Anderson et al. present the radio afterglow of short gamma-ray burst (GRB) 230217A, which was detected less than 1 day after the gamma-ray prompt emission with the ATCA and the Karl G. Jansky Very Large Array. The ATCA rapid-response system automatically triggered an observation of GRB 230217A following its detection by the Neil Gehrels Swift Observatory and began observing the event just 32 minutes postburst at 5.5 and 9 GHz. Dividing the 7 hr observation into three time-binned images allowed the team to obtain radio detections 1, 2.8, and 5.2 hr postburst, the first of which represents the earliest radio detection of any GRB to date. The decline of the light curve is consistent with reverse shock emission, making GRB 230217A the fifth short GRB with radio detections attributed to a reverse shock at early times (<1 day postburst). Following brightness temperature arguments, the team use the early radio detections to place the highest minimum Lorentz factor constraints (Gmin > 50 at ∼1 hr) on a GRB in the radio band. The results demonstrate the importance of rapid radio follow-up observations with long integrations and good sensitivity for detecting the fast-evolving radio emission from short GRBs and probing their reverse shocks.

The figure above shows the radio light curve of GRB 230217A at 5.5 (ATCA observations; yellow squares/stars), 6 (VLA observations; yellow diamonds), and 9 GHz (ATCA observations; green circles/stars). The stars show the afterglow detections from splitting the first ATCA observation into three and two time-binned images at 5.5 and 9 GHz, respectively. The average flux density over the first ATCA observation and all subsequent ATCA observations are plotted as squares and circles at 5.5 and 9 GHz, respectively. The solid data points show detections, whereas the open data points are force-fitted flux densities at the known position of the GRB when there was no significant detection. The non-detections, are also plotted with the 3σ upper limits as downward pointing triangles. A nearby check source detected in all ATCA 5.5 GHz and VLA 6 GHz images is also plotted in gray to indicate the absolute flux density scale between the ATCA and VLA measurements. Overplotted is the powerlaw fit to the ATCA 5.5 GHz and VLA 6 GHz data using the average flux density from the first ATCA observation.