Detecting light axions from supernovae in nearby galaxies

Axionlike particles (ALPs) coupled to nucleons can be efficiently produced in core-collapse supernovae (SNe) and then, if they couple to photons, convert into gamma rays in cosmic magnetic fields, generating short gamma-ray bursts. Though ALPs from a Galactic SN would induce an intense and easily detectable gamma-ray signal, such events are exceedingly rare. In contrast, a few SNe per year are expected in nearby galaxies within ∼ O ( 10 ) Mpc , where strong magnetic fields can enable more efficient ALP-photon conversions than in the Milky Way, offering a promising extragalactic target. This circumstance motivates full-sky gamma-ray monitoring, ideally combined with decihertz gravitational-wave detectors to enable time-triggered searches from nearby galaxies. We show that, under realistic conditions, a decade of coverage could reach sensitivities to the product of the ALP-proton and ALP-photon couplings g a p × g a γ ≳ 10 − 24 GeV − 1 for ALP masses m a ≲ 10 − 9 eV . This sensitivity would allow one to probe a large, currently unexplored region of the parameter space below the long-standing SN 1987A bound.