Expectation for the MeV Gamma-Ray Emission from Pair-Instability Supernovae

Pair-instability supernovae (PISNe) are predicted thermonuclear explosions of massive stars with helium core masses exceeding $\sim 65M_\odot$ and synthesize substantial amounts of radioactive $\mathrm{^{56}Ni}$ ($M(\mathrm{^{56}Ni})\sim60M_\odot$ 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 $\mathrm{^{56}Ni}\to\mathrm{^{56}Co}\to\mathrm{^{56}Fe}$. We find that key gamma-ray lines (847 and 1238 keV) from $\mathrm{^{56}Co}$ decay in the $130M_\odot$ helium core model can be detected up to 300-400 Mpc by next-generation MeV gamma-ray telescopes. In contrast, the signals from the $100M_\odot$ 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.