Optical rebrightening of Swift J1357.2-0933 observed during the 2021 outburst decline

ATel #14729; Maria Cristina Baglio (NYU Abu Dhabi), Eleonora Caruso (NYU Shanghai), Dave M. Russell, Payaswini Saikia , D. M. Bramich (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 20 Jun 2021; 19:09 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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The black hole candidate X-ray binary Swift J1357.2-0933 was first discovered during an outburst in 2011 (ATel #3138). After that, it underwent two more outbursts in 2017 (ATel #10297) and 2019 (ATel #12816, ATel #12821, ATel #12867). The new outburst was first detected in the optical on 2021 April 10 with the Zwicky Transient Facility (ZTF; ATel #14539), with the optical magnitude brightening from g'>19.5 (soon before the start of the new outburst) to g'~18.7. A few days later, follow-up observations performed with the Swift satellite between April 11 and 24 revealed an increased activity in the X-rays and UV bands (ATel #14573), thus confirming the start of a new outburst and catching for the first time the very early stages of an outburst from Swift J1357.2-0933. Optical observations performed with the Las Cumbres Observatory (LCO) network of telescopes put a lower limit to the start date of the new outburst at optical wavelengths on MJD 59310 (2021 April 6); according to the same monitoring, the peak was reached around MJD 59337 (2021 May 3; ATel #14623). We report on optical follow-up observations performed with the LCO 2-m and 1-m robotic telescopes (Lewis et al. 2008); for the analysis of the LCO data we are making use of the "X-ray Binary New Early Warning System" pipeline (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 for details). After the source reached the peak in the optical around MJD 59337, with magnitudes g'=17.09+/-0.01, r'=17.20+/-0.01, i'=21.87+/-0.15, I=16.78+/-0.01, z=17.31+/-0.01, the outburst started declining, reaching its minimum flux on MJD 59373 (2021 June 8) with the following magnitudes: g'=19.88+/-0.04, r'=19.41+/-0.02, i'=19.23+/-0.03, z=19.06+/-0.04. These values were however brighter with respect to the faintest quiescent magnitudes that were reached before the start of the 2021 outburst (g'=22.90+/-0.20, i'=21.91+/- 0.23, r'=22.60+/-0.19; ATel#14623).

On MJD 59374 (2021 June 9), the start of a rebrightening was detected in all optical bands, with magnitudes: g'=19.45+/-0.02, r'=19.13+/-0.02, i'=18.96+/-0.03, z=18.81+/-0.04. The brightest magnitudes reached so far during the rebrightening are: g'=18.61+/-0.02, r'=18.43+/-0.02, I=17.72+/-0.03, z=18.30+/-0.05 (MJD 59380; 2021 June 15). The rebrightening is still ongoing.

We will continue monitoring the source with LCO; follow-up multi-wavelength observations are encouraged.

The LCO observations are part of an on-going monitoring campaign of ~ 50 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Optical light curve of Swift J1357.2-0933