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 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.