Relativistic Jets in Core-collapse Supernovae

After decades of extensive research the mechanism driving core-collapse supernovae (CCSNe) is still unclear. One common mechanism is a neutrino-driven outflow, but others have been proposed. Among those, a long-standing idea is that jets play an important role in supernova (SN) explosions. Gamma-ray bursts (GRBs) that accompany hypernovae, rare and powerful CCSNe, involve relativistic jets. A GRB jet punches a hole in the stellar envelope and produces the observed gamma-rays far outside the progenitor star. While SNe and jets coexist in long GRBs (LGRBs), the relationship between the mechanisms driving the hypernova and the jet is unknown. Also unclear is the relationship between the rare hypernovae and the more common CCSNe. Here we present observational evidence that indicates that choked jets are active in CCSNe that are not associated with GRBs. A choked jet deposits all its energy in a cocoon. The cocoon eventually breaks out from the star, releasing energetic material at very high, yet sub-relativistic, velocities. This fast-moving material engulfs the star leading to a unique detectable very broad line absorption signature in early time SN spectra. We find a clear evidence for this signature in several CCSNe, all involving progenitors that have lost all, or most, of their hydrogen envelope prior to the explosion. These include CCSNe that do not harbor GRBs or any other relativistic outflows. Our findings suggest a continuum of central engine activities in different types of CCSNe and call for rethinking of the explosion mechanism of CCSNe.