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 $\mathcal{O}(100)$~MeV photons. These photons can form a fireball with characteristic temperature smaller than $1\rm\, MeV$. 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 $1$-$400\,\rm MeV$. We exclude couplings down to $5\times 10^{-11}\,\rm GeV^{-1}$, complementing and surpassing existing constraints. Our approach can be extended to any feebly-interacting particle decaying into photons.