Dynamical Effects of Cosmic Rays on the Medium Surrounding Their Sources

TL;DR

This paper demonstrates through simulations that cosmic rays can create cavities around their sources by exciting instabilities, significantly suppressing diffusion and challenging previous flux-tube assumptions.

Contribution

The study reveals that nonresonant streaming instabilities lead to cavity formation around CR sources, invalidating the flux-tube model in nonlinear regimes.

Findings

CRs excite nonresonant instabilities creating cavities

Diffusivity is strongly suppressed within these cavities

Results explain suppressed CR diffusion around Galactic sources

Abstract

Cosmic rays (CRs) leave their sources mainly along the local magnetic field; in doing so they excite both resonant and nonresonant modes through streaming instabilities. The excitation of these modes leads to enhanced scattering and in turn to a large pressure gradient that causes the formation of bubbles of gas, CRs, and self-generated magnetic fields expanding into the interstellar medium. By means of hybrid Particle-In-Cell simulations, we show that, by exciting the nonresonant instability, CRs excavate a cavity around their source where the diffusivity is strongly suppressed. This finding invalidates the so far largely adopted flux-tube assumption, under which particles move along magnetic lines even in the nonlinear regime. This phenomenon is general and is expected to occur around any sufficiently powerful CR source in the Galaxy. Our results might provide a physical explanation of the numerous claims of suppressed CR diffusion around Galactic sources such as supernova remnants, pulsar wind nebulae, and stellar clusters.