High-energy neutrinos from extragalactic jets

Abstract

In this paper I calculate the diffuse background of high-energy neutrinos from extragalactic jets emerging from active galactic nuclei (AGN). I assume that radio-quiet AGN have jets that are disrupted by turbulence in the vicinity of an accretion disk surrounding a central black hole and remain invisible in the radio band. In contrast, radio-loud AGN have high Mach number jets which dissipate in a strong shock far away from the AGN. It is assumed that protons are accelerated to ultra-high energies in jets. Photo-production of pions induces neutrino emission and electromagnetic cascades. The cascade spectra are in agreement with the most recent X- and γ-ray observations of AGN; in particular with the lack of γ-ray emission from radio-quiet AGN and the powerful γ-ray emission from flat-spectrum radio-loud AGN. A high energy neutrino background from radio-quiets is predicted assuming that they produce the cosmic diffuse X-ray background. However, the flux limits from the Fréjus proton decay experiment are violated by this prediction. On the other hand, recent γ-ray observations make it very plausible that the diffuse γ-ray background at least above 100 MeV is due to radio-loud AGN. A striking similarity exists between the energy fluxes of diffuse γ-rays above 100 MeV and cosmic ray protons above the ankle. This is an independent argument for proton acceleration in radio jets consistent with the explanation of the individual γ-ray spectra by hadronically induced cascades. The corresponding prediction of a neutrino flux at ultra-high energies therefore rests on a firm basis.