In the neutron star (NS) merger events the short gamma-ray burst (sGRB) jet heats up part of the merger ejecta producing the cocoon component.
The cocoon is expected to produce a bright early electromagnetic (EM) counterpart. However, in GW170817, sky localization took $\sim$10 hours and early EM counterparts were missed. Here, in anticipation of future GW170817-like events, we analytically model the cocoon.
Then, we calculate its EM cooling emission. We find that the cocoon outshines the r-process powered kilonova/macronova at early times (10–1000 s), peaking at UV bands. In particular, later engine activity makes the cocoon emission brighter and longer. We show that the relativistic velocity of the cocoon's photosphere is measurable with instruments such as Swift, ULTRASAT and LSST. Also, we show that energetic cocoons, including failed jets, can be detected as X-ray flashes.
Our model clarifies the physics and parameter dependence, enabling the extraction of important physical information (about the jet and the merger ejecta) with future multi-messenger observations of NS mergers.