4U 1543-47: The Brightest Black Hole X-ray Binary Observed simultaneously 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 characteristics during the entire outburst, tracking the disk, coronal, and reflection spectra. 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, probed by both the reflection spectrum and the thermal continuum.