NICER observations of 4U 1543-47: brightest (NICER) black hole target to date

ATel #14725; R. Connors (CalTech), J. Steiner (SAO, CfA), J. Homan (Eureka Scientific), J. Garcia (CalTech), G. Mastroserio (CalTech), K. Gendreau (NASA GSFC), Z. Arzoumanian (NASA GSFC)
on 17 Jun 2021; 14:51 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Riley Connors (rconnors@caltech.edu)

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The NICER telescope has followed up on the recently reported MAXI/GSC detection of the outbursting black hole binary 4U 1543-47 (Negoro et al. ATel #14701). The source has evolved into the brightest black hole observed by NICER to date. It has been in a soft, thermal state from the onset of NICER observations on June 12th until present, and there is no requirement for a power-law component from the NICER data alone. These trends agree well with the timeline of MAXI observations in a similar observing window (see ATel #14701 and ATel #14708). Observations have also been made in the UV with Swift/UVOT (ATel #14715) and in the optical with both the MASTER telescope (ATel #14719) and Las Cumbres Observatory (ATel #14721). We see evidence for an iron emission line in the fit residuals around 6.4 keV. The PDS shows low power (<~1% rms from 0.1-10 Hz), with no QPO features in evidence throughout.

The MAXI/GSC nova alert system detected the source at 12:00 on June 11, 2021, and NICER subsequently began observing the source at 06:37:24 on June 12. To prevent telemetry saturation, NICER is turning off many of its detectors, currently operating with 7 FPMs on, one per MPU---the telemetered count rates have been maintained at one to several times 10^4/s. At the time NICER first observed 4U 1543-47, its spectrum was already soft and bright, with an inner disk temperature (kTin) of 1.100 +/- 0.002 keV, disk normalization 5780 +\- 50, and hydrogen column density Nh/[10^22 cm^-2] = 0.439 +/- 0.002 (adopting the model tbfeo*diskbb, with the Wilms et al. 2000 abundances, with oxygen/iron abundances free to vary). Throughout observations on June 12th, the flux of the source evolved from 3.6 Crab to 5.4 Crab (2-10 keV), and kTin increased to ~1.18 keV.

Assuming that 4U 1543-47 follows the same spectral state evolution pattern as seen in the majority of other black hole transients, the fact that it was already in the soft state during the first NICER observations indicates that it likely evolved through the hard and intermediate states in just a few days and at much lower bolometric luminosity than is typical. In general such a state evolution lasts several weeks. The 2002 outburst of 4U 1543-47 showed a similarly rapid initial state evolution (Park et al. 2004, ApJ, 610,378).

Subsequent observations on June 14th (MJD 59379.7) revealed a further increase in kTin to 1.315 +/- 0.005 keV, and normalization to 6270 +/- 90. Towards the latter part of observations on June 14th kTin was at 1.321 +/- 0.002 keV, normalization 6120 +/- 30. The 2-10 keV flux had by this stage increased to 9.2 Crab, after which (June 15th) the flux and kTin appeared to plateau. As of June 16th, the 2-10 keV flux dropped to 8.3 Crab, and kTin ~1.27 keV. The overall variations in disk normalization (using diskbb) imply inner disk radius deviations on the order of < 5%. Observations are ongoing to track the evolution of the source. A first round of simultaneous NICER (2 ks) + NuSTAR (20 ks) observations were scheduled for 2021-06-17 at 10:10 UT, and are currently underway. There will be 3 additional observations roughly a week apart from then onwards.

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.