Evidence for $\gtrsim{4}$ Gyr timescales of neutron star mergers from
  Galactic archaeology
  The nucleosynthetic site of the rapid ($r$) neutron-capture process is
currently being debated. The direct detection of the neutron star merger
GW170817, through gravitational waves and electromagnetic radiation, has
confirmed such events as important sources of the $r$-process elements.
However, chemical evolution models are not able to reproduce the observed
chemical abundances in the Milky Way when neutron star mergers are assumed to
be the only $r$-process site and realistic time distributions of such events
are taken into account. Now for the first time, we combine all the available
observational evidence of the Milky Way and its dwarf galaxy satellites to show
that the data can only be explained if there are (at least) two distinct
$r$-process sites: a quick source with timescales comparable to core-collapse
supernovae, $t_{quick}\lesssim10^8$ yr, and a delayed source with
characteristic timescales $t_{delayed}\gtrsim4$ Gyr. The delayed $r$-process
source most probably originates in neutron star mergers, as the timescale fits
well with that estimated for GW170817. Given the short timescales of the quick
source, it is likely associated with massive stars, though a specific
fast-track channel for compact object mergers cannot be excluded at this point.
Our approach demonstrates that only by looking at all the available data will
we be able to solve the puzzle that is the $r$-process.
