Neutron stars as photon double-lenses: constraining resonant conversion into ALPs

TL;DR

This paper proposes a novel method using neutron stars, specifically magnetars, to detect axion-like particles through energy-dependent photon conversion resonances, potentially surpassing current experimental constraints.

Contribution

It introduces a new astrophysical approach leveraging magnetar environments to improve sensitivity to axion-photon coupling over a broad mass range.

Findings

Sensitivity to |g_{aγ}| ~ 10^{-12} GeV^{-1} for m_a < 10^{-6} eV

Potential to improve existing constraints by over an order of magnitude

Method competes with future laboratory experiments like ALPS-II and IAXO

Abstract

Axion-photon conversion is a prime mechanism to detect axion-like particles that share a coupling to the photon. We point out that in the vicinity of neutron stars with strong magnetic fields, magnetars, the effective photon mass receives comparable but opposite contributions from free electrons and the radiation field. This leads to an energy-dependent resonance condition for conversion that can be met for arbitrary light axions and leveraged when using systems with detected radio component. Using the magnetar SGR J1745-2900 as an exemplary source, we demonstrate that sensitivity to $|g_{a\gamma}| \sim 10^{-12}\,\rm{GeV^{-1}}$ or better can be gained for $m_a \lesssim 10^{-6}\,\rm eV$, with the potential to improve current constraints on the axion-photon coupling by more than one order of magnitude over a broad mass range. With growing insights into the physical conditions of magnetospheres of magnetars, the method hosts the potential to become a serious competitor to future experiments such as ALPS-II and IAXO in the search for axion-like particles.