Gamma-Ray Emission Produced by r-process Elements from Neutron Star Mergers

Abstract

Abstract 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 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.