The Potential for Hadronic Particle Acceleration in Galactic Pulsar Wind Nebulae

TL;DR

This paper reviews the potential for hadronic particle acceleration in galactic pulsar wind nebulae, discussing theoretical models, recent observational hints, and future constraints, to evaluate their role in cosmic ray production at very high energies.

Contribution

It provides a comprehensive review of the theoretical and observational prospects for hadronic acceleration in PWNe, highlighting recent developments and future research directions.

Findings

Recent LHAASO results suggest possible hadronic contributions.

Theoretical models support hadron acceleration in PWNe under certain conditions.

Observational constraints are being developed to test hadronic scenarios.

Abstract

Pulsar wind nebulae (PWNe), formed when the wind originating from a rapidly rotating neutron star flows out into its surroundings, have now been observed across the electromagnetic spectrum from the radio to the PeV gamma-ray regime. For most of these sources, leptonic processes, where electrons interacting with background photon fields produce high-energy photons through inverse Compton scattering, are believed to be the origin of associated very-high-energy gamma-ray emission. As such, these objects cannot contribute significantly to the galactic hadronic cosmic ray flux at ~TeV-PeV energies. However, in a handful of cases, the possibility for an energetically sub-dominant hadron population being accelerated and producing very to ultra-high energy gamma-rays through pion decay has not yet been comprehensively excluded. Such scenarios have received renewed attention in the light of recent results from the Large High Altitude Air Shower Observatory (LHAASO). In this review we explore the theoretical background positing hadronic acceleration in galactic PWNe, considering cases where the hadrons escape from the pulsar surface and/or are accelerated in the wind, as well as potential 'shock mixing' scenarios. We also explore current and future possible constraints on a hadronic component to PWNe from observations.