B2 1811+31 multiwavelength lightcurves and SEDs

We propose an interpretative leptonic model for the observed radiative high state. During the 2020 flaring state, the synchrotron peak frequency shifted to higher values and reached the limit of the IBL classification. Variability in timescales of few hours in the high-energy (HE; 100MeV<E<100GeV) gamma-ray band poses an upper limit of 6x10^14^delta_D_cm to the size of the emission region responsible for the gamma-ray flare, delta_D being the relativistic Doppler factor of the region.

During the 2020 high state, the average spectrum became harder in the HE {gamma}-ray band compared to the low states. A similar behaviour has been observed in X rays. Conversely, during different activity periods, we find harder-when-brighter trends in X-rays and a hint of softer-when-brighter trends at HE {gamma} rays.

Long-term HE gamma-ray and optical correlation indicates the same emission regions dominate the radiative output in both ranges, whereas the long-term evolution at 15GHz shows no correlation with the fluxes at higher frequencies. We test one-zone and two- zone synchrotron-self-Compton models for describing the broad-band spectral energy distribution during the 2020 flaring state and investigate the self- consistency of the proposed scenario.