Monday

Long gamma-ray bursts (GRBs) are the most energetic stellar-scale phenomena in the Universe. In the past two years, a new sub-group of GRBs have been discovered based on their detection of very high energy (VHE) photons at hundreds of GeV by ground-based Cherenkov detectors. As a result of this discovery, it has become apparent that we have been missing half of the energy produced in the afterglow of GRBs. Studying the afterglows of GRBs with detected VHE counterparts is vital if we are to determine what is producing the VHE emission and whether these events are rare or just rarely observed? I will present the results of a multi-frequency radio campaign to observe the most distant VHE GRB to date, GRB 201216C. Combined with optical data, our observing campaign shows evidence of a jet, with two separate components, propagating through a very low density circum-burst medium. Early time emission is dominated by an on-axis, narrow, more energetic jet. The narrow jet fades quickly, giving way to a slower, wide-angled outflow that dominates at radio and X-ray wavelengths on a timescale of tens of days. Given that a two-component jet has also been used to describe the afterglow of VHE GRB 190829A, we explore the possibility that such a jet structure is required to produce the VHE emission.