2022ApJ...933..111T

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 10⁶ yr old. The nucleosynthesis calculation of 4070 nuclei is adopted to provide the elemental composition ratios of nuclei with an electron fraction Y_e between 0.10 and 0.45. The decay processes of 3237 unstable nuclei are simulated to extract the gamma-ray spectra. As a result, the NSMs have different spectral colors in the gamma-ray band from various other astronomical objects at less than 10⁵ yr old. In addition, we propose a new line diagnostic method for Y_e that uses the line ratios of either ^137mBa/⁸⁵K or ²⁴³Am/^60mCo, which become larger than unity for young and old r-process sites, respectively, with a low-Y_e environment. From an estimation of the distance limit for gamma-ray observations as a function of age, the high sensitivity in the sub-megaelectronvolt 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 Wu et al. 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 an NSM rate of 30 Myr^–1. The feasibility of gamma-ray missions observing Galactic NSMs is also studied.