Shock acceleration in gamma-ray bursts
Résumé
Gamma-ray bursts offer a rather unique window on the fundamental astrophysics of particle acceleration. Sources of high-energy gamma rays, they are also likely sources of cosmic rays, possibly of the so-called ultra-high energy cosmic rays, and they may well turn out to be the strongest sources of high energy neutrinos. Through the interaction of their outflow with the circum-burst medium, these explosions generate ultra-relativistic shock waves that convert part of the bulk kinetic energy into particle energy, ultimately giving rise to the impressive photon power law spectra of the afterglow. The prompt emission may well occur through the interactions of disturbances moving with mildly relativistic relative velocity within the flow itself. However, the detailed acceleration mechanism is not yet understood. This chapter discusses the progress made in the past decade in our understanding of relativistic shock acceleration and its relation to gamma-ray burst phenomenology. It notably discusses the intimate relationship between the electromagnetic micro-instabilities upstream of the collisionless shock and the accelerated particles. It also briefly discusses the possibility of accelerating particles to ultra-high energies and the production of secondary neutrino signals. It concludes with a list of open questions and some perspectives.