Panel a: epoch-folding (black, left y-axis) and Lomb–Scargle Periodogram (green, right y-axis) of all Swift/UVOT data. Panel b: same as panel a but for the Swift/XRT data. The dotted vertical line marks the strongest period of the UVOT data (which is consistent with the implied orbital period from the pulse period analysis), the dashed line the period of the XRT data. The dashed horizontal line marks the 99.9% false alarm probability of the epoch folding results based on an F-distribution corrected for number of trial periods (Davies 1990).