Electromagnetic Counterparts of Compact Object Mergers Powered by the
  Radioactive Decay of R-process Nuclei
  The most promising astrophysical sources of kHz gravitational waves (GWs) are
the inspiral and merger of binary neutron star(NS)/black hole systems.
Maximizing the scientific return of a GW detection will require identifying a
coincident electro-magnetic (EM) counterpart. One of the most likely sources of
isotropic EM emission from compact object mergers is a supernova-like transient
powered by the radioactive decay of heavy elements synthesized in ejecta from
the merger. We present the first calculations of the optical transients from
compact object mergers that self-consistently determine the radioactive heating
by means of a nuclear reaction network; using this heating rate, we model the
light curve with a one dimensional Monte Carlo radiation transfer calculation.
For an ejecta mass ~1e-2 M_sun[1e-3 M_sun] the resulting light curve peaks on a
timescale ~ 1 day at a V-band luminosity nu L_nu ~ 3e41[1e41] ergs/s (M_V =
-15[-14]); this corresponds to an effective "f" parameter ~3e-6 in the
Li-Paczynski toy model. We argue that these results are relatively insensitive
to uncertainties in the relevant nuclear physics and to the precise early-time
dynamics and ejecta composition. Due to the rapid evolution and low luminosity
of NS merger transients, EM counterpart searches triggered by GW detections
will require close collaboration between the GW and astronomical communities.
NS merger transients may also be detectable following a short-duration
Gamma-Ray Burst or "blindly" with present or upcoming optical transient
surveys. Because the emission produced by NS merger ejecta is powered by the
formation of rare r-process elements, current optical transient surveys can
directly constrain the unknown origin of the heaviest elements in the Universe.
