The Observable Signatures of GRB Cocoons

As a long gamma-ray burst (GRB) jet propagates within the stellar atmosphere it creates a cocoon composed of an outer Newtonian shocked stellar material and an inner (possibly relativistic) shocked jet. The jet deposits {10}⁵¹{--}{10}⁵² erg into this cocoon. This is comparable to the energies of the GRB and of the accompanying supernova, yet the cocoon’s signature has been largely ignored. The cocoon radiates a fraction of this energy as it expands, following the breakout from the star, and later as it interacts with the surrounding matter. We explore the possible signatures of this emission and outline a framework to calculate them from the conditions of the cocoon at the time of the jet breakout. The cocoon signature depends strongly on the, currently unknown, mixing between the shocked jet and shocked stellar material. With no mixing the γ-ray emission from the cocoon is so bright that it should have been already detected. The lack of such detections indicates that some mixing must take place. For partial and full mixing the expected signals are weaker than regular GRB afterglows. However, the latter are highly beamed while the former are wider. Future optical, UV, and X-ray transient searches, like LSST, ZTF, ULTRASAT, ISS-Lobster, and others, will most likely detect such signals, providing a wealth of information on the progenitors and jets of GRBs. While we focus on long GRBs, analogous (but weaker) cocoons may arise in short GRBs. Their signatures might be the most promising electromagnetic counterparts for gravitational wave signals from compact binary mergers.