Swift J1357.2-0933 reaches its optical peak in the 2021 outburst

ATel #14623; Eleonora Caruso (NYU Shanghai), David M. Russell, Maria Cristina Baglio, Payaswini Saikia, D. M. Bramich (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 13 May 2021; 09:32 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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Swift J1357.2-0933 (Swift J1357) is a black hole candidate low-mass transient X-ray binary, first discovered during its 2011 outburst (ATel #3138). The source then had second and third outbursts in 2017 and 2019 (ATel #10297, #12796, #12867). Swift J1357 is currently undergoing its fourth outburst, first reported by the Zwicky Transient Factory (ZTF) which detected a significant optical re-brightening of the source on MJD 59314 (2021 April 10; ATel #14539).

We report on optical monitoring of Swift J1357's faint quiescent period preceding the rise, the early stages of the rise and the new outburst peak. We have been monitoring the source with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes, as part of an on-going monitoring campaign of ~ 50 low-mass X-ray binaries (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). Here we report on data from January to May 2021.

Since the 2019 outburst, the source showed a decreasing trend in quiescence luminosity that culminated with a significant dip in January 2021. We recorded the faintest optical values ever reached during quiescence in g', r', and i' filters, with magnitudes g'=22.9+/-0.20 on MJD 59237 (January 23), r'=22.6+/-0.19 on MJD 59236 (January 22), and i'=21.9+/- 0.23 on MJD 59262 (February 17). These can be considered the brightest possible magnitudes of the companion star in the system (the companion's contribution is uncertain).

After this dip, we detected a gradual brightening of the source, which may be a precursor to the fast outburst rise. In g', the re-brightening seems to have started between MJD 59262 and MJD 59286 (March 13), while in r' and I bands the source had already brightened by MJD 59279 (March 6). Therefore, this slow rise likely began between MJD 59262 and MJD 59279. The source brightened from g'=22.77+/-0.18 to g'=21.95+/-0.12 and from i'=21.91+/-0.22 to i'=20.81+/-0.08 between MJD 59262 and MJD 59310 (April 6). These represent quite slow rising rates of ~0.017 mag/day and ~0.022 mag/day in the g' and i' bands respectively.

We detected the following fast outburst rise on MJD 59316 (April 12), with magnitudes g'=18.61+/-0.01 , r'=18.57+/-0.01, I=18.29+/-0.01, and zs=18.73+/-0.02. There is a gap between our last detection of the slow rise (MJD 59310) and the first of the fast rise. Because our magnitude value r'=21.14+/-0.06 on MJD 59310 looks similar to the values from ~10 days before and to the value r'~21.1 reported by ZTF for the day after, MJD 59311 (April 7), we constrain the start of the outburst fast rise to between MJD 59310 and MJD 59316. For the outburst rise, we measured a rise rate of ~0.17 mag/day in g' band (MJD 59310-59337, April 6 - May 3). The outburst peak was likely reached around MJD 59337 with I=16.78+/-0.01, and it seems to be currently plateauing with the most recent magnitude being I=16.90+/-0.02 on MJD 59341 (May 7). This outburst peak is slightly fainter than the previous outburst in 2019, and the source displays a decreasing trend in outburst peak magnitude, the first outburst being the brightest. Specifically, the outburst's peak magnitudes were I=16.64 in 2019, I=16.26 in 2017, and I=16.20 in 2011 (all the four brightest measurements have errors 0.01 mag). A similar decreasing trend has been reported in X-ray peak luminosities (Beri et al. 2019; ATel #14573).

We will continue monitoring the source during the outburst decay; we encourage multiwavelength observations. We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Optical light curve of Swift J1357.2-0933