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The black hole candidate Swift J1753.5-0127 is back in quiescence

ATel #10562; Guobao Zhang, David M. Russell, Federico Bernardini, Joseph D. Gelfand (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 7 Jul 2017; 16:09 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

Subjects: Radio, Optical, X-ray, Binary, Black Hole, Transient

Referred to by ATel #: 10664

We report on new optical observations showing that after a period of intense activity at the end of its 11-year long outburst, the black hole candidate Swift J1753.5-0127 appears to be back in quiescence. We have detected the source in quiescence with the 2-m Faulkes and 1-m Las Cumbres Observatory (LCO) network telescopes.

After spending almost 11 years in outburst, Swift J1753.5-0127 approached optical quiescence for the first time in 2016 November. Since then, the source unexpectedly reactivated at all wavelengths. In particular, the optical flux peaked for a first time in 2017 February and then dropped towards quiescence a second time, reaching its lowest level in 2017 April. It then became bright again just a few days after that (ATel #9708, #9735, #9739, #9741, #9758, #9765, #10075, #10081, #10097, #10110, #10114, #10288, #10325).

Since then, we have continued monitoring Swift J1753.5-0127 in i', R, and V bands with the 2-m Faulkes Telescopes North and South, and the 1-m LCO network of telescopes. The optical flux peaked a second time in late April - early May, with i' = 17.27 +/- 0.01 mag on 2017 April 30 (MJD 57873) and V = 17.90 +/- 0.03; R = 17.46 +/- 0.03; i' = 17.53 +/- 0.03 on May 1 (MJD 57874). The source then started decaying in all three bands (the third decay we recorded since the steady flux that ended in 2016). During this decay, intense variability was seen in the form of several flares. This flaring activity was not recorded during the previous fading phases.

A dramatic drop of flux was seen between 2017 May 26 (MJD 57899; i' = 18.71 +/- 0.04) and our next detection on June 4 (MJD 57908; i' = 20.91 +/- 0.29). Since then we increased exposure times to 300 seconds and have detected the source significantly (> 3 sigma) in six i'-band images and one V-band image by July 5 (MJD 57934). By combining our quiescent data into deep V and i'-band images, we measure an average quiescent magnitude during June 2 to July 5 of V = 22.17 +/- 0.25; i' = 21.00 +/- 0.14 (from data totaling exposure times of 39 minutes in V-band and 41 mins in i'-band). These are the faintest detections ever made of the source, even fainter than in 2016 November (ATel #9708, #9739, #9741), and fully consistent with the quiescent upper limits derived from observations taken before the beginning of the outburst in 2005 (Cadolle Bel et al. 2007, ApJ, 659, 549). This suggests that the source is currently in quiescence, but a third re-brightening or further fading could still be possible.

We will continue monitoring the source at optical wavelengths. Multi-wavelength observations are encouraged during quiescence to constrain the companion type and orbital parameters, and the quiescent X-ray and radio luminosity.

The Faulkes Telescope 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). A link to the updated light curves of Swift J1753.5-0127 is included below.

Swift J1753.5-0127 Faulkes Telescope light curve