Chastain et al. present broadband radio modeling of GRB 240205B, using observations with the ATCA and MeerKAT radio telescopes. The observations include an automatically triggered early-time ATCA observation that began approximately 13 minutes after the gamma-ray signal and continued for 12 hours, resulting in the earliest detected GRB radio afterglow to date at about 35 minutes post-burst. Following this initial detection, an extensive radio follow-up campaign was conducted for more than 5 months. Although the observations beyond one day post-burst are well described by a standard forward shock model, the observation before one day post-bust reveals an additional synchrotron component, which can be explained as the reverse shock. This component would have been missed without the automated ATCA rapid-response trigger. A combined reverse and forward shock model in a stellar wind medium best describes the radio afterglow. The team constrain the spectral breaks due to synchrotron self-absorption and the minimum electron energy, and use the light-curve peaks to constrain the microphysical parameters. The study also places GRB 240205B in the context of the growing sample of GRBs with radio detections in the first hours after the gamma-ray trigger. Using the rapid response observation, the team estimate the highest model independent constraint on a GRB minimum bulk Lorentz factor of around 100 at about 35 minutes post burst. They also discuss future prospects of detecting similar long GRBs at centimetre wavelengths, as well as potential improvements to future strategies for targeting their radio afterglows.

The plots above show the GRB 240205B afterglow light curves for four radio observing frequency bands. The open circles show data from the (u,v)-fitting of the initial 9 GHz observation. The solid lines show the best fit for a combined forward plus reverse shock model for a stellar wind density profile, and the dashed lines show a fit using the forward shock only. The 68% confidence intervals for the combined fits are indicated by the shaded regions around the lines. The dotted lines indicate the reverse shock component of the combined fit and the dash-dotted lines indicate the forward shock component of the combined fit. The vertical shaded regions indicate the duration of the observation.