  The short Gamma-Ray Burst, GRB170817A, that followed the binary neutron star
merger gravitational waves signal, GW170817, is not a usual sGRB. It is weaker
by three orders of magnitude than the weakest sGRB seen before and its spectra,
showing a hard early signal followed by a softer thermal spectrum, is unique.
We show, first, that the $\gamma$-rays must have emerged from at least mildly
relativistic outflow, implying that a relativistic jet was launched following
the merger. We then show that the observations are consistent with the
predictions of a mildly relativistic shock breakout: a minute $\gamma$-ray
energy as compared with the total energy and a rather smooth light curve with a
hard to soft evolution. We present here a novel analytic study and detailed
numerical 2D and 3D relativistic hydrodynamic and radiation simulations that
support the picture in which the observed $\gamma$-rays arose from a shock
breakout of a cocoon from the merger's ejecta (Kasliwal 2017). The cocoon can
be formed by either a choked jet which does not generate a sGRB (in any
direction) or by a successful jet which generates an undetected regular sGRB
along the system's axis pointing away from us. Remarkably, for the choked jet
model, the macronova signal produced by the ejecta (which is partially boosted
to high velocities by the cocoon's shock) and the radio that is produced by the
interaction of the shocked cocoon material with the surrounding matter, agree
with the observed UV/optical/IR emission and with current radio observations.
Finally, we discuss the possibility that the jet propagation within the ejecta
may photodissociate some of of the heavy elements and may affect the
composition of a fraction of ejecta and, in turn, the opacity and the early
macronova light.
