Echo mapping of Swift J1753.5-0127

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 it consistent correlation with all immediate response and an extended tail lasting similar to 5 s. The properties of the variability and the correlation are consistent with thermal reprocessing in all accretion disc. The shortness of the lag suggests it 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 con unction 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).