Short Gamma-Ray Burst (SGRB) 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.
Post GW170817 papers
Illuminating gravitational waves: A concordant picture of photons from a neutron star merger, Kasliwal et al. The cocoon emission – an electromagnetic counterpart to gravitational waves from neutron star mergers, Gottlieb et al. The γ -rays that accompanied GW170817 and the observational signature of a magnetic jet breaking out of NS merger ejecta, Bromberg et al. A radio counterpart to a neutron star merger, Hallian et al. A mildly relativistic wide-angle outflow in the neutron star merger GW170817, Mooley et al. From γ to Radio - The Electromagnetic Counterpart of GW 170817, Nakar et al. Superluminal motion of a relativistic jet in the neutron star merger GW170817, Mooley, Deller, Gottlieb et al. Detectability of neutron star merger afterglows, Gottlieb et al. Electromagnetic signals from the decay of free neutrons in the first hours of neutron star mergers, Gottlieb & Loeb