Multimessenger Constraints on Radiatively Decaying Axions from GW170817

The metastable hypermassive neutron star produced in the coalescence of two neutron stars can copiously produce axions that radiatively decay into $O\left(100\right)\mathrm{MeV}$ photons. These photons can form a fireball with characteristic temperature smaller than 1 MeV. By relying on x-ray observations of GW170817/GRB 170817A with CALET CGBM, Konus-Wind, and Insight-HXMT/HE, we present new bounds on the axion-photon coupling for axion masses in the range 1–400 MeV. We exclude couplings down to $5\times {10}^{-11}{\mathrm{GeV}}^{-1}$, complementing and surpassing existing constraints. Our approach can be extended to any feebly interacting particle decaying into photons.