B2 1811+31 long-term lightcurve collected from 2005 to 2023. From top to bottom panels: HE γ-ray flux above 100 MeV from Fermi-LAT monthly binned data, X-ray flux in the 0.3−10 keV energy range from Swift-XRT, optical R-band data and radio data. The dashed red line marks the Fermi-LAT high-state detection on October 1, 2020 (MJD 59123). The shaded light orange, light blue, and green bands in the top panel indicate the “Pre-flare”, “Flare” and “Post-flare” periods, respectively (Table 2).