outburst lasted months since rapidly reaching its peak, making this current outburst distinct in luminosity and time from previous ones. I will present a detailed analysis of the spectral and time-variability characteristics during the entire outburst. The unprecedented signal-to-noise of the spectrum reveals complex reflection characteristics that are challenging the most advanced reflection models available. In particular, there are rich emission features in the iron K region, which suggests a non-canonical illumination geometry. We measure key properties of the accretion disk, such as the gas density and ionization, as well as black hole spin.