Optical (flux) light curve of IC 3599 from the Catalina sky survey in arbitrary units (red dots), together with the Swift/XRT light curve (open squares) converted into flux using the spectral fit model, in arbitrary units. The Gaussian fit refers to the optical data. The vertical dashed line marks the time (MJD) of the optical peak, the dotted line the peak of the X-ray flux according to the multi-peak modelling. The continuous line marks the time at which the optical light curve rises by 10% from the quiescent level.