Magnetic fingerprint of a cosmic explosion detected for the first time

Astronomers have made a series of landmark observations of one of the universe’s most violent events. Using the U.S. National Science Foundation Very Large Array (NSF VLA) radio telescope, which is operated by the U.S. National Science Foundation National Radio Astronomy Observatory (NSF NRAO), the team detected polarized light from a gamma-ray burst (GRB) afterglow for the first time at radio wavelengths.

It also marks the first time scientists have detected Faraday rotation in a GRB, a phenomenon in which magnetic fields cause the polarization of light to twist as it travels through space, revealing how the magnetic environment of these explosions interacts with the light they produce. The findings, led by researchers at the University of Arizona and the University of Utah, offer a new window into the extreme physics driving these titanic explosions.

The paper has been submitted to The Astrophysical Journal Letters and is available on the arXiv preprint server.

What are gamma-ray bursts?

Gamma-ray bursts are the most powerful explosions in the universe, releasing in a matter of seconds as much energy as the sun will emit over its entire lifetime. They are thought to launch narrow jets of particles accelerating to nearly the speed of light, and those jets produce a radio “afterglow” that can linger for months. Despite decades of study, the magnetic fields that are believed to accompany these jets and their local environments have remained stubbornly difficult to measure, until now.

GRB 260310A reveals polarized radio waves

The burst in question, designated GRB 260310A, was relatively nearby to Earth by cosmic standards, making its radio afterglow one of the brightest seen in decades. That brightness gave astronomers an extraordinary opportunity. By pointing the NSF VLA at the fading explosion, the team found that the radio waves were polarized, meaning the light waves were oscillating in a preferred direction, much like sunlight reflecting off the surface of water, which polarized sunglasses are designed to filter out.

Faraday rotation in a gamma-ray burst

This alone would have been an exciting first for the NSF VLA. But the team made an even more extraordinary discovery: The polarization signal changed across different wavelengths, a phenomenon known as Faraday rotation. Never before detected in a gamma-ray burst, this effect acts like a magnetic fingerprint, encoding information about the strength and structure of the fields the light passed through. Just as a prism bends different colors of visible light by different amounts, a magnetized plasma can rotate the polarization angle of radio waves. The faster that rotation changed with wavelength, the stronger the magnetic field the light passed through.

“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,” said Tanmoy Laskar, assistant professor at the University of Utah. “By detecting polarized radio emission, we can now directly measure the magnetic environment of one of the universe’s most violent events. Our new GRB observations allow us to use the universe as our laboratory to test our understanding of how physics operates in such extreme conditions.”

The NSF VLA data revealed a magnetic field along the light’s path that was thousands of times stronger than what could be explained by our own galaxy or the space between galaxies. Instead, it points to an exceptionally dense, magnetized cloud of gas surrounding the star that exploded to produce GRB 260310A.

Clues for GRB origins

That cloud is what astronomers call an HII region, a bubble of ionized hydrogen gas shaped by powerful ultraviolet radiation and stellar winds from a massive young star. The fact that GRB 260310A appears to have exploded inside such a region is consistent with GRBs arising from the deaths of the most massive stars and may help scientists understand precisely what kinds of stars and environments are capable of producing these extreme events.

“Previous searches for polarization in GRBs used facilities like the Atacama Large Millimeter/submillimeter Array (ALMA) telescope that measure shorter wavelengths and had to happen early, before the afterglow light faded,” said Collin Christy, a graduate student at the University of Arizona and lead author of the study. “Now, with the NSF VLA, we’ve pushed into the centimeter bands and made the first-ever measurement of Faraday rotation in a GRB. Each new observation reveals another layer of the magnetic story these explosions are telling us.”

Why it matters

“Future monitoring of GRB afterglows with the NSF VLA and other radio telescopes will allow scientists to watch magnetic field structures evolve in real time,” said Dr. Kate Denham Alexander, an assistant professor and Christy’s Ph.D. adviser. “This is a capability that could transform our understanding of how relativistic jets form, how they are powered, and how magnetic energy is released in the most extreme environments the universe has to offer.”

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Lisa Lock

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Andrew Zinin

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