Pulsar Nulling and Vacuum Radio Emission from Axion Clouds

TL;DR

This paper explores how axion clouds around pulsars can influence their radio emissions, causing nulling or producing narrow radio lines, with implications for understanding pulsar magnetospheres and axion physics.

Contribution

It introduces a novel mechanism where axion clouds affect pulsar radio emissions, including nulling and narrow line production, expanding the understanding of pulsar magnetosphere dynamics.

Findings

Axion clouds can periodically screen electric fields, causing pulsar nulling.

Short-lived vacuum regions near neutron stars can emit narrow radio lines.

Axion back-reaction effects may lead to non-linear deviations in pulsar electrodynamics.

Abstract

Non-relativistic axions can be efficiently produced in 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 $m_a \lesssim 10^{-7}$ 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 back-reaction 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 work 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 back-reaction becomes non-linear.