Variable Gamma-ray Emission Induced by Ultra-High Energy Neutral Beams: Application to 4C +21.35

TL;DR

This paper explores how ultra-high energy cosmic rays in the quasar 4C +21.35 can produce variable gamma-ray emissions through photohadronic processes, suggesting that detection of high-energy neutrinos could confirm this mechanism.

Contribution

It introduces a model linking UHECRs to variable gamma-ray emissions in 4C +21.35, including detailed calculations of secondary radiations and their beaming effects, which is a novel approach.

Findings

High Doppler factors (>100) are needed for rapid variability in gamma rays.

Secondary gamma-ray spectra fit the observed flaring spectrum of 4C +21.35.

Detection of >100 TeV neutrinos would support the proposed UHECR-induced emission scenario.

Abstract

The flat spectrum radio quasar (FSRQ) 4C +21.35 (PKS 1222+216) displays prominent nuclear infrared emission from ~1200 K dust. A 70 -- 400 GeV flare with ~10 min variations during half an hour of observations was found by the MAGIC telescopes, and GeV variability was observed on sub-day timescales with the Large Area Telescope on Fermi. We examine 4C +21.35, assuming that it is a source of ultra-high energy cosmic rays (UHECRs). UHECR proton acceleration in the inner jet powers a neutral beam of neutrinos, neutrons and gamma rays from photopion production. The radiative efficiency and production spectra of neutrals formed through photohadronic processes with isotropic external target photons of the broad line region and torus are calculated. Secondary radiations made by this process have a beaming factor ~\delta^5, where \delta is the Doppler factor. The pair-production optical depth for gamma rays and the photopion efficiency for UHECR neutrons as they pass through external isotropic radiation fields are calculated. If target photons come from the broad line region and dust torus, large Doppler factors, \delta >~100 are required to produce rapidly variable secondary radiation with isotropic luminosity >~1e47 erg/s at the pc scale. The \gamma-ray spectra from leptonic secondaries are calculated from cascades initiated by the UHECR neutron beam at the pc-scale region and fit to the flaring spectrum of 4C +21.35. Detection of >~100 TeV neutrinos from 4C +21.35 or other VHE blazars with IceCube or KM3NeT would confirm this scenario.