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

TL;DR

This study analyzes the 2021 outburst of black hole candidate 4U 1543-47 using Insight-HXMT and NICER data to test accretion disk models at high luminosities and to refine spin measurements.

Contribution

It compares thin and slim disk models at high accretion rates and identifies optimal epochs for accurate spin measurement via continuum fitting.

Findings

The outburst exceeded twice the Eddington luminosity.

Slim disk models better fit high luminosity data than thin disk models.

Two optimal epochs for spin measurement were identified.

Abstract

4U 1543--47 is one of a handful of known black hole candidates located in the Milky Way Galaxy, and has undergone a very bright outburst in 2021, reaching a total of $\sim$9 Crab, as observed by the Monitor of All-sky Image (MAXI), and exceeding twice its Eddington luminosity. 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 previous spin measurements of 4U 1543--47 have been largely inconsistent with each other. 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, as well as 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 for which it is most suitable to measure the spin with continuum fitting.