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Optical spectroscopy and photometry of the black hole transient SWIFT J1753.5-0127 at the faintest ever recorded level.

ATel #10664; Vitaly Neustroev (FINCA, University of Turku, University of Oulu), Sergey Zharikov (IA UNAM), Antonio Cabrera-Lavers (GTC)
on 23 Aug 2017; 15:32 UT
Credential Certification: Vitaly Neustroev (vitaly@neustroev.net)

Subjects: Optical, Black Hole, Transient

The black hole transient SWIFT J1753.5-0127 had been in outburst for more than 11 years, from 2005 to 2016. It started to decline in 2016 September (ATel #9708) and reached quiescence in 2016 November (ATel #9735, #9739, #9741, #9758, #9765). The reported optical magnitude at this time was about V=21.2 and the source showed very red colours, e.g. V-I=1.4 mag (ATel #9741). However, in 2017 February and April, SWIFT J1753.5-0127 exhibited two rebrightenings (ATel #10075, #10081, #10097, #10110, #10114, #10288, #10325). In particular, on May 1 the source reached V = 17.9 and then started slowly declining (ATel #10562). Zhang et al. (2017, ATel #10562) reported a dramatic drop of flux between 2017 May 26 and June 4, and measured an average quiescent magnitude during June 2 to July 5 of V = 22.17+/-0.25.

We observed SWIFT J1753.5-0127 on 2017 June 16, July 17 and Aug 19 using OSIRIS on the Gran Telescopio CANARIAS (GTC). The acquisition images, which were taken with the Sloan filter g' during centering of the target on the slit, were used for photometric measurements. On June 16 and July 17 we found consistent g' magnitudes to be 22.44+/-0.12 and 22.48+/-0.14, respectively. However, on Aug 19 we took three images, separated by a few minutes, in which the target was significantly fainter. The first two images give the exact same g' magnitude of 22.64+/-0.08, whereas the image taken 3 min later gives g'=22.90+/-0.08, indicating a possible short-term variability.

The spectra exhibit a very red continuum and a strong, very broad and double-peaked emission line of H_alpha (EW ~90 Angstrom, peak-to-peak separation ~2800 km/s, FWHM ~3800 km/s). No other emission or absorption lines are clearly seen.

In order to compare the current V-I colour index with previous observations, we used our flux-calibrated spectra. For the August observations, we estimated V-I to be ~2 mag. It is redder than was measured in 2016 November (ATel #9741).