A drop in X-ray/UV/Optical flux from Swift J1753.5-0127

ATel #10288; A. W. Shaw (U. Alberta), J. A. Tomsick (SSL/UCB), G. Zhang, D. M. Russell (NYU Abu Dhabi), R. M. Plotkin, J. C.A. Miller-Jones (ICRAR-Curtin), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 19 Apr 2017; 17:50 UT
Credential Certification: Aarran Shaw (aarran@ualberta.ca)

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Swift J1753.5-0127 (hereafter J1753) is a candidate black hole X-ray transient that was in outburst for 11 years, before reaching quiescence in November 2016 (ATel #9735, #9739, #9765). It was reported to be back in outburst in February 2017 (ATel #10075, #10081, #10097, #10110, #10114), triggering a large multi-wavelength observing campaign. We report here on the decay in flux across multiple wavebands since the initial re-brightening.

As part of an ongoing monitoring campaign, we have obtained Swift observations of J1753 with an average cadence of three days since 2017 Feb. 16. The observations have been performed in both Photon Counting mode and Windowed Timing mode, dependent on the source count rate. The last reported X-ray flux was (7.4+/-0.3)E-11 erg/cm2/s (0.3-10 keV unabsorbed) on 2017 Feb. 22 (ATel #10114). Since then the flux has decreased to (1.6+0.5/-0.4)E-12 erg/cm2/s as of 2017 Apr. 18 (from a power law fit to the X-ray spectrum with Γ = 1.9+/-0.5). The reported uncertainties are 90% confidence.

J1753 was also observed with Swift/UVOT in all filters and the magnitudes (Vega) as of 2017 Apr. 18 are as follows: V = 18.62+/-0.34+/-0.01, B = 18.87+/-0.18+/- 0.02, U = 18.49+/-0.19+/-0.02, UVW1 = 18.84+/-0.25+/-0.03, UVM2 > 20.24, UVW2 > 20.4, where the first and second error values are statistical and systematic 1σ uncertainties. This is a drop of ~1.5 magnitudes in the optical bands and ~2 in the UV bands since 2017 Feb. 22.

We have also maintained regular monitoring of J1753 with the 2-m Faulkes Telescopes North and South and the 1-m Las Cumbres Observatory (LCO) network of telescopes. We confirm the decay in both the V and i'-bands. We find that the rate of the decay seems to increase after MJD ~ 57835 - 57850 (2017 Mar. 23 - Apr. 7), at a rate of 0.075 mag/day in i' and 0.061 mag/day in V, compared to 0.0034 mag/day (i') and 0.0049 mag/day (V) at MJD < 57835. The current optical rapid decay is ~1.7 times faster than during the previous decay in 2016 Sep. - Nov. (ATel #9708). The latest magnitudes are V = 18.49+/-0.04, i' = 18.25+/-0.04 on MJD 57862 (2017 Apr. 19).

The above observations indicate that J1753 may be returning to quiescence once more after a short re-brightening. We encourage multi-wavelength follow up and we will continue our Swift and Faulkes Telescope monitoring.

We thank the Swift team for scheduling our observations. 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 LCO.