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Constraints on light QCD and CP-violating axions from the death line of rotation-powered pulsars

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, 063057 – Published 28 September, 2026

DOI: https://doi.org/10.1103/jdmb-rws8

Abstract

Dense nuclear matter can modify the effective potential of axions, displacing them from their vacuum minimum and sourcing large external field gradients (“axion hair”). In the case of neutron stars, axion hair directly modifies the electrodynamic processes operating on the open field-line region, strongly enhancing or suppressing the acceleration experienced by ambient charges. As a result, the point in the neutron star lifetime at which pair cascades cease—known as pulsar “death”—can be dramatically altered, allowing for much older pulsars to emit observable radio emission. We study the pair discharge process in the presence of axion hair using semianalytic techniques and particle-in-cell simulations and use these results alongside pulsar demographics to derive new constraints on light QCD axions with non-negligible axion-photon coupling and CP-violating axion-nucleon interactions. We also illustrate how nearly orthogonal rotators, where emission is observed from both poles (such as in the case of PSR J1906+0746), provide a complementary probe of axion hair.

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Phys. Rev. D 114, 063057– Published 28 September, 2026
  • Received 9 February 2026
  • Accepted 4 August 2026
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