Reconnection-driven particle acceleration in relativistic shear flows

TL;DR

This study uses particle-in-cell simulations to reveal how Kelvin-Helmholtz instabilities in relativistic shear flows create reconnection layers that energize particles, explaining jet boundary emissions.

Contribution

It demonstrates, through first-principles simulations, that KH vortices induce reconnection layers which efficiently accelerate particles, providing a new injection mechanism for shear-driven acceleration.

Findings

KH vortices generate kinetic-scale reconnection layers

Reconnection layers efficiently energize jet particles

Supports spine-sheath models of jet emission

Abstract

Particle energization in shear flows is invoked to explain non-thermal emission from the boundaries of relativistic astrophysical jets. Yet, the physics of particle injection, i.e., the mechanism that allows thermal particles to participate in shear-driven acceleration, remains unknown. With particle-in-cell simulations, we study the development of Kelvin-Helmholtz (KH) instabilities seeded by the velocity shear between a relativistic magnetically-dominated electron-positron jet and a weakly magnetized electron-ion ambient plasma. We show that, in their nonlinear stages, KH vortices generate kinetic-scale reconnection layers, which efficiently energize the jet particles, thus providing a first-principles mechanism for particle injection into shear-driven acceleration. Our work lends support to spine-sheath models of jet emission - with a fast core/spine surrounded by a slower sheath - and can explain the origin of radio-emitting electrons at the boundaries of relativistic jets.