Gamma-ray bursts remain among the most powerful explosions in the Universe, and their magnetic fields have never previously been measured directly. Astronomers have now obtained this opportunity for the first time thanks to the polarized radio light of the afterglow of GRB 260310A. Observations with the U.S. National Science Foundation’s Very Large Array radio interferometer also detected Faraday rotation, indicating an extremely dense, magnetized cloud surrounding the star that exploded.
A New Window into the Magnetism of Gamma-Ray Bursts
Polarized radio emission from a gamma-ray burst afterglow has been detected for the first time. This means that the radio waves oscillated in a single direction rather than randomly. Previously, researchers searched for polarization in gamma-ray bursts using ALMA at shorter wavelengths, and such observations had to be conducted quickly before the afterglow faded.
A team led by Collin Christie of the University of Arizona and Tanmoy Laskar of the University of Utah used the VLA antenna array of the U.S. National Radio Astronomy Observatory to observe at centimeter wavelengths. This made it possible to detect the polarization and trace how the signal changed at different wavelengths. It was this variation that revealed Faraday rotation—the phenomenon in which magnetized plasma rotates the orientation of polarized waves, with longer wavelengths experiencing a stronger rotation.
The Imprint of the Environment
Faraday rotation acts like a magnetic fingerprint. The nature of the rotation can be used to reconstruct the strength of the magnetic field along the path traveled by the light. In the case of GRB 260310A, the field proved to be thousands of times stronger than could be explained by the signal passing through the Milky Way or intergalactic space.
This indicates that the effect was not caused by the magnetic field of our Galaxy but originated in the immediate surroundings of the explosion itself. Astronomers concluded that the burst occurred inside an H II region—a bubble of ionized hydrogen created by the intense ultraviolet radiation and stellar wind of a massive young star. The presence of such a structure confirms the connection between gamma-ray bursts and the deaths of the most massive stars, while also providing a new way to study the environments in which these extreme events form.
The Burst That Created the Opportunity
GRB 260310A produced one of the brightest radio afterglows observed in recent decades and was also located comparatively nearby by cosmic standards. These circumstances made the burst an ideal target for observation.
No previous gamma-ray burst had allowed Faraday rotation to be measured. This requires a signal that is both bright and sufficiently long-lasting, combined with a sensitive instrument. The paper presenting the results has been submitted to The Astrophysical Journal, and its preprint is already available on arXiv, according to the University of Utah.
Kate Denham Alexander, Christie’s academic supervisor at the University of Arizona, emphasizes that continued monitoring of afterglows with the VLA and other radio telescopes will allow scientists to observe the evolution of magnetic structures in real time. This may transform our understanding of how relativistic jets form, which processes supply their energy, and how magnetic energy is released under the most extreme conditions in the Universe.
