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Pulsar Nulling and Vacuum Radio Emission from Axion Clouds

Andrea Caputo1,*, Samuel J. Witte2,3,†, Alexander A. Philippov4, and Ted Jacobson4

  • 1Department of Theoretical Physics, CERN, Esplanade des Particules 1, P.O. Box 1211, Geneva 23, Switzerland
  • 2Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom
  • 3Departament de Física Quàntica i Astrofísica and Institut de Ciencies del Cosmos (ICCUB), Universitat de Barcelona, Diagonal 647, E-08028 Barcelona, Spain
  • 4Department of Physics, University of Maryland, College Park, Maryland 20742, USA

  • *Contact author: andrea.caputo@cern.ch
  • †Contact author: samuel.witte@physics.ox.ac.uk
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Phys. Rev. Lett. 133, 161001 – Published 17 October, 2024

DOI: https://doi.org/10.1103/PhysRevLett.133.161001

Abstract

Nonrelativistic axions can be efficiently produced in the polar caps of pulsars, resulting in the formation of a dense cloud of gravitationally bound axions. Here, we investigate the interplay between such an axion cloud and the electrodynamics in the pulsar magnetosphere, focusing specifically on the dynamics in the polar caps, where the impact of the axion cloud is expected to be most pronounced. For sufficiently light axions ma≲10−7  eV, we show that the axion cloud can occasionally screen the local electric field responsible for particle acceleration and pair production, inducing a periodic nulling of the pulsar’s intrinsic radio emission. At larger axion masses, the small-scale fluctuations in the axion field tend to suppress the backreaction of the axion on the electrodynamics; however, we point out that the incoherent oscillations of the axion in short-lived regions of vacuum near the neutron star surface can produce a narrow radio line, which provides a complementary source of radio emission to the plasma-resonant emission processes identified in previous work. While this Letter focuses on the leading order correction to pair production in the magnetosphere, we speculate that there can exist dramatic deviations in the electrodynamics of these systems when the axion backreaction becomes nonlinear.

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Phys. Rev. Lett. 133, 161001– Published 17 October, 2024
  • Received 13 December 2023
  • Revised 4 June 2024
  • Accepted 23 July 2024
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