4U 1543-47 at quiescent levels before the recent re-brightening

ATel #15719; Jwaher Alnaqbi, Payaswini Saikia, David M. Russell, M. C. Baglio, Kevin Alabarta, D. M. Bramich (NYU Abu Dhabi) F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 25 Oct 2022; 15:03 UT
Credential Certification: Payaswini Saikia (ps164@nyu.edu)

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4U 1543-47 is a transient low-mass X-ray binary system with a black hole as a compact object. The system is ~7.5 kpc away and it was discovered during an outburst in 1971 (Matilsky et al. 1972). In June 2021, we detected the brightest outburst of this source to date in the optical with Las Cumbers Observatory (LCO, ATel #14721), which was also the brightest XRB outburst ever detected with NICER (ATel #14725). Hereafter, we refer to this outburst as the "primary outburst". We also detected and reported a mini-outburst starting on 9th February 2022 (MJD 59619) after the primary outburst reached quiescence, with magnitudes rising from i'~16.3 mag to i'~15.9 mag (ATel #15254). Multi-wavelength measurements of the re-brightening were also reported (ATels #14878, #14888, #15157, #15253, #15255, #15261). A new re-brightening of 4U 1543-47 on 21st October 2022 (MJD 59873) was recently reported by MAXI/GSC and Swift/BAT (ATel #15715, #15717). In this ATel we report on optical data of the mini-outburst prior to the new re-brightening.

The optical mini-outburst reached its peak on 13 February 2022 (MJD 59623) with i'=14.56 mag, V=15.17 mag and started to decay until 11-14th April 2022 (MJD 59680-59683) with i'=15.70, R=15.71, V=16.11, B=16.73 mag, after which there was a brightening that reached a peak of R=15.10, V=15.57, B=16.17 mag on 26th April 2022 (MJD 59695), and i'= 14.91 mag on 7th May 2022 (MJD 59706). The average statistical uncertainty associated with all magnitude measurements is less than 0.01 mag. The decay continued until 21st-23rd May 2022 (MJD 59720-59722) with i'=15.99, R=16.06, V=16.34, B=17.02 mag, and then we detected a second brightening on 4-6th June 2022 (MJD 59734-59736) with i'=15.08, R=15.15, V=15.55, B=16.28 mag. The mini-outburst then decayed until the source reached quiescence on 18th August 2022 (MJD 59809) with i'=16.32, V=16.62 mag, consistent with the quiescent level prior to the primary outburst (i'~16.3 and V~16.6 mag on 19th May 2021 MJD 59353; ATel #14721). The mini-outburst spent a total of ~190 days in a fluctuating decay before it finally reached quiescence. The source was still at quiescent levels on 3rd October 2022 (MJD 59855), which is ~18 days before the reported re-brightening. Currently, it is not possible to monitor 4U 1543-47 at optical wavelengths due to Sun constraints. Since we find that the source had reached quiescence prior to the recent X-ray re-brightening (ATel #15715, #15717), we speculate that this event could be another mini-outburst (Zhang et al. 2019).

We monitor 4U 1543-27 with the LCO using the 1-m and 2-m robotic telescopes, in optical i', R, V, and B bands as part of an ongoing campaign to monitor ~50 low-mass X-ray binaries (Lewis et al. 2008). The photometric analysis and calibration is performed using the "X-ray Binary New Early Warning System" pipeline (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details).

Optical light curve of the 2021-2022 outburst of 4U 1543-47