Gamma-ray emission of relativistic jets as a supercritical process

TL;DR

This paper explores how supercritical photon breeding in relativistic jets can lead to high radiation efficiency and jet deceleration, with implications for gamma-ray burst and active galactic nucleus phenomena.

Contribution

It introduces the concept of supercritical photon breeding in astrophysical jets and demonstrates through simulations how it causes efficient energy dissipation and jet deceleration.

Findings

Jets with Lorentz factor ~20 can radiate up to 50% of their energy.

Jets with Lorentz factor 40 can radiate up to 80% of their energy.

Outer jet layers decelerate to Lorentz factors 2-4, while the spine remains at 10-20.

Abstract

Supercriticality of the same kind as that in a nuclear pile can take place in high-energy astrophysical objects producing a number of impressive effects. For example, it could cause an explosive release of the energy of a cloud of ultrarelativistic protons into radiation. More certainly, supercriticality should be responsible for energy dissipation of very energetic relativistic fluids such as ultrarelativistic shocks in gamma-ray bursts and jets in active galactic nuclei (AGNs). In this case, the photon breeding process operates. It is a kind of the converter mechanism with the high-energy photons and e^+ e^- pairs converting into each other via pair production and inverse Compton scattering. Under certain conditions, which should be satisfied in powerful AGNs, the photon breeding mechanism becomes supercritical: the high-energy photons breed exponentially until their feedback on the fluid changes its velocity pattern. Then the system comes to a self-adjusting near-critical steady state. Monte-Carlo simulations with the detailed treatment of particle propagation and interactions demonstrate that a jet with the Lorentz factor Gamma ~ 20 can radiate away up to a half of its total energy and for Gamma=40 the radiation efficiency can be up to 80 per cent. Outer layers of the jet decelerate down to a moderate Lorentz factor 2-4, while the spine of the jet has the final Lorentz factor in the range 10-20 independently on the initial Gamma. Such sharp deceleration under the impact of radiation must cause a number of interesting phenomena such as formation of internal shocks and an early generation of turbulence.