Insights from leptohadronic modelling of the brightest blazar flare

ABSTRACT The blazar 3C 454.3 experienced a major flare in 2010 November, making it the brightest $\gamma$-ray source in the sky of the Fermi-Large Area Telescope (LAT). We obtain seven daily consecutive spectral energy distributions (SEDs) of the flare in the infrared, optical, ultraviolet, X-ray, and $\gamma$-ray bands with publicly available data. We simulate the physical conditions in the blazar and show that the observed SEDs are well reproduced in the framework of a ‘standing feature’ where the position of the emitting region is almost stationary, located beyond the outer radius of the broad-line region and into which fresh blobs of relativistically moving magnetized plasma are continuously injected. Meanwhile, a model with a single ‘moving blob’ does not describe the data well. We obtain a robust upper limit to the amount of high-energy protons in the jet of 3C 454.3 from the electromagnetic SED. We construct a neutrino light curve of 3C 454.3 and estimate the expected neutrino yield at energies $\ge 100$ TeV for 3C 454.3 to be up to $6 \times 10^{-3}$ $\nu _{\mu }$ per year. Finally, we extrapolate our model findings to the light curves of all Fermi-LAT flat-spectrum radio quasars. We find that next-generation neutrino telescopes are expected to detect approximately one multimessenger ($\gamma + \nu _{\mu }$) flare per year from bright blazars with neutrino peak energy in the hundreds TeV–hundreds PeV energy range and show that the electromagnetic flare peak can precede the neutrino arrival by months to years.