We present a detailed analysis of the observations with the Hard X-ray Modulation
Telescope of the black hole X-ray transient 4U 1543−47 during its outburst in 2021.
We find a clear state transition during the outburst decay both in the hardness-intensity
diagram and light curve of the source. Using previous measurements of the black-hole
mass and distance to the source, we find that the source luminosity during this transition
is around Eddington. The light curves before and after the state transition can be well
fitted by two exponential functions with short (∼ 16 days) and long (∼ 130 days) decay
time scales, respectively. During the super-Eddington period the hard component in the
X-ray spectra has a large photon index Γ ∼ 4.5, similar to the "hypersoft" state of the
2005 outburst of the black-hole transient GRO J1655−40. We detect strong reflection
features in all observations that can be well fitted with the model RELXILLNS with a
temperature kTbb ∼ 1 keV of the black-body incident spectrum. This indicates that the
emission of the accretion disk plays an important role in shaping the reflection spectra.
The fluxes of the RELXILLNS and the GAUSSIAN components show different correlations
before and after the transition. The best-fitting emissivity index of the disk changes
significantly during the state transition, from ∼ 3 − 4.5 in the super-Eddington period
to ∼ 8.0 in the sub-Eddington period. These results suggest that the accretion disk may
have changed significantly near the Eddington limit.
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