Maximum synchrotron frequency for shock-accelerated particles

TL;DR

This paper challenges the traditional 50 MeV limit for synchrotron photon energy in shock acceleration, showing that under specific conditions in gamma-ray burst shocks, higher energy photons can be produced near the shock front.

Contribution

It demonstrates that magnetic field decay downstream of shocks allows for synchrotron photons exceeding 50 MeV, revising the maximum energy limit in relativistic shock environments.

Findings

Higher energy synchrotron photons are possible near shock fronts.

Magnetic field decay is crucial for exceeding the 50 MeV limit.

Electrons can reach higher Lorentz factors in weaker, extended magnetic fields.

Abstract

It is widely believed that the maximum energy of synchrotron photons when electrons are accelerated in shocks via the Fermi process is about 50 MeV (in plasma comoving frame). We show that under certain conditions, which are expected to be realized in relativistic shocks of gamma-ray bursts, synchrotron photons of energy much larger than 50 MeV (comoving frame) can be produced. The requirement is that magnetic field should decay downstream of the shock front on a length scale that is small compared with the distance traveled by the highest energy electrons before they lose half their energy; photons of energy much larger than 50 MeV are produced close to the shock front whereas the highest Lorentz factor that electrons can attain is controlled by the much weaker field that occupies most of the volume of the shocked plasma.