Spin measurement of 4U 1543-47 with Insight-HXMT and NICER from its 2021 outburst A test of accretion disk models at high luminosities

Context. 4U 1543-47 is one of a handful of known black hole candidates located in the Milky Way galaxy. It underwent a very luminous outburst in 2021, reaching a peak intensity of similar to 9 Crab, as observed by the Monitor of All-sky Image (MAXI), and exceeding twice its Eddington luminosity. Aims. The unprecedented bright outburst of 4U 1543-47 provides a unique opportunity to test the behavior of accretion disk models at high luminosities and accretion rates. In addition, we explore the possibility of constraining the spin of the source at high accretion rates, given that the previous spin measurements of 4U 1543-47 are largely inconsistent with each other. Methods. We measure the spectral evolution of the source throughout its outburst as observed by Insight-HXMT, and compare the behavior of both the thin-disk model kerrbb2 and the slim disk model slimbh up to the Eddington limit for two different values of disk alpha-viscosity. In addition, given the behavior of these two models, we identify two "golden" epochs in which it is most suitable to measure the spin with the continuum fitting method. Results. We find evidence of a disk state transition from a thicker slim disk to a thin disk occurring around 1 L-Edd from fits to the luminosity-temperature relation. We obtain consistent and constant spin measurements from both slimbh and kerrbb2 as the luminosity varies towards the Eddington limit, implying the recovery of thin-disk solutions above the traditional thin-disk criterion of 30% L-Edd. We constrain the spin to a(*) = 0.65(-0.24)(+0.14), assuming an alpha-viscosity = 0.01 from both Insight-HXMT and NICER observations from the above-mentioned "golden" epochs where the condition of the disk being truncated at the innermost stable circular orbit is most closely met.