Finding the Milky Way's Last Neutron Star Mergers

Prof. Brian Metzger

Columbia University

The discovery of a binary neutron star merger (NSM) through both its gravitational wave and electromagnetic emission has revealed these events to be key sites of heavy element nucleosynthesis through the rapid neutron-capture process (r-process). I'll describe the prospects of finding the remnants of the last Galactic NSM by detecting the gamma-ray decay lines from their radioactive ejecta. We find that 126 Sn (Tin 126), which has several lines in the energy range 410-695 keV and resides close to the second r-process peak, is the most promising isotope, because of its half-life ~2e5 yr being comparable to the ages of recent NSMs. We predict that multiple remnants may be detectable as individual sources by next-generation γ-ray telescopes such as AMEGO. Given the unknown locations of the remnants, the most promising search strategy is a systematic survey of the Galactic plane and bulge extending to high Galactic latitudes. Individual known supernova remnants which may be misclassified NSM remnants could also be targeted, especially those located outside the Galactic plane. The discovery and characterization of such remnants would address many currently open question about the r-process and the role of neutron star mergers in its production.