Strong constraints on hadronic models of blazar activity from Fermi and IceCube stacking analysis
Résumé
Context. High-energy emission from blazars is produced by electrons that are either accelerated directly, under the assumption of leptonic models of blazar activity, or produced in interactions of accelerated protons with matter and radiation fields, under the assumption of hadronic models. The hadronic models predict that γ-ray emission is accompanied by neutrino emission with comparable total energy. Aims. We derive constraints on the hadronic models of activity of blazars imposed by non-detection of neutrino flux from a population of γ-ray emitting blazars. Methods. We stacked the γ-ray and muon neutrino flux from 749 blazars situated in the declination strip above − 5°. Results. Non-detection of neutrino flux from the stacked blazar sample rules out the proton-induced cascade models in which the high-energy emission is powered by interactions of a shock-accelerated proton beam in the active galactic nucleus (AGN) jet with the ambient matter or radiation field of the black hole accretion disk. The result also remains valid for the case of interactions in the scattered radiation field in the broad line region. The IceCube constraint could be avoided if the spectrum of accelerated protons is sharply peaking in the ultra-high-energy cosmic ray range, as in the models of acceleration in the magnetic reconnection regions or in the vacuum gaps of black hole magnetospheres. Models based on these acceleration mechanisms are only consistent with the data if characteristic energies of accelerated protons are higher than 1019 eV. The constraint could also be avoided if the hadronic emission component of γ-ray flux is largely sub-dominant compared to the leptonic component and/or if it only appears during flaring activity episodes, which provide negligible contribution to the time-averaged source flux. Key words: galaxies: active / gamma rays: galaxies / neutrinos