Small Progenitors, Large Couplings: Type Ic Supernova Constraints on Radiatively Decaying Particles.

Supernova (SN) 1987A provides classic bounds on gamma-ray flashes from the radiative decay of sub-GeV particles, but the latter may decay so rapidly as to be shielded by the stellar envelope. Using axionlike particles with photon coupling as a benchmark, we show that Type Ic core-collapse supernovae largely evade this attenuation due to their compact progenitors. We identify two regimes. At small couplings, while individual flashes may be missed by Fermi-LAT, the high Type Ic rate enables a stacking analysis of nondetections. At larger couplings, decay photons trigger fireball formation; the absence of a signal from the rare broad-lined Type Ic SN 1998bw excludes this scenario. The resulting bounds significantly exceed those from SN 1987A for short decay lengths.