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Axion hair and pulsar electrodynamics: Modeling, discharge dynamics, and particle-in-cell simulations

Samuel J. Witte1,2,3,*, Andrea Caputo4,5,6,†, Stefan Stelzl7,8,‡, Alexander Chernoglazov9, Alexander A. Philippov10, and Surjeet Rajendran11

  • *Contact author: Samuel.Witte@physics.ox.ac.uk
  • †Contact author: Andrea.Caputo@cern.ch
  • ‡Contact author: SStelzl@ifae.es
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Phys. Rev. D 114, 055040 – Published 22 September, 2026

DOI: https://doi.org/10.1103/2pdn-3n31

Abstract

In a companion paper, we demonstrated that static axion field gradients sourced by dense nuclear matter (axion hair) can dominate the near-field electrodynamics of old rotation-powered pulsars, leading to new constraints on light QCD axions and on CP-violating axion-nucleon interactions. This article provides the extended theoretical and numerical framework underlying those results. We begin by providing a detailed description of the sourcing of axion hair from dense nuclear matter, computing self-consistent field profiles for each interaction across the relevant parameter space. We then study the modification to the electrodynamics induced in the polar gap region by these axion gradients; this is done at the analytic level by studying the modification induced by axion field gradients on the effective discharge parameter (computed in the force-free limit of the split monopole magnetic field configuration, and looking at leading deviations from the force-free limit for dipolar field configurations), and numerically by developing dedicated 1D particle-in-cell simulations which capture the leading order dynamical behavior near the star. Our results demonstrate that axion hair serves to either enhance acceleration, or enhance screening, where the relevant effect changes between the northern and southern hemispheres of the star, and between the field lines which support out-flowing and return currents.

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Phys. Rev. D 114, 055040– Published 22 September, 2026
  • Received 9 February 2026
  • Accepted 19 August 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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