Long-term evolution of the (quiescent level) bolometric luminosity. The y-axis scales have been defined in the caption of Fig. 12. In the upper panel we show the original light curve from all XMM-Newton and Swift observations. The lower panel shows a re-binned version of the same light curve (see text for details). In both panels, a vertical dashed line separates two different phases of the evolution: Phase1 comprises the initial X-ray peak and subsequent decay during the first ≃9 yr, while Phase2 is associated with the late-time X-ray re-brightening.