Gamma-ray Diagnostics of r-process Nucleosynthesis in the Remnants of Galactic Binary Neutron-Star Mergers

Abstract

We perform a full nuclear-network numerical calculation of the $r$-process nuclei in binary neutron-star mergers (NSMs), with the aim of estimating $\gamma$-ray emissions from the remnants of Galactic NSMs up to $10^6$ years old. The nucleosynthesis calculation of 4,070 nuclei is adopted to provide the elemental composition ratios of nuclei with an electron fraction $Y_{\mathrm{e}}$ between 0.10 and 0.45 . The decay processes of 3,237 unstable nuclei are simulated to extract the $\gamma$-ray spectra. As a result, the NSMs have different spectral color in $\gamma$-ray band from various other astronomical objects at less than $10^5$ years old. In addition, we propose a new line-diagnostic method for $Y_{\mathrm{e}}$ that uses the line ratios of either ${}^{137\mathrm{m}}$Ba/${}^{85}$K or ${}^{243}$Am/${}^{60\mathrm{m}}$Co, which become larger than unity for young and old $r$-process sites, respectively, with a low $Y_{\mathrm{e}}$ environment. From an estimation of the distance limit for $\gamma$-ray observations as a function of the age, the high sensitivity in the sub-MeV band, at approximately $10^{-9}$ photons s${}^{-1}$ cm${}^{-2}$ or $10^{-15}$ erg s${}^{-1}$ cm${}^{-2}$, is required to cover all the NSM remnants in our Galaxy if we assume that the population of NSMs by \citet{2019ApJ...880...23W}. A $\gamma$-ray survey with sensitivities of $10^{-8}$--$10^{-7}$ photons s${}^{-1}$ cm${}^{-2}$ or $10^{-14}$--$10^{-13}$ erg s${}^{-1}$ cm${}^{-2}$ in the 70--4000 keV band is expected to find emissions from at least one NSM remnant under the assumption of NSM rate of 30 Myr${}^{-1}$. The feasibility of $\gamma$-ray missions to observe Galactic NSMs are also studied.

17 pages in double column, 14 figures, 1 table, Accepted for publication in ApJ

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Cited by in corpus (4)

• Nucleosynthesis and observation of the heaviest elements
• Directed search for continuous gravitational waves from the possible kilonova remnant G4.8+6.2
• How Do Nuclear Isomers Influence the Gamma-Ray Bursts in Binary Neutron Star Mergers?
• Gamma-ray Signatures of r-Process Radioactivity from the Collapse of Magnetized White Dwarfs