Hadronic jet models today

The matter content of relativistic jets in AGNs is dominated by a mixture of protons, electrons, and positrons. During dissipative events these particles tap a significant portion of the internal and/or kinetic energy of the jet and convert it into electromagnetic radiation. While leptons - even those with only mildly relativistic energies - can radiate efficiently, protons need to be accelerated up to energies exceeding $10^{16-19}$ eV to dissipate radiatively a significant amount of energy via either trigerring pair cascades or direct synchrotron emission. Here I review various constraints imposed on the role of hadronic non-adiabatic cooling processes in shaping the high energy spectra of blazars. It will be argued that protons, despite being efficiently accelerated and presumably playing a crucial role in jet dynamics and dissipation of the jet kinetic energy to the internal energy of electrons and positrons, are more likely to remain radiatively passive in AGN jets.

💡 Research Summary

The paper provides a comprehensive review of the role of hadronic processes in shaping the high‑energy emission of blazars. It begins by summarizing the current consensus that relativistic jets in active galactic nuclei (AGN) consist of a mixture of protons, electrons, and positrons, with protons carrying the bulk of the kinetic energy. During dissipative events—such as internal shocks, magnetic reconnection, or shear‑driven turbulence—these particles tap a fraction of the jet’s internal or bulk kinetic energy and convert it into radiation.

Electrons (and positrons) radiate efficiently even at modest Lorentz factors (γ≈10⁴–10⁶) via synchrotron and inverse‑Compton processes, producing the characteristic two‑hump spectral energy distribution (SED) observed in blazars. Protons, by contrast, are much less radiatively efficient because of their large mass. To contribute significantly to the observed γ‑ray output, protons must be accelerated to ultra‑high energies (E ≳ 10¹⁶–10¹⁹ eV). At those energies two non‑adiabatic cooling channels become relevant: (1) photohadronic interactions that trigger electron‑positron pair cascades, and (2) direct proton synchrotron radiation.

The author derives the acceleration timescale τ_acc for typical shock or reconnection scenarios and compares it with the cooling timescales τ_cool for the two channels. The analysis shows that τ_cool is generally longer than τ_acc unless the target photon field is extremely dense or the magnetic field exceeds ∼10–100 G, conditions that are rarely met in most blazar zones. Consequently, the proton radiative efficiency η_p (the fraction of jet power radiated by protons) is constrained to be well below 10 % in order to reproduce the observed SED shapes.

Observational constraints are then examined in detail. First, broadband SED modeling of both flat‑spectrum radio quasars (FSRQs) and BL Lac objects indicates that the high‑energy hump can be fully accounted for by leptonic processes; adding a substantial hadronic component would over‑predict the γ‑ray flux and distort the spectral curvature. Second, variability studies reveal that blazars can vary on timescales of minutes to hours, whereas proton acceleration and cooling typically require days to weeks, making it difficult for protons alone to explain rapid flares. Third, recent IceCube detections of high‑energy neutrinos coincident with blazar activity are used to set upper limits on the photohadronic interaction rate. The inferred neutrino fluxes imply that only a tiny fraction of the jet power can be channeled into proton‑photon collisions, reinforcing the low η_p estimate.

Putting these pieces together, the paper argues that while protons are indispensable for jet dynamics—providing inertia, pressure support, and a reservoir of kinetic energy—they are likely to remain radiatively passive in most AGN jets. Their main contribution to observable signatures may be indirect, such as seeding the electron‑positron population through cascade processes or producing a modest neutrino background. The author concludes by emphasizing the need for next‑generation neutrino telescopes and ultra‑high‑resolution VLBI observations to tighten the constraints on hadronic activity and to test whether any subclass of blazars can host a genuinely proton‑dominated radiative output.