Astronomers detect first magnetic fingerprint of a gamma-ray burst

Astronomers detect first magnetic fingerprint of a gamma-ray burst

Astronomers have achieved two landmark firsts in the study of the universe's most violent events: the first detection of polarized radio-wavelength emission from a gamma-ray burst (GRB) afterglow and the first measurement of Faraday rotation in a GRB. These findings, led by researchers at the University of Utah and the University of Arizona, provide a direct method to measure the magnetic environments driving these titanic explosions.

Using the U.S. National Science Foundation Very Large Array (NSF VLA) radio telescope, the team observed GRB 260310A. This specific burst occurred relatively close to Earth by cosmic standards, producing one of the brightest radio afterglows seen in decades. By pointing the NSF VLA at the fading event, researchers discovered that the radio waves were polarized, meaning the light waves oscillated in a preferred direction rather than vibrating randomly.

The team further identified Faraday rotation, a phenomenon where magnetic fields twist the orientation of polarized light as it travels through space. This effect serves as a magnetic fingerprint, encoding the strength and structure of the magnetic fields encountered by the light. Because magnetized plasma rotates polarized radio waves by different amounts depending on the wavelength, the speed of that rotation indicates the strength of the magnetic field.

Data from the NSF VLA revealed a magnetic field along the light's path that is thousands of times stronger than what would be expected from the space between galaxies or passage through the Milky Way. The researchers attribute this to an H II region—a dense, magnetized cloud of ionized hydrogen created by the stellar winds and ultraviolet radiation of a massive young star. This suggests that GRB 260310A exploded within such a region, supporting the theory that these bursts result from the deaths of the most massive stars.

"GRBs are the most powerful explosions in the universe, and magnetic fields are thought to play a central role in powering them, but probing those fields has been extraordinarily difficult,"

Tanmoy Laskar, assistant professor of physics and astronomy at the University of Utah, via National Radio Astronomy Observatory

Previous attempts to find polarization in GRBs utilized facilities like the Atacama Large Millimeter/submillimeter Array (ALMA), which measures shorter wavelengths and requires observations to occur early before the afterglow fades. By pushing into centimeter bands, the NSF VLA team was able to capture the Faraday rotation. Collin Christy, a University of Arizona graduate student and lead author of the study, noted that each new observation reveals more of the magnetic story these explosions tell.

Gamma-ray bursts are thought to launch narrow jets of particles traveling at nearly the speed of light, releasing as much energy in seconds as the sun will emit over its entire lifetime. While these jets produce radio afterglows that can linger for months, the magnetic fields within them have remained difficult to measure until these recent observations.

The discovery opens the door for astronomers to track the evolution of magnetic fields in real time. According to assistant professor Kate Denham Alexander of the University of Arizona, this capability could transform the scientific understanding of how relativistic jets form, how they are powered, and how magnetic energy is released in extreme environments.

The study has been submitted to The Astrophysical Journal and is available on arXiv.