Rapid optical/infrared brightening of 4U 1543-47 after the recent outburst, observed with REM and LCO

ATel #15254; Jwaher Alnaqbi, Payaswini Saikia (NYU Abu Dhabi), Piergiorgio Casella, Vincenzo Testa (INAF-OAR), David M. Russell, M. C. Baglio, Kevin Alabarta, D. M. Bramich (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), Federico Vincentelli (Villanova University), Thomas J. Maccarone (Texas Tech University), Tomaso Belloni (INAF), and Emrah Kalemci (Sabanci University)
on 2 Mar 2022; 19:29 UT
Credential Certification: Payaswini Saikia (ps164@nyu.edu)

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4U 1543-47 is a transient black hole X-ray binary that was discovered during an outburst on 17 August 1971 (MJD 41180; Matilsky et al. 1972, ApJL, 174, 53). The source had a new outburst that was detected by MAXI/GSC on 2021 June 11, and confirmed by Swift/BAT (MJD 59376, ATel #14701). Several measurements at different wavelengths were reported in the following days (ATels #14708, #14715, #14719, #14721, #14725, #14749).

Our optical data with the Las Cumbres Observatory (LCO) 2-m (Faulkes Telescope South) and 1-m robotic telescopes, and infrared data with the 60cm Rapid Eye Mount (REM) robotic telescope (La Silla, Chile) show that, after reaching its brightest magnitude in June 2022, the source had a smooth decay. After a gap in our coverage due to Sun constraint, it reached optical magnitudes of i'=16.14, V=16.50, and R=16.14 (uncertainties below 0.01 mag) on 2022 January 10 (MJD 59589), consistent with the pre-outbursts quiescence levels (ATel #14721), and infrared magnitudes of J~15.6 and H~15.4 (uncertainties ~0.1-0.2 mag). Based on radio and X-ray observations, it was reported that the source had transitioned to the hard state a week before, around 2022 January 3 (MJD 59582, ATel #15157).

Recently, we observed a new, rapid brightening of the source, with optical magnitudes rising from i' = 16.28 on 2022 January 23 (MJD 59602, optical magnitudes consistent with the quiescent level), to i'= 15.88 on 2022 February 9 (MJD 59619), and i'= 14.46 on 2022 February 18 (MJD 59628). The most recent observations on 2022 March 1 (MJD 59639) show that the flare already started to decay with magnitudes i'=14.69 and V=15.31. The recent brightening is also evident at infrared wavelengths, as seen with REM. The source had a rapid brightening from J~15.6 and H~15.4 (the K filter was temporarily unavailable) on 2022 February 10 (MJD 59620) to J~12.8, H~12, and K~10.8 on 2022 February 20 (MJD 59630). The reflaring is also observed in the X-rays with NICER, MAXI/GSC and Swift/BAT (ATel #15253).

We will continue monitoring the source with LCO and REM. Further multi-wavelength observations are encouraged. The LCO observations are part of a monitoring campaign of ~50 low-mass X-ray binaries (Lewis et al. 2008) since 2008. All LCO photometric analysis and data calibration were performed using the "X-ray Binary New Early Warning System" pipeline (XB-NEWS; see Russell et al. 2019 and Goodwin et al. 2020 for details).

LCO and REM light curve of 4U 1543-47