Swift J1753.5-0127 is heading to quiescence after an 11-year outburst

ATel #9708; David M. Russell, Aisha AlMannaei, Ahlam Al Qasim (NYU Abu Dhabi), A. W. Shaw (U. Alberta), P. A. Charles (IAC & U. Southampton), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 4 Nov 2016; 06:18 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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The black hole candidate X-ray binary Swift J1753.5-0127 began an outburst in June 2005 (ATel #546). After an initial outburst peak, the source settled into a hard state, and was persistent for the next 11 years. It made excursions into the hard-intermediate state (Soleri et al. 2013) and in March 2015 an anomalous low-luminosity soft state (Shaw et al. 2016), followed by a return to the hard state by March 2016 (ATel #8782). Here, we report a dramatic optical fading of the source in the last two months.

We have been monitoring the optical counterpart of Swift J1753.5-0127 with the 2-m robotic Faulkes Telescope North (located at Haleakala on Maui) and Faulkes Telescope South (at Siding Spring, Australia) since April 2010 (see Shaw et al. 2013). From 2010 to 2014 the source remained remarkably steady in the optical at V = 16.8; i' = 16.6, with intrinsic variability of amplitude no greater than +- 0.2 mag in V and i'. It gradually faded by 0.6 mag in both V and i' during the 2015 low-luminosity soft state, and then recovered to V = 17.0; i' = 16.7 by April 2016 when it was back in the hard state (ATel #8782).

On 19 September 2016, the magnitudes were V = 17.8, i = 17.4, indicating a fading had started some time after the previous observation on 22 August 2016. Since then it has faded more rapidly, to V = 19.15 +- 0.07; i' = 18.62 +- 0.03 on 20 October, and i' = 19.52 +- 0.11 on 1 November (no V-band data on this date).

If we assume an exponential decay of the flux (linear decay of the magnitude), between 19 September and 1 November the average rate of decay was 0.043 mag/day in i'-band. The V-i' colour has increased during the decay, with V-i' = 0.2, 0.3, 0.4 and 0.5 on 23 July, 22 August, 19 September and 20 October respectively, becoming redder, likely due to a decrease in temperature of the accretion disc during the fade. There are no pre-outburst detections of the quiescent counterpart to compare to, but DSS limits yield V > 21, so the source may reach and surpass this limit very soon. Our light curves can be found at the link below.

The source will soon not be visible from the ground due to the Sun. Multi-wavelength observations are encouraged during the fade and in quiescence, to measure the quiescent magnitudes and check for re-flares. We have been granted Swift observations on 6 and 7 November so any multi-wavelength follow-up that coincides with those dates is encouraged. 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).

Swift J1753.5-0127 Faulkes Telescope light curve