2009MNRAS.399..281H

We present two epochs of coordinated X-ray-optical timing observations of the black hole candidate Swift J1753.5-0127 during its 2005 outburst. The first epoch in July occurred at outburst peak. Two consecutive nights of observations using the McDonald Observatory Argos camera with the Rossi X-ray Timing Explorer show a consistent correlation with an immediate response and an extended tail lasting ∼5s. The properties of the variability and the correlation are consistent with thermal reprocessing in an accretion disc. The shortness of the lag suggests a short orbital period consistent with that recently claimed. The second epoch in August used the Very Large Telescope Focal Reducer/low dispersion Spectrograph 2 (FORS2) High Time resolution (HIT) mode again in conjunction with RXTE. Again a repeatable correlation is seen between two independent subsets of the data. In this case, though, the cross-correlation function has an unusual structure comprising a dip followed by a double peak. We suggest that this may be equivalent to the dip plus single-peak structure seen by Kanbach et al. (2001) in XTE J1118+480 and attributed there to synchrotron emission; a similar structure was seen during later activity of Swift J1753.5-0127 by Durant et al. (2008).