Faulkes Telescope monitoring of Swift J1357.2-0933 shows it has been in outburst since 21 March

ATel #12803; D. M. Russell (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), P. Gandhi, J. A. Paice, P. A. Charles (U. Southampton), Y.-J. Yang (IHEP, CAS, Beijing), P. Casella (INAF-OAR), D. M. Bramich (NYU Abu Dhabi), D. A.H. Buckley (SAAO)
on 24 May 2019; 13:16 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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The black hole candidate X-ray binary Swift J1357.2-0933 had outbursts in 2011 and 2017. A new outburst has been detected at optical wavelengths by the Zwicky Transient Facility (ZTF) in March to May 2019 (ATel #12796), and confirmed at X-ray energies by a NICER observation on 2019 May 23 (ATel #12801). Here we report long term optical monitoring with the 2-m Faulkes Telescopes North (at Haleakala Observatory, Maui, Hawaii, USA) and South (at Siding Spring Observatory, Australia).

Images have been taken in Bessel I-band since the 2017 outburst. In 2018 the source remained at its quiescent level of I = 20.0 +/- 0.4 where the error encompasses the high amplitude variability known to be prominent in this source in quiescence (Russell et al. 2018). From 2019 January 5 to March 19 (MJD 58488 - 58561) the source was in quiescence, with the average flux fainter than in 2018 (I = 20.5 +/- 0.4).

On March 21 a flux increase is detected; I = 19.61 +/- 0.15 (MJD 58563.5). The source continued to brighten until April 14 (MJD 58587), with magnitudes I = 18.17 +/- 0.01 on March 28 (MJD 58570.6), I = 16.86 +/- 0.01 on April 4 (MJD 58577.6), I = 16.61 +/- 0.01 on April 14 (MJD 58587.6). The source remained at I ~ 16.6 until our latest measurement, when it faded to I = 16.90 +/- 0.01 on May 22 (MJD 58625.5). The source may be starting to fade, and multi-wavelength observations are encouraged during the remainder of the outburst. A link to our light curves is provided below.

Due to the two observations on March 19 (quiescence) and March 21 (brightening), we can pinpoint the date of the start of the optical outburst, to MJD 58562.44 +/- 1.06 (2019 March 20 at 10:30 UT +/- 1 day). This is slightly earlier than that reported from ZTF in the r_ZTF and g_ZTF bands (ATel #12796). Intriguingly, it is also ~ 3 days after weak X-ray 2-20 keV activity (on MJD 58558-58559) detected by MAXI (ATel #12801). According to disc instability models, an optical rise preceding the X-ray brightening is expected at the start of new LMXB outbursts (e.g. Lasota 2001, Russell et al. 2019). Here, instead there appears to be a brief X-ray brightening (although the MAXI detections are 3 sigma only) followed a few days later by the optical brightening of the new outburst. There are no subsequent MAXI detections (>2.5 sigma) since the start of this outburst, but the source is in a faint X-ray outburst as confirmed by the NICER detection (ATel #12801).

The peak magnitudes of the 2011, 2017 and 2019 outbursts are I = 16.2, I = 16.1 and I = 16.6 respectively. The 2019 brightening is a new outburst (not a mini-outburst) adopting the classification of Zhang et al. (2019). It is unusual to observe outbursts so soon after previous outbursts because it usually takes time for matter to build up in the accretion disc between outbursts. However there are exceptions, such as recurrent systems like GX 339-4, and XTE J1550-564 has some similarities to Swift J1357.2-0933 as it had bright outbursts in 1998 and 2000 followed by fainter outbursts in 2001, 2002 and 2003 (Curran & Chaty 2013). We will monitor the new outburst of Swift J1357.2-0933 in four optical filters with the 2-m Faulkes and 1-m Las Cumbres Observatory (LCO) telescopes.

The Faulkes observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. This work makes use of observations from the Las Cumbres Observatory (LCO).

Optical LCO light curves of Swift J1357.2-0933