Erratum: updated measurements (now including uncertainties) for ATel 15253
ATel #15255; Jingyi Wang (MIT), Erin Kara (MIT), James F. Steiner (SAO, CfA), Keith Gendreau, Zaven Arzoumanian (NASA/GSFC), Guglielmo Mastroserio (Caltech), Kevin Alabarta (New York University Abu Dhabi), Diego Altamirano (Southampton), for the NICER team
on 2 Mar 2022; 20:12 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jingyi Wang (jingyiw@mit.edu)
Subjects: X-ray, Black Hole, Transient
After ~17 years of quiescence, the black hole X-ray binary 4U 1543-475 was detected in a bright soft state starting in June 2021 (ATels #14701, #14708). Its luminosity quickly increased, making it the brightest black hole binary observed by NICER until now (5.4 Crab in 2-10 keV; ATel #14725). Then it decayed in the soft state and transitioned back to the hard state by Jan 2022 (ATel #15157).
On 2022-02-15, there was an increase of the X-ray count rate in both MAXI/GSC and Swift/BAT light curves, likely due to a secondary reflare after the major outburst. We triggered NICER Target of Opportunity observations starting on 2022-02-23 (MJD 59633). By 2022-03-01 (MJD 59639) we had obtained a total exposure of ~18 ks. During this period the source gradually declined its flux, with the 0.5-10 keV NICER mean count rate decreasing from 336.7+/-0.8 counts/s to 87.4+/-0.1 counts/s. We preliminarily fit the flux-energy spectra with a model including a disk blackbody and a power-law (diskbb+powerlaw). The fitted column density, using Wilms et al. 2000 abundances, is (5.65+/-0.08)E21 cm. The modeled flux decreases from (1.12+/-0.01)E-9 to (3.23+/-0.02)E-10 erg cm s, corresponding to 37.3+/-0.3 and 10.77+/-0.07 mCrab, respectively. The disk temperature decreases from 0.210+/-0.005 keV to 0.170+/-0.003 keV, the photon index Gamma is consistent with being 1.70+/-0.02, and weak Fe K line features can be seen.
The 0.3-10 keV power spectra show an integrated 1-10 Hz fractional rms of 22%. Combined with spectral features including the detection of relatively cool thermal disk emission and the hard power-law spectrum, the X-ray activity indicates that 4U 1543-475 is in the hard state and is reflaring.
NICER will continue monitoring; the next observation is on 2022-03-05 (MJD 59643). We encourage further multi-wavelength observations if the rebrightening activity continues. We note that in some black hole X-ray binaries, there could be multiple reflares with even state transitions (e.g., Cuneo et al. 2020). NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
