First Results from the PanRadio Gamma-Ray Burst Collaboration: The 400-day Afterglow of GRB 230815A

We introduce the PanRadio Gamma-ray Burst (GRB) program carried out on the Australia Telescope Compact Array: a systematic, multiyear, radio survey of all southern Swift GRB events, comprehensively following the multifrequency evolution of their afterglows from within an hour to years postburst. We present the results of the 400-day observing campaign following the afterglow of the long-duration (collapsar) GRB 230815A—the first one detected through this program. Typically, GRB 230815A would not otherwise receive traditional radio follow-up, given it has no known redshift and lacks comprehensive multiwavelength follow-up, due to its high line-of-sight extinction with AV = 2.3. We found its early X-ray jet break at ∼0.1 days postburst to be at odds with the evolution of the multifrequency radio light curves that were traced over an unusually long duration of 400 days. The radio light curves approximately evolved (with minor deviations) according to the standard self-similar expansion for a relativistic blast wave in a homogeneous environment prior to the jet break, showing no evidence for evolutions of the microphysical parameters describing the electron acceleration processes. We reconcile these features by proposing a two-component jet: the early X-ray break originates from a narrow component with a half-opening angle of ∼2 .° 1, while the evolution of the radio afterglow stems from a wider component with a half-opening angle of ⪆35°. The PanRadio GRB program will establish a sample of comprehensively followed GRBs, where a rigorous inspection of their microphysical and dynamical parameters can be performed, thereby revealing the diversity of features in their outflows and environments.