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arXiv · 2512.11023

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

Samuel J. Witte·Andrea Caputo·Stefan Stelzl·Alexander Chernoglazov·Alexander A. Philippov·Surjeet Rajendran
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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 semi-analytic 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), provides a complementary probe of axion hair.

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Samuel J. Witte, Andrea Caputo, Stefan Stelzl, Alexander Chernoglazov, Alexander A. Philippov, Surjeet Rajendran. 2025-12-11. Constraints on light QCD and CP-violating axions from the death line of rotation-powered pulsars. https://arxiv.org/abs/2512.11023

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