  Synergetic observations of multi-band coincidence signals from merging
neutron stars have definitively marked the significance of multi-messenger
astronomy. We present a new generic signature of neutron star mergers, positron
emission and the associated 511 keV radiation, produced from ejected
neutron-rich radioactive merger material. Accounting for historical neutron
star mergers within the Milky Way allows to readily explain the origin of the
long-observed 511 keV emission line from the Galactic Center. Further, we draw
a direct link between heavy element production ($r$-process nucleosynthesis)
and 511 keV emission, which signifies the surprising recent observations of
Reticulum II ultra-faint dwarf spheroidal galaxy as a smoking gun of our
proposal. This novel tracer of neutron star mergers provides a distinct handle
for exploring binary merger history.
