2021ApJ...919...59C

The observation of a radioactively powered kilonova AT 2017gfo associated with the gravitational wave event GW170817 from a binary neutron star merger proves that these events are ideal sites for the production of heavy r-process elements. The gamma-ray photons produced by the radioactive decay of heavy elements are unique probes for the detailed nuclide compositions. Based on the detailed r-process nucleosynthesis calculations and considering radiative transport calculations for the gamma rays in different shells, we study the gamma-ray emission in a merger ejecta on a timescale of a few days. It is found that the total gamma-ray energy generation rate evolution is roughly depicted as {dot}E∝t^–1.3. For the dynamical ejecta with a low electron fraction (Y_e <= 0.20), the dominant contributors of gamma-ray energy are the nuclides around the second r-process peak (A ∼ 130) and the decay chain of ¹³²Te (t_1/2 = 3.21 days) - ¹³²I (t_1/2 = 0.10 days) - ¹³²Xe produces gamma-ray lines at 228, 668, and 773 keV. For the case of a wind ejecta with Y_e >= 0.30, the dominant contributors of gamma-ray energy are the nuclides around the first r-process peak (A ∼ 80) and the decay chain of ⁷²Zn (t_1/2 = 1.93 days) - ⁷²Ga (t_1/2 = 0.59 days) - ⁷²Ge produces gamma-ray lines at 145, 834, 2202, and 2508 keV. The peak fluxes of these lines are 10^–9 ∼ 10^–7 ph cm^–2 s^–1, which are marginally detectable with the next-generation MeV gamma-ray detector ETCC if the source is at a distance of 40 Mpc.