When a jet propagates inside an external medium, a forward-reverse shock structure is created at its head. Matter that interacts with the shock is then pushed around the jet to form a hot cocoon which applies pressure on the jet and collimates it.

In the afterglow phase of Long Gamma-Ray Bursts (LGRBs), radiation is emitted as a result of the interaction between the ambient medium and the burst outflow. This radiation has been extensively examined, but mostly by considering just the jet contribution, rather than the radiation that is released from the cocoon. The former can only be discovered when facing the small angle covered by the jet, whereas the latter can potentially be discovered regardless of the observer's angle in respect to the jet.

By using relativistic hydrodynamic simulations of the collapsar model, we present preliminary numerical results which describe the cocoon evolution and emission. Our results show the importance of its radiation which is prominent from hours to days after the break-out. We find that it is distributed isotropically, and we derive an estimation of its light curve and radiation spectrum which is located in the range of x-ray to visible.

We predict that the cocoon radiation can be detected observationally in future surveys, possibly even in present-day ones. Those observations can be used as probes of the total LGRBs rate, and also indicate the progenitor and jet characteristics.