Time evolution of the QPE intensity since first detection. The QPE intensity is measured as the average of the best-fitting Gaussian normalisation in each observation (i.e. we ignore here the difference between strong and weak QPEs). The colour scheme is the same as in Fig. 1. The dotted line represents the best-fitting linear relation (corresponding to a decay of about 0.22 cts s⁻¹ per 100 d). We also show (cross) the continuum level during the first observation with no detected QPEs (XMM7).