Temporal evolution of the variance-stabilized dark signal (T′ = 7.4 s) in pixel (1561,1341), over-plotted with the piecewise constant fit (dashed line) that results from the reconstruction with the unbalanced Haar technique. This represents a vertical cut in Fig. 4a and the corresponding one in Fig. 4b. Note that the dark signal oscillates between two values after the pixel has become hot. This multi-level random telegraph noise is expected (cf. Sect. 1.3). The drop in dark current observed in this curve around August 2011 also illustrates and demonstrates that hot pixels cool down sometimes, even if they do not recover fully.