Hadronic jet models today

TL;DR

This paper reviews the role of hadronic processes in AGN jets, concluding that protons are likely passive radiators despite their importance in jet dynamics and energy dissipation.

Contribution

It provides a comprehensive review of constraints on hadronic cooling processes and argues that protons remain radiatively passive in AGN jets.

Findings

Protons need extremely high energies (>10^{16-19} eV) to radiate efficiently.

Leptons radiate efficiently even at mildly relativistic energies.

Protons are more likely to be dynamically important but radiatively passive.

Abstract

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.