First Results from the PanRadio Gamma-Ray Burst Collaboration: The 400-day Afterglow of GRB 230815A
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arXiv
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2025
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| author | Leung, James K. Anderson, Gemma E. van der Horst, Alexander J. Morley, Claire Schneider, Benjamin De Colle, Fabio Salafia, Om Sharan Ghirlanda, Giancarlo Chastain, Sarah I. Goodwin, Adelle J. Gulati, Ashna Rhodes, Lauren Ryder, Stuart D. Chrimes, Ashley A. D'Elia, Valerio de Lavergne, Mathieu de Bony De Pasquale, Massimiliano Postigo, Antonio de Ugarte Hartmann, Dieter H. Gompertz, Benjamin P. Levan, Andrew J. Murphy, Tara Rowell, Gavin P. Russell, Thomas D. Schüssler, Fabian Starling, Rhaana L. C. Tanvir, Nial R. Vergani, Susanna D. Wijers, Ralph A. M. J. |
| author_facet | Leung, James K. Anderson, Gemma E. van der Horst, Alexander J. Morley, Claire Schneider, Benjamin De Colle, Fabio Salafia, Om Sharan Ghirlanda, Giancarlo Chastain, Sarah I. Goodwin, Adelle J. Gulati, Ashna Rhodes, Lauren Ryder, Stuart D. Chrimes, Ashley A. D'Elia, Valerio de Lavergne, Mathieu de Bony De Pasquale, Massimiliano Postigo, Antonio de Ugarte Hartmann, Dieter H. Gompertz, Benjamin P. Levan, Andrew J. Murphy, Tara Rowell, Gavin P. Russell, Thomas D. Schüssler, Fabian Starling, Rhaana L. C. Tanvir, Nial R. Vergani, Susanna D. Wijers, Ralph A. M. J. |
| contents | 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 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 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 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 ${\sim}2.1^{\circ}$, while the evolution of the radio afterglow stems from a wider component with a half-opening angle of $\gtrapprox 35^{\circ}$. 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2511_07644 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | First Results from the PanRadio Gamma-Ray Burst Collaboration: The 400-day Afterglow of GRB 230815A Leung, James K. Anderson, Gemma E. van der Horst, Alexander J. Morley, Claire Schneider, Benjamin De Colle, Fabio Salafia, Om Sharan Ghirlanda, Giancarlo Chastain, Sarah I. Goodwin, Adelle J. Gulati, Ashna Rhodes, Lauren Ryder, Stuart D. Chrimes, Ashley A. D'Elia, Valerio de Lavergne, Mathieu de Bony De Pasquale, Massimiliano Postigo, Antonio de Ugarte Hartmann, Dieter H. Gompertz, Benjamin P. Levan, Andrew J. Murphy, Tara Rowell, Gavin P. Russell, Thomas D. Schüssler, Fabian Starling, Rhaana L. C. Tanvir, Nial R. Vergani, Susanna D. Wijers, Ralph A. M. J. High Energy Astrophysical Phenomena 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 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 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 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 ${\sim}2.1^{\circ}$, while the evolution of the radio afterglow stems from a wider component with a half-opening angle of $\gtrapprox 35^{\circ}$. 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. |
| title | First Results from the PanRadio Gamma-Ray Burst Collaboration: The 400-day Afterglow of GRB 230815A |
| topic | High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2511.07644 |