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 γ-ray emissions from the remnants of Galactic NSMs up to 106 years old. The nucleosynthesis calculation of 4,070 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 3,237 unstable nuclei are simulated to extract the γ-ray spectra. As a result, the NSMs have different spectral color in γ-ray band from various other astronomical objects at less than 105 years old. In addition, we propose a new line-diagnostic method for Y e that uses the line ratios of either 137 mBa/85K or 243Am/60 mCo, 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 γ-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 2019ApJ...880...23W. A γ-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 γ-ray missions to observe Galactic NSMs are also studied.
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