Probing axion-like particles with multimessenger observations of neutron star mergers

Abstract

Axion-like particles (ALPs) can be copiously produced in binary neutron star (BNS) mergers through nucleon-nucleon bremsstrahlung if the ALP-nucleon couplings $g_{aN}$ are sizable. The ALP-photon coupling $g_{a{\hat{I}}^3}$ may trigger conversions of ultralight ALPs into photons in the magnetic fields of the merger remnant and of the Milky Way. This effect would lead to a potentially observable short gamma-ray signal, in coincidence with the gravitational-wave signal produced during the merging process. This event could be detected through multi-messenger observation of BNS mergers employing the synergy between gravitational-wave detectors and gamma-ray telescopes. Here, we study the sensitivity of current and proposed MeV gamma-ray experiments to detect such a signal. As an explicit example, we consider ALP couplings related as in the Kim-Shifman-Vainshtein-Zakharov (KSVZ) axion model, and show that in this case the proposed instruments can reach a sensitivity down to $g_{a{\hat{I}}^3}\gtrsim \text{few}\times 10^{-13}{\text{GeV}}^{-1}$ for $m_a\lesssim 10^{-9}$ eV, comparable with the SN 1987A limit.

18 pages, 6 figures. Matches the version published on PRD. Changes: we add a discussion on how of the location of the BNS merger event may affect our results