The brightest optical outburst of 4U 1543-47

ATel #14721; Payaswini Saikia, David M. Russell, Maria Cristina Baglio, D. M. Bramich (NYU Abu Dhabi) and Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 16 Jun 2021; 11:29 UT
Credential Certification: Payaswini Saikia (ps164@nyu.edu)

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4U 1543-47 (IL Lup) is a transient black hole X-ray binary ~7.5 kpc away, discovered during an outburst on 1971 August 17 (MJD 41180; Matilsky et al. 1972, ApJL, 174, 53). Since then, it has undergone three additional outbursts almost every 10 years - in 1983, 1992 and 2002, although no outburst was observed around 2012. Recently, a new outburst of the source was detected, almost 19 years after the last outburst, by MAXI/GSC on 2021 June 11 (MJD 59376), confirmed also by the Swift/BAT light curve (ATel #14701). It is reported to be reaching the Eddington luminosity, with a flux of 1.96e-7 erg/cm2/s (~8.2 Crab) in the 2-10 keV MAXI/GSC band (ATel #14708) and a UV magnitude of 15.77+/-0.10 in the uvw1 band of Swift/UVOT (ATel #14715).

We have been monitoring 4U 1543-47 since February 2008 with the Las Cumbres Observatory (LCO) 2-m (Faulkes) and 1-m robotic telescopes, as part of an on-going monitoring campaign of ~50 low-mass X-ray binaries (Lewis et al. 2008). To analyse the LCO data, we use the "X-ray Binary New Early Warning System" pipeline (XB-NEWS; see Russell et al. 2019 and Goodwin et al. 2020 for details).

4U 1543-47 has been in quiescence since the start of our optical monitoring (February 2008), with no hints of significant optical brightening, with the last quiescent magnitudes before the outburst measured as V=16.56 and i'=16.30 on 2021 May 19 (MJD 59353.7). All the uncertainties associated with the optical magnitudes are ~0.01 mag. Around the same time as the start of the X-ray outburst (MJD 59376, ATel #14701), we also see a significant enhancement in its optical magnitudes (with an increase of ~ 2.3 magnitudes compared to the quiescent values in all bands), with B=14.83, V=14.29, R=13.99 and i'=14.02, on 2021 June 12 (MJD 59377.2). The source is still brightening, with recent magnitudes of B=14.53, V=13.90, R=13.54 and i'=13.60 on 2021 June 15 (MJD 59380.5). Comparing this to the 2002 outburst which had dense optical monitoring by Buxton & Bailyn (2004), the peak magnitude in 2002 was V ~ 14.8, which is ~1 mag fainter than its current magnitude. In fact, this appears to be the brightest optical magnitude ever reported for the source, as far as we are aware (although optical coverage was poor in outbursts before 2002; Forman & Liller 1973, ApJ, 183, L117; IAUC # 3858 ; IAUC # 5512 ; IAUC # 5520 ).

We will continue monitoring the source during its outburst in the optical wavelengths, and encourage follow-up multi-wavelength observations. We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Longtime optical light curve of 4U 1543–47