  The discovery by Advanced LIGO/Virgo of gravitational waves from the binary
neutron star (NS) merger GW170817, and subsequently by astronomers of transient
counterparts across the electromagnetic (EM) spectrum, has initiated the era of
multi-messenger astronomy. Given the slew of papers appearing on this event, I
thought it useful to summarize the EM discoveries in the context of theoretical
models and present my views on the major take-away lessons from this watershed
event. The weak GRB discovered in close time coincidence with GW170817, and
potential evidence for a more powerful off-axis relativistic jet (initially
beamed away from our line of sight) from the delayed rise of a non-thermal
X-ray and radio orphan afterglow, provides the most compelling evidence yet
that cosmological short GRBs originate from binary NS mergers. The luminosity
and colors of the early optical emission discovered within a day of the merger
agrees strikingly well with original predictions (Metzger et al. 2010) for
"kilonova" emission powered by the radioactive decay of r-process nuclei, the
NS merger origin of which was initially proposed by Lattimer & Schramm 1974.
The transition of the spectral energy distribution to NIR wavelengths on
timescales of days matches predictions by Barnes & Kasen 2013 and Tanaka &
Hotokezaka 2013 if a portion of the ejecta contains heavy r-process nuclei with
higher opacities. The "blue" and "red" ejecta components may possess distinct
origins (e.g. dynamical ejecta versus accretion disk outflows), with key
implications for the merger physics and the properties of neutron stars. I
outline the diversity in the EM emission expected from additional
mergers-observed with different binary masses and viewing angles-discovered
once LIGO/Virgo reach design sensitivity and NS mergers are discovered as
frequently as once per week.
