  Short Gamma-Ray Bursts (SGRBs) are believed to arise from compact binary
mergers (either neutron star-neutron star or black hole-neutron star). If so
their jets must penetrate outflows that are ejected during the merger. As a jet
crosses the ejecta it dissipates its energy, producing a hot cocoon which
surrounds it. We present here 3D numerical simulations of jet propagation in
mergers' outflows and we calculate the resulting emission. This emission
consists of two components: the cooling emission, the leakage of the thermal
energy of the hot cocoon, and the cocoon macronova that arises from the
radioactive decay of the cocoon's material. This emission gives a brief (~ one
hour) blue, wide angle signal. While the parameters of the outflow and jet are
uncertain, for the configurations we have considered the signal is bright (~
-14 $-$ -15 absolute magnitude) and outshines all other predicted UV-optical
signals. The signal is brighter when the jet breakout time is longer and its
peak brightness does not depend strongly on the highly uncertain opacity. A
rapid search for such a signal is a promising strategy to detect an
electromagnetic merger counterpart. A detected candidate could be then followed
by deep IR searches for the longer but weaker macronova arising from the rest
of the ejecta.
