Diffuse Neutrino Intensity from the Inner Jets of Active Galactic Nuclei: Impacts of External Photon Fields and the Blazar Sequence [HEAP]


We study high-energy neutrino production in the inner jets of radio-loud active galactic nuclei (AGN), taking into account effects of external photon fields and the blazar sequence. We show that the resulting diffuse neutrino intensity is dominated by quasar-hosted blazars, in particular, flat spectrum radio quasars, and that PeV neutrino production due to photohadronic interactions with broadline radiation is unavoidable if the inner jets of blazars are cosmic-ray sources. The resulting neutrino spectrum has a cutoff feature around PeV energies since the main target photons are Ly$\alpha$ emission. Because of infrared photons provided by the dust torus, neutrino spectra above PeV energies are too hard to be consistent with the IceCube data unless the proton spectral index is steeper than 2.5, or the maximum proton energy is $\lesssim100$ PeV. Thus, although the cumulative background can be as high as $E_\nu^2\Phi_\nu\sim{10}^{-8}~{\rm GeV}~{\rm cm}^{-2}~{\rm s}^{-1}~{\rm sr}^{-1}$ for the cosmic-ray loading factor of $\xi_{\rm cr}\sim10$-$100$, it is difficult to explain the IceCube signal in the inner jet model unless more complicated situations are invoked. Even so, interestingly, future detectors such as the Askaryan Radio Array can detect $\sim0.1$-$1$ EeV neutrinos. We find that the diffuse neutrino intensity from radio-loud AGN is dominated by blazars with gamma-ray luminosity of $\gtrsim10^{48}~{\rm erg}~{\rm s}^{-1}$, and the arrival directions of $\sim1$-$100$ PeV neutrinos correlate with the luminous blazars detected by Fermi.

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K. Murase, Y. Inoue and C. Dermer
Tue, 18 Mar 14