Optical and X-ray re-brightening in Swift J1745-26

ATel #5084; D. M. Russell (IAC, Tenerife), F. Lewis (Faulkes Telescope Project, University of South Wales), T. Munoz-Darias (University of Southampton), E. Kalemci (Sabanci University)
on 24 May 2013; 17:37 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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The black hole candidate X-ray binary Swift J1745-26 (Swift J174510.8-262411) is currently in the hard state and fading towards quiescence (ATel #4760, #4782, #4804, #4921). We have been monitoring the source with the Faulkes Telescope South (FTS; located at Siding Spring, Australia) in SDSS i'-band, Bessel R and Bessel V filters (ATel #4456). In the last few months the optical counterpart has brightened, from i' >19.74 on 17 March (MJD 56368.8) to i' = 19.45 +- 0.15 on 9 April (MJD 56391.8) and has remained at this level, with a recent magnitude of i' = 19.50 +- 0.14 on 17 May (MJD 56429.8). The R-band has also brightened, but the V-band has faded. On these three dates the magnitudes were R = 20.67 +- 0.24, R = 20.45 +- 0.21, R = 20.20 +- 0.13, and V > 21.19, V = 21.58 +- 0.48, V = 22.94 +- 0.55, respectively. The current magnitudes are 2 mag fainter than the outburst peak, i' = 17.6 (Munoz-Darias et al. 2013, MNRAS, in press) but still more than ~3 mag brighter than the likely quiescent level, r' > 23.1+-0.5 (ATel #4417). The fade in V-band implies the spectrum is redder in May than it was in March - April and redder than outburst peak. A link the light curve is provided below.

On inspection of the Swift BAT (15-50 keV) light curve of the source, a hard X-ray re-brightening is also apparent, and is coincident with the optical re-brightening. The X-ray flux is now fading again. It also appears that the transition to the hard state may have occurred at the beginning of December 2012 (just before a gap in the coverage due to Sun constraints) when there was an increase in the hard X-ray flux followed by a monotonic decay that lasted until this re-brightening. Multi-wavelength observations are encouraged during this re-brightening stage and decay to quiescence.

The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 30 low-mass X-ray binaries (Lewis et al. 2008). The Faulkes Telescope South is maintained and operated by Las Cumbres Observatory Global Telescope Network. FL acknowledges support from the Dill Faulkes Educational Trust.

Faulkes Telescope optical light curve