Top: time distribution of ASCA (green squares, “A”-marks), RXTE (pink diamonds, “R”-marks), BeppoSAX (brown stars, “S”-marks), and Suzaku (blue triangles, “Sz”-marks) observations (see Tables 1–4). Bottom: Swift/XRT light curve of BL Lac in the 0.3−10 keV range during 2005–2016. Red points mark the source signal (with 2-σ detection level) and blue arrows show the MJD of Suzaku. Note, that rate-axis is related to RTXE/PCA count rate which are not comparable with other instruments (ASCA, BeppoSAX and Suzaku). For clarity, the error bars are omitted.