Rocking the BOAT: the ups and downs of the long-term radio light curve for GRB 221009A

TL;DR

This paper presents an extensive radio observation dataset of GRB 221009A over 475 days, revealing that its afterglow is best explained by three synchrotron components, challenging standard models and suggesting complex shock interactions.

Contribution

It provides the most detailed radio afterglow data for a GRB to date and introduces a three-component synchrotron model to explain the observed evolution.

Findings

Three synchrotron components explain the afterglow.

Standard models cannot fully account for the observations.

No evidence of late-time jet break was observed.

Abstract

We present radio observations of the long-duration gamma-ray burst (GRB) 221009A which has become known to the community as the Brightest Of All Time or the BOAT. Our observations span the first 475 days post-burst and three orders of magnitude in observing frequency, from 0.15 to 230GHz. By combining our new observations with those available in the literature, we have the most detailed radio data set in terms of cadence and spectral coverage of any GRB to date, which we use to explore the spectral and temporal evolution of the afterglow. By testing a series of phenomenological models, we find that three separate synchrotron components best explain the afterglow. The high temporal and spectral resolution allows us to conclude that standard analytical afterglow models are unable to explain the observed evolution of GRB 221009A. We explore where the discrepancies between the observations and the models are most significant and place our findings in the context of the most well-studied GRB radio afterglows to date. Our observations are best explained by three synchrotron emitting regions which we interpret as a forward shock, a reverse shock and an additional shock potentially from a cocoon or wider outflow. Finally, we find that our observations do not show any evidence of any late-time spectral or temporal changes that could result from a jet break but note that any lateral structure could significantly affect a jet break signature.