Gamma-Ray Bursts, Collisionless Shocks and Synthetic Spectra

TL;DR

This paper uses advanced simulations to explore the microphysics of collisionless shocks in gamma-ray burst afterglows, revealing new magnetic field generation and electron acceleration mechanisms that produce synthetic spectra aligning with observations.

Contribution

It introduces a self-consistent simulation approach to study shock microphysics, uncovering a novel electron acceleration process and generating synthetic spectra consistent with GRB afterglow data.

Findings

Magnetic fields up to a few percent of equipartition are generated in shocks.

A new non-thermal electron acceleration mechanism is identified.

Synthetic spectra differ from synchrotron but match GRB afterglow observations.

Abstract

The radiation from afterglows of gamma-ray bursts (GRB) is generated in collisionless plasma shocks. The two main ingredients behind the radiation are high-energy, non-thermal electrons and a strong magnetic field. I argue that in order to make the right conclusions about gamma-ray burst and afterglow parameters from observations, it is crucial to have a firm understanding of the microphysics of collisionless shock. I present the results of self-consistent, three-dimensional particle-in-cell computational simulations of the collision of weakly magnetized plasma shells: The experiments show how a plasma instability generates a magnetic field in the shock. The field has strength up to percents of the equipartition value. The experiments also reveal a new, non-thermal electron acceleration mechanism that differs substantially from Fermi acceleration. Finally, I present the results from a new numerical tool that enables us to extract synthetic radiation spectra directly from the experiments. The preliminary results differ from synchrotron radiation but are consistent with GRB afterglow observations. I conclude that strong magnetic field generation, non-thermal particle acceleration and the emission of radiation that is consistent with GRB afterglow observations, are all unavoidable consequences of the collision between two relativistic plasma shells.