Optical brightening of Swift J1753.5-0127 observed with the Faulkes Telescope North

ATel #10075; Ahlam Al Qasim, Aisha AlMannaei, David M. Russell (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), Guobao Zhang, Joseph D. Gelfand (NYU Abu Dhabi)
on 14 Feb 2017; 14:39 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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We have continued monitoring Swift J1753.5-0127, the black hole candidate X-ray binary, with the 2-m Faulkes Telescope North after it became once again visible from the ground. Last year, we reported an optical fading of the source towards quiescence (ATel #9708, #9739). Follow-up radio (ATel #9765), X-ray (ATel #9735) and optical (ATel #9741, #9758) observations confirmed the source was very faint compared to the last 11 years.

We report our new magnitudes of the object in the i'-band and the V-band as follows. V = 18.37 +- 0.06; i' = 17.92 +- 0.05 on 30 January 2017 (MJD 57783.7) and V = 17.27 +- 0.05; i' = 16.95 +- 0.02 on 13 February (MJD 57797.6). The latest magnitudes are similar to that before the fading in August 2016; we observe that it is now as bright as it was during July 2016. During the recent brightening, the V-i' color of the source seems to have shifted back to what it originally was before fading away, getting bluer as it has brightened, probably due to an increase in temperature of the accretion disc. The source is 4.0 magnitudes brighter in V-band since it was detected at V = 21.25 +- 0.03 on 8 November 2016 with the Nordic Optical Telescope (ATel #9741).

The re-brightening exhibits surprising behavior since it was expected to fade into quiescence after its dramatic fading, and perhaps become undetectable. Re-flares are not uncommon in X-ray binary outburst decays, but returning to the previous bright flux level is highly unusual (another case was the neutron star system IGR J00291+5934; Lewis et al. 2010). It is unclear whether the source will continue to brighten, remain steady like it was before the fade, or exhibit flares before fading once more into quiescence. Multi-wavelength observations are encouraged during this time. We are requesting Swift observations spanning the next two weeks. 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