Swift/BAT light curves in the 50–100 keV (bottom panel), 20–50 keV (middle panel), and full band (top panel) for the sources we studied. Solid circles and open squares indicate the 2 d (1 d in the case of NGC 4151), and the 20 d-rebinned light curves, respectively. The time axis counts days since the start of the Swift/BAT observations. The dashed vertical lines in the NGC 4151 plot indicate the case of a flux variation whose evolution in the hard band is faster than the respective flux change in the soft band (see Sect. 3.1 for details).