Abstract:
Merging neutron stars offer an excellent laboratory for simultaneously studying strong-
field gravity and matter in extreme environments. We establish the physical association of
an electromagnetic counterpart (EM170817) with gravitational waves (GW170817) detected
from merging neutron stars. By synthesizing a panchromatic data set, we demonstrate
that merging neutron stars are a long-sought production site forging heavy elements
by r-process nucleosynthesis. The weak gamma rays seen in EM170817 are dissimilar to
classical short gamma-ray bursts with ultrarelativistic jets. Instead, we suggest that
breakout of a wide-angle, mildly relativistic cocoon engulfing the jet explains the low-
luminosity gamma rays, the high-luminosity ultraviolet-optical-infrared, and the delayed
radio and x-ray emission. We posit that all neutron star mergers may lead to a wide-angle
cocoon breakout, sometimes accompanied by a successful jet and sometimes by a
choked jet.
