2025ApJ...988...10S

Pair-instability supernovae (PISNe) are predicted thermonuclear explosions of massive stars with helium core masses exceeding ∼65M_⊙, and they synthesize substantial amounts of radioactive ⁵⁶Ni (M(⁵⁶Ni) ∼ 60 M_⊙ in extreme cases). To investigate their observational signatures, we developed a multi-D Monte Carlo radiation transport code, assuming spherical symmetry in the background medium and the photon sources distribution, and performed simulations of gamma-ray and hard X-ray emissions from the decay chain ⁵⁶Ni→⁵⁶Co→⁵⁶Fe. We find that key gamma-ray lines (847 and 1238 keV) from ⁵⁶Co decay in the 130 M_⊙ helium core model can be detected up to 300-400 Mpc by next-generation MeV gamma-ray telescopes. In contrast, the signals from the 100 M_⊙ model remain below the detection limits. Our results provide the template for gamma-ray follow-up observations of PISNe. Considering theoretical predictions and observational constraints, we estimate PISN event rates within 300 Mpc to be approximately 0.01-0.1 events per year, highlighting their rarity but also emphasizing their feasibility as targets for future gamma-ray observations over the decade.