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

Abstract

The observation of a radioactively powered kilonova AT~2017gfo associated with the gravitational wave-event GW170817 from 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. Basing 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 $\dot{E}\propto t^{-1.3}$. For the dynamical ejecta with a low electron fraction ($Y_{\mathrm{e}}\lesssim 0.20$), the dominant contributors of gamma-ray energy are the nuclides around the second $r$-process peak ($A\sim 130$), and the decay chain of ${}^{132}$Te ($t_{1/2}=3.21$~days) $\to$ ${}^{132}$I ($t_{1/2}=0.10$~days) $\to$ ${}^{132}$Xe produces gamma-ray lines at $228$ keV, $668$ keV, and $773$ keV. For the case of a wind ejecta with $Y_{\mathrm{e}}\gtrsim 0.30$, the dominant contributors of gamma-ray energy are the nuclides around the first $r$-process peak ($A\sim 80$), and the decay chain of ${}^{72}$Zn ($t_{1/2}=1.93$~days) $\to$ ${}^{72}$Ga ($t_{1/2}=0.59$~days) $\to$ ${}^{72}$Ge produces gamma-ray lines at $145$ keV, $834$ keV, $2202$ keV, and $2508$ keV. The peak fluxes of these lines are $10^{-9}\sim 10^{-7}$~ph~cm${}^{-2}$ s${}^{-1}$, which are marginally detectable with the next-generation MeV gamma-ray detector \emph{ETCC} if the source is at a distance of $40$~Mpc.

24 pages, 7 figures, 2 tables, accepted for publication in ApJ

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• Systematic exploration of heavy element nucleosynthesis in protomagnetar outflows
• Impact of nuclear mass models on $r$-process nucleosynthesis and heavy element abundances in $r$-process enhanced metal-poor stars
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• Gamma-ray Signatures of r-Process Radioactivity from the Collapse of Magnetized White Dwarfs
• Gamma-ray emission from decays of boosted nuclei in protomagnetar jets
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