Gamma-ray Pulsar Halos in the Galaxy

TL;DR

Pulsar halos are extended gamma-ray sources caused by electrons and positrons escaping pulsar wind nebulae, providing insights into cosmic-ray propagation, particle escape mechanisms, and related astrophysical phenomena.

Contribution

This review summarizes recent advances in understanding pulsar halos, their characteristics, mechanisms of slow diffusion, and their significance in cosmic-ray and gamma-ray studies.

Findings

Cosmic-ray diffusion in pulsar halos is much slower than the Galactic average.

Pulsar halos help probe particle escape and cosmic-ray propagation.

Implications for cosmic positron and gamma-ray excesses.

Abstract

Pulsar halos are extended gamma-ray structures generated by electrons and positrons escaping from pulsar wind nebulae (PWNe), considered a new class of gamma-ray sources. They are ideal indicators for cosmic-ray propagation in localized regions of the Galaxy and particle escape process from PWNe. The cosmic-ray diffusion coefficient inferred from pulsar halos is more than two orders of magnitude smaller than the average value in the Galaxy, which has been arousing extensive discussion. We review the recent advances in the study of pulsar halos, including the characteristics of this class of sources, the known pulsar halos, the possible mechanisms of the extremely slow diffusion, the critical roles of pulsar halos in the studies of cosmic-ray propagation and electron injection from PWNe, and the implications on the problems of the cosmic positron excess and the diffuse TeV gamma-ray excess. Finally, we give prospects for the study in this direction based on the expectation of a larger sample of pulsar halos and deeper observations for bright sources.