Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

The Potential for Hadronic Particle Acceleration in Galactic Pulsar Wind Nebulae

Alison M. W. Mitchell, Samuel T. Spencer

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.

The Potential for Hadronic Particle Acceleration in Galactic Pulsar Wind Nebulae

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.
Paper Structure (13 sections, 4 equations, 3 figures, 1 table)

This paper contains 13 sections, 4 equations, 3 figures, 1 table.

Table of Contents

  1. Introduction to pulsars and the formation of Pulsar Wind Nebulae
  2. Ion escape from the pulsar and acceleration in the wind
  3. Ions originating from the pulsar
  4. Ions in the pulsar wind
  5. Hadrons originating external to the PWN
  6. Shock Mixing from the ambient environment
  7. Hadronic population of unknown origin in the PWN
  8. Observational constraints on the presence of hadrons in PWNe
  9. Neutrinos in coincidence with PWNe
  10. Gamma-ray spatial and spectral signatures
  11. Ultra High Energy Cosmic Rays
  12. Implications for the galactic cosmic ray flux
  13. Outlook

Figures (3)

  • Figure S1: The potential acceleration sites in a PWN, as seen for the Crab Nebula by Webb and Chandra. Image credits: Webb: NASA, ESA, CSA, STScI, T. Temim (Princeton University), Chandra: NASA, CXC, SAO. The inset shows solely the X-ray emission.
  • Figure S2: Example spectral energy distribution for a Crab-like PWN, with arbitrary flux normalisation. A potential hadronic contribution could emerge at the highest energies, beyond the Klein-Nishina cut-off.
  • Figure S3: Cumulative energy in electrons and protons originating from pulsars in the Milky Way, assuming that 10% of the energy in particles goes into protons.