Neutral Beams from Blazar Jets

Abstract

We treat the production of neutrons, photons, and neutrinos through photomeson interactions of relativistic protons with ambient photons in the compact inner jets of blazars. We show that the presence of the external UV radiation field makes possible strong energy losses already for protons with energies > 1 PeV, while without this component effective energy losses of protons begin only at E > 10^{18} eV. We develop a model describing the production and escape of neutrons from a comoving spherical blob, which continue to interact with the ambient external radiation field on the parsec-scale broad line region (BLR). Neutrons may carry ~10% of the overall energy of the protons accelerated beyond E ~ 1 PeV outside the BLR. Ultra-high energy (UHE) gamma rays produced in photomeson interactions of neutrons outside the blob can also escape the BLR. The escaping neutrons, gamma rays and neutrinos form a collimated neutral beam with a characteristic opening angle ~ 1/Gamma, where Gamma is the bulk Lorentz factor of the inner jet. The energy and the momentum of such beam is then mostly deposited in the extended jet due to neutron decay at distances ~ (E_n/10^{17} eV}) kpc, and through pair-production attenuation of gamma rays with energies E_g ~ 10^{15}-10^{18} eV which can propagate to distances beyond (10-100) kpc. In this scenario, neutral beams of UHE gamma rays and neutrons can be the reason for straight extended jets such as in Pictor A. Fluxes of neutrinos detectable with km-scale neutrino telescopes are predicted from flat spectrum radio quasars such as 3C 279.