Resonant Axion-Photon Conversion in the Early Inspiral of Neutron Star Binaries

TL;DR

This paper explores how binary neutron star inspirals can be used to detect axion-photon conversion through electromagnetic signals modulated by gravitational waves, offering a new multimessenger approach to axion physics.

Contribution

It models the resonant axion-photon conversion in binary neutron star magnetospheres and predicts observable electromagnetic signatures linked to axion properties.

Findings

Conversion occurs on evolving peanut-shaped surfaces.

Electromagnetic power shows characteristic dependence on axion mass.

Potential detectability within current radio observation sensitivities.

Abstract

We consider the early binary neutron star inspiral phase as a scenario to probe environmental axion--photon resonant conversion. For this we approximately model the merger site electromagnetic fields as the superposition of two rotating dipolar stellar magnetic fields at the thousand--km scale when both magnetospheres are not largely distorted. We capture the time-sliced near-zone magnetospheric geometry relevant for axion--photon mixing. Plasma effects are incorporated through an effective Goldreich--Julian charge density, used to determine the effective plasma frequency and the location of resonant conversion surfaces. Our results show that axion--photon resonant conversion in binary magnetospheres mostly occurs on extended peanut-shaped surfaces whose global geometry evolves as the binary inspiral evolves. As a consequence, the total electromagnetic power emitted through axion--photon conversion exhibits a characteristic dependence on axion mass and a slow temporal modulation correlated with the gravitational wave frequency emission. This feature is potentially detectable for $m_a \in [50,170] \,\rm \mu eV$ and set $g_{a \gamma} \lesssim 10^{-11}\rm \,GeV^{-1}$ as it lies within the sensitivity limits of current or planned radio observation missions. In light of our results we discuss the opportunity of binary neutron star inspirals as time-dependent, multimessenger probes of axion physics, and motivate coordinated searches combining gravitational wave observations with radio and millimeter wavelength electromagnetic measurements.