GRB 220706A: a gamma-ray burst with a month-long engine and a luminous supernova

While the progenitors of many long gamma-ray bursts (GRBs) are well established as the core collapse of very massive, envelope-stripped, rapidly-rotating stars, it has been suggested that bursts at the extremely long end of the duration distribution may be a separate population of `ultra-long' GRBs. With durations of thousands of seconds or more, these bursts are difficult to reconcile with the engine timescales expected from the compact Wolf-Rayet stars that are typically assumed to produce more `standard' long GRBs. Here, we present observations of GRB 220706A, where X-ray follow-up reveals flaring episodes that last until about 51 days after trigger. At the measured redshift of z = 0.8577, this corresponds to 27 days in the GRB rest frame, and represents the latest central engine activity ever observed in a GRB by a margin of about 21 rest-frame days. We also identify a likely supernova (SN) which, when accounting for the inferred optical extinction of 0.9 at most A V at most 3.6 mag (constrained by indirect arguments), has a peak absolute magnitude of M r at most-20.25, resembling the energetic SN,2011kl found accompanying ultra-long GRB,111209A, and consistent with super-luminous SNe. We discuss the implications GRB 220706A has for ultra-long GRB progenitor models and possible powering mechanisms for the extremely late central engine activity.