Axion-sourced fireballs from supernovae

New feebly interacting particles would emerge from a supernova core with 100-MeV-range energies and produce ? rays by subsequent decays. These would contribute to the diffuse cosmic ?-ray background or would have shown up in the Solar Maximum Mission satellite from SN 1987A. However, we show for the example of axionlike particles that, even at distances beyond the progenitor star, the decay photons may not escape and can instead form a fireball, a plasma shell with T ?1 MeV. Thus, existing arguments do not exclude axionlike particles with few 10 MeV masses and a two-photon coupling of a few 10(-10) GeV-1. However, the energy would have showed up in sub-MeV photons, which were not seen from SN 1987A in the Pioneer Venus Orbiter, closing again this new window. A careful reassessment is required for other particles that were constrained in similar ways.