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The optical flux of Swift J1753.5-0127 strikes back

ATel #10325; Federico Bernardini, Guobao Zhang, David M. Russell, Joseph D. Gelfand, Ahlam Al Qasim, Aisha AlMannaei (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), A. W. Shaw (U. Alberta), J. A. Tomsick (SSL/UCB), R. M. Plotkin (ICRAR-Curtin)
on 28 Apr 2017; 05:38 UT
Credential Certification: Federico Bernardini (bernardini@nyu.edu)

Subjects: Optical, Ultra-Violet, X-ray, Binary, Black Hole, Transient

Referred to by ATel #: 10562, 10664

We report on a second re-brightening at optical wavelengths of the transient black hole candidate Swift J1753.5-0127 (hereafter J1753). J1753 was in outburst for 11 years from May 2005 until November 2016 when it was first observed to fade at optical wavelengths (ATel #9708, #9739). Soon after that, it reached the lowest recorded magnitude, V = 21.25 (ATel #9741), which is very close to the quiescent V-band upper limit (V > 21 mag; Cadolle Bel et al. 2007, ApJ, 659, 549), and it was no longer detected at radio and X-ray wavelengths (ATel #9735, #9765). Just two months later, on 30th January 2017, it was found back in outburst (V = 18.37 mag) after becoming once again visible from the ground (ATel #10075). Radio, UV and X-ray follow-up observations confirmed the renewed activity (ATel #10081, #10097, #10110, #10114). Then, a second drop at optical/UV/X-ray wavelengths was reported, with the optical flux fading at a much greater rate (a factor of 1.7 faster) compared to the previous decay in Sep. - Nov. 2016. On 19th April 2017 the V-band magnitude was V = 18.49 (ATel #10288). Since then, we have continued to monitor the source with the 2-m Faulkes Telescopes North and South, and the 1-m Las Cumbres Observatory (LCO) network of telescopes. We collected multiband photometry in i', R and V bands. We observed a second, unexpected, increase of the optical flux in all covered bands. Indeed, after reaching a minimum of V = 20.49 +- 0.20; R = 19.37 +- 0.06; i' = 19.26 +- 0.09 on 23rd April 2017 (at 23:40-23:50 UT), the source brightened over the next 3.6 days by 1.4, 0.6 and 0.8 mag in V, R and i'-bands respectively, to V = 19.10 +- 0.05; R = 18.81 +- 0.03; i' = 18.45 +- 0.05 on 27th April (14:40-15:00 UT). The optical flux of the source may peak again soon, or keep rising to a new (mini-)outburst, before fading again to quiescence. There is an approved Swift/XRT ToO scheduled for 29th April. Multi-wavelength observations during this re-brightening phase are strongly encouraged. The Faulkes Telescope / LCO observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008). This work makes use of observations from the Las Cumbres Observatory (LCO). Below is a link to the updated light curves of J1753.

Swift J1753.5-0127 Faulkes/LCO light curve