First results from the PanRadio GRB Collaboration: the 400-day afterglow of GRB~230815A
Authors
James K. Leung
Gemma E. Anderson
Alexander J. van der Horst
Claire Morley
Benjamin Schneider
Fabio De Colle
Om Sharan Salafia
Giancarlo Ghirlanda
Sarah L. Chastain
Adelle J. Goodwin
Ashna Gulati
Lauren Rhodes
Stuart D. Ryder
Ashley A. Chrimes
Valerio D'Elia
Mathieu de Bony de Lavergne
Massimiliano De Pasquale
Antonio de Ugarte Postigo
Dieter H. Hartmann
Benjamin P. Gompertz
Andrew J. Levan
Tara Murphy
Gavin P. Rowell
Thomas D. Russell
Fabian Schüssler
Rhaana L. C. Starling
Nial R. Tanvir
Susanna D. Vergani
Ralph A. M. J. Wijers
Abstract
We introduce the PanRadio Gamma-ray Burst (GRB) program carried out on the Australia Telescope Compact Array: a systematic, multi-year, radio survey of all southern \textit{Swift} GRB events, comprehensively following the multi-frequency evolution of their afterglows from within an hour to years post-burst. We present the results of the 400-day observing campaign following the afterglow of 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 multi-wavelength follow-up due to its high line-of-sight extinction with $A_V = 2.3$. We found its early X-ray jet break at ${\sim}0.1$ days post-burst to be at odds with the evolution of the multi-frequency 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 jet break, showing no evidence for evolution in 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 ${\sim}2.1^{\circ}$, while the evolution of the radio afterglow stems from a wider component with a half-opening angle $\gtrapprox 35^{\circ}$. 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.
