Radiative Signatures of Relativistic Shocks

TL;DR

This paper investigates the radiative signatures of relativistic shocks, showing how particle acceleration limits and magnetic fluctuation strength influence gamma-ray emission mechanisms in astrophysical phenomena.

Contribution

It introduces a model linking magnetic fluctuation strength to radiation mechanisms and photon energy limits in relativistic shocks, clarifying conditions for jitter versus synchrotron radiation.

Findings

Jitter radiation is ruled out for a<1 as a gamma-ray source.

Maximum photon energy scales linearly with fluctuation strength a for a>1.

Radiation saturation occurs due to radiative losses limiting particle acceleration.

Abstract

(Abbreviated) Particle-in-cell simulations of relativistic, weakly magnetized collisionless shocks show that particles can gain energy by repeatedly crossing the shock front. This requires scattering off self-generated small length-scale magnetic fluctuations. The radiative signature of this first-order Fermi acceleration mechanism is important for models of both the prompt and afterglow emission in gamma-ray bursts and depends on the strength parameter "a" of the fluctuations. For electrons (and positrons), acceleration saturates when the radiative losses produced by the scattering cannot be compensated by the energy gained on crossing the shock. We show that this sets an upper limit on both the electron Lorentz factor and on the energy of the photons radiated during the scattering process. This rules out "jitter" radiation on self-excited fluctuations with a < 1 as a source of gamma-rays, although high-energy photons might still be produced when the jitter photons are upscattered in an analog of the synchrotron self-Compton process. In fluctuations with a > 1, radiation is generated by the standard synchrotron mechanism, and the maximum photon energy rises linearly with a, until saturating at approximately 70 MeV.