Edited from a release by the National Radio Astronomy Observatory.
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, operated by the U.S. National Science Foundation National Radio Astronomy Observatory (NSF NRAO), the team achieved two firsts: the first detection of polarized radio-wavelength emission from a gamma-ray burst (GRB) afterglow and the first detection of Faraday rotation in a GRB.
Faraday rotation occurs when magnetic fields twist the orientation of polarized light as it travels through space. The effect acts like a magnetic fingerprint, encoding information about the strength and structure of the fields the light passed through. 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 and is available on arXiv.
What are gamma-ray bursts?
Gamma-ray bursts release in seconds as much energy as the sun will emit over its entire lifetime. They are thought to launch narrow jets of particles traveling at nearly the speed of light, producing a radio "afterglow" that can linger for months. Despite decades of study, the magnetic fields within these jets and their immediate surroundings have remained stubbornly difficult to measure, until now.
"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 of physics and astronomy 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 burst, GRB 260310A, reveals polarized radio waves
GRB 260310A occurred relatively close to Earth by cosmic standards, giving astronomers an extraordinary opportunity to study one of the brightest radio afterglows seen in decades. 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, rather than vibrating randomly. The same property is exploited by polarized sunglasses, which reduce glare by blocking partially polarized light reflected from water and other smooth surfaces.
Faraday rotation in a gamma-ray burst
Detecting polarized radio emission alone would have been an exciting milestone for the NSF VLA. But the team made an even more remarkable discovery: the polarization signal changed across different wavelengths, revealing Faraday rotation for the first time in a gamma-ray burst. Just as a prism bends different colors of visible light by different amounts, magnetized plasma rotates polarized radio waves by different amounts depending on their wavelength. The faster that rotation changes with wavelength, the stronger the magnetic field the light passed through.
The NSF VLA data revealed a magnetic field along the light's path thousands of times stronger than what could be explained by passage through our Milky Way 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
Astronomers call this type of cloud an H II region-a bubble of ionized hydrogen carved out by the powerful ultraviolet radiation and stellar winds of a massive young star. The findings suggest that GRB 260310A exploded within one of these regions, supporting the idea that GRBs arise from the deaths of the most massive stars and offering new clues about the environments that produce 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
With this discovery, astronomers can begin to track how magnetic fields evolve in the aftermath of gamma ray bursts.
"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 assistant professor Kate Denham Alexander, Christy's PhD advisor at U of A. "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."
About NRAO
The National Radio Astronomy Observatory (NRAO) is a facility of the U.S. National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
About ALMA
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI). ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
