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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi mathvariant="script">O</a:mi><a:mo stretchy="false">(</a:mo><a:mn>100</a:mn><a:mo stretchy="false">)</a:mo><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>MeV</a:mi></a:mrow></a:math> 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 <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mrow><f:mn>5</f:mn><f:mo>×</f:mo><f:msup><f:mrow><f:mn>10</f:mn></f:mrow><f:mrow><f:mo>−</f:mo><f:mn>11</f:mn></f:mrow></f:msup><f:mtext> </f:mtext><f:mtext> </f:mtext><f:msup><f:mrow><f:mi>GeV</f:mi></f:mrow><f:mrow><f:mo>−</f:mo><f:mn>1</f:mn></f:mrow></f:msup></f:mrow></f:math>, complementing and surpassing existing constraints. Our approach can be extended to any feebly interacting particle decaying into photons.