Gamma-ray halos around pulsars as the key to understanding cosmic ray transport in the Galaxy

TL;DR

Gamma-ray halos around pulsars reveal crucial insights into cosmic ray propagation in the Galaxy, offering new observational constraints and informing models of particle transport.

Contribution

This paper reviews recent observations of gamma-ray halos around pulsars and discusses their implications for understanding cosmic ray transport mechanisms.

Findings

Gamma-ray halos indicate diffusion of electrons and positrons from pulsars.

Observations constrain models of cosmic ray propagation.

Future facilities will enhance understanding of these halos.

Abstract

Pulsars are factories of relativistic electrons and positrons that propagate away from the pulsar, permeating later our Galaxy. The acceleration and propagation of these particles are a matter of intense debate. In the last few years, we had the opportunity to directly observing the injection of these particles into the interstellar medium through the discovery of gamma-ray halos around pulsars. This new type of gamma-ray source is produced by electrons and positrons diffusing out of the pulsar wind nebula and scattering ambient photon fields to produce gamma rays. This new field of study comes with a number of observations and constraints at different wavelengths and a variety of theoretical models explaining the characteristics of these halos. We examine the characteristics of the propagation of cosmic rays inferred from the observations of halos and their local and global implications on particle transport in the Galaxy. We also discuss the prospects for observations of these sources with facilities such as LHAASO, or CTA or SWGO in the near future.