X-ray light curves observed at different epochs. Left panel: the evolution of the host-subtracted optical luminosity at the V-band is also shown (gray points, in units of 10⁴²erg s⁻¹). The solid line shows the best exponential fit, while the dotted line is the fit with the canonical t^−5/3 decay law. The two best-fit lines are scaled down by a factor of 100 to match the X-ray light curve.