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

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 \(Y_{\rm e}\) 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 \(Y_{\rm e}\) that uses the line ratios of either \(^{137{\rm m}}\)Ba/\(^{85}\)K or \(^{243}\)Am/\(^{60{\rm m}}\)Co, which become larger than unity for young and old \(r\)-process sites, respectively, with a low \(Y_{\rm e}\) 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.