Radio Emission from the Cocoon of a GRB Jet: Implications for Relativistic Supernovae and Off-axis GRB Emission

Abstract

Abstract Relativistic supernovae constitute a subclass of Type Ic supernovae (SNe). Their nonthermal, radio emission differs notably from that of regular Type Ic supernovae as they have a fast expansion speed (with velocities ∼0.6–0.8 c) which cannot be explained by a “standard” spherical SN explosion, but advocates for a quickly evolving, mildly relativistic ejecta associated with the SN. In this paper, we compute the synchrotron radiation emitted by the cocoon of a long gamma-ray burst jet (GRB). We show that the energy and velocity of the expanding cocoon, and the radio nonthermal light curves and spectra are consistent with those observed in relativistic SNe. Thus, the radio emission from this events is not coming from the SN shock front, but from the mildly relativistic cocoon produced by the passage of a GRB jet through the progenitor star. We also show that the cocoon radio emission dominates the GRB emission at early times for GRBs seen off-axis, and the flux can be larger at late times compared with on-axis GRBs if the cocoon energy is at least comparable with respect to the GRB energy.