γ rays and neutrinos from a powerful cosmic accelerator

Possibly the powerful radio quasar 3C273 will reveal its nature as an efficient proton accelerator up to energies of order ${10}^{11}$ GeV in the near future. It is shown in this paper that the shock-accelerated protons expected to be present in the quasar’s plasma jet induce an unsaturated synchrotron cascade with electromagnetic radiation emerging in the x-ray and γ-ray ranges. While (including the synchrotron emission from the accelerated primary electrons) the broadband nonthermal emission from 3C273 can be explained over the observed 18 orders of magnitude, a flattening of the spectrum at the highest observed energies (a few GeV) is predicted that could be falsified by the Energetic Gamma Ray Experiment Telescope on board the Compton Gamma Ray Observatory. Above ≊100 GeV the cascade spectrum dramatically steepens again due to the absorption of the γ-ray photons by the host galaxy’s strong infrared photon field from extended dust clouds, in accordance with the nondetection of 3C273 by Cherénkov telescopes. However, neutrinos from the hadronic interactions initiating the cascade are not damped and reach terrestrial experiments without any modification of their injected flux. In contrast with the neutrino flux from pp interactions, which are energetically unimportant in jets, pγ interactions generate a flat neutrino flux. Therefore it is emphasized that one must not simply normalize the expected neutrino flux by the observed γ-ray flux. Hence it is shown that the expected neutrino flux in the energy range relevant for underwater or under-ice detectors is much lower than assumed by many authors. On the other hand, with an increasing number of cosmic γ-ray sources at known positions, their neutrino detection should be feasible when it is realized that the angular resolution is the crucial design property for neutrino detectors.