Astronomers make first detection of polarized radio waves in gamma ray burst jets

Good fortune and cutting-edge scientific equipment have allowed scientists to observe a Gamma Ray Burst jet with a radio telescope and detect the polarisation of radio waves within it for the first time - moving us closer to an understanding of what causes the universe's most powerful explosions.

Gamma Ray Bursts (GRBs) are the most energetic explosions in the universe, beaming out mighty jets which travel through space at over 99.9% the speed of light, as a star much more massive than our sun collapses at the end of its life to produce a black hole.

Studying the light from Gamma Ray Burst jets as we detect it travelling across space is our best hope of understanding how these powerful jets are formed, but scientists need to be quick to get their telescopes into position and get the best data. The detection of polarised radio waves from a burst's jet, made possible by a new generation of advanced radio telescopes, offers new clues to this mystery.

The light from this particular event, known as GRB 190114C, which exploded with the force of millions of suns' worth of TNT about 4.5 billion years ago, reached NASA's Neil Gehrels Swift Observatory on Jan 14, 2019.

A rapid alert from Swift allowed the research team to direct the Atacama Large Millimeter/Sub-millimeter Array (ALMA) telescope in Chile to observe the burst just two hours after Swift discovered it. Two hours later the team was able to observe the GRB from the Karl G. Jansky Very Large Array (VLA) telescope when it became visible in New Mexico, USA.

Combining the measurements from these observatories allowed the research team to determine the structure of magnetic fields within the jet itself, which affects how the radio light is polarised.

Theories predict different arrangements of magnetic fields within the jet depending on the fields' origin, so capturing radio data enabled the researchers to test these theories with observations from telescopes for the first time.

The research team, from the University of Bath, Northwestern University, the Open University of Israel, Harvard University, California State University in Sacramento, the Max Planck Institute in Garching, and Liverpool John Moores University discovered that only 0.8% of the jet light was polarised, meaning that jet's magnetic field was only ordered over relatively small patches - each less than about 1% of the diameter of the jet. Larger patches would have produced more polarised light.

These measurements suggest that magnetic fields may play a less significant structural role in GRB jets than previously thought.

This helps us narrow down the possible explanations for what causes and powers these extraordinary explosions. The study is published in Astrophysical Journal Letters.

First author Dr Tanmoy Laskar, from the University of Bath's Astrophysics group, said: "We want to understand why some stars produce these extraordinary jets when they die, and the mechanism by which these jets are fuelled - the fastest known outflows in the universe, moving at speeds close to that of light and shining with the incredible luminosity of over a billion suns combined.

"I was in a cab on my way to O'Hare airport in Chicago, following a visit with collaborators when the burst went off. The extreme brightness of this event and the fact that it was visible in Chile right away made it a prime target for our study, and so I immediately contacted ALMA to say we were going to observe this one, in the hope of detecting the first radio polarisation signal.

"It was fortuitous that the target was well placed in the sky for observations with both ALMA in Chile and the VLA in New Mexico. Both facilities responded quickly and the weather was excellent. We then spent two months in a painstaking process to make sure our measurement was genuine and free from instrumental effects. Everything checked out, and that was exciting.

Dr Kate Alexander, who led the VLA observations, said: "The lower frequency data from the VLA helped confirm that we were seeing the light from the jet itself, rather than from the interaction of the jet with its environment."

Dr Laskar added: "This measurement opens a new window into GRB science and the studies of energetic astrophysical jets. We would like to understand whether the low level of polarisation measured in this event is characteristic of all GRBs, and if so, what this could tell us about the magnetic structures in GRB jets and the role of magnetic fields in powering jets throughout the universe."

Professor Carole Mundell, Head of Astrophysics at the University of Bath, added: "The exquisite sensitivity of ALMA and rapid response of the telescopes has, for the first time, allowed us to swiftly and accurately measure the degree of polarisation of microwaves from a GRB afterglow just two hours after the blast and probe the magnetic fields that are thought to drive these powerful, ultrafast outflows."

The research team plans to hunt for more GRBs to continue to unravel the mysteries of the biggest explosions in the universe.

Northwestern University

Astronomers uncover first polarized radio signals from gamma-ray burst
Evanston IL (SPX) Jun 21 - An international team of astronomers has captured the first-ever polarized radio waves from a distant cosmic explosion.

This explosive event (known as gamma-ray burst GRB 190114C) is part of a class of the most energetic explosions in the universe. It was produced when a star - much more massive than our sun - collapsed to form a black hole.

Gamma ray bursts produce powerful jets that travel close to the speed of light and shine with the incredible luminosity of more than a billion suns combined. Astronomers have struggled to understand how these jets are formed and why they seem to appear only in gamma ray bursts - but not other explosions, such as ordinary supernovae.

Because these jets are extremely bright at radio wavelengths, the discovery of polarized radio signals may offer new clues to help solve this mystery. Polarization is a property of light that indicates how a magnetic field is organized and structured in a jet.

"We know that only a very tiny fraction (less than 1%) of massive stars form jets when they collapse," said Northwestern University's Raffaella Margutti, who contributed to the study. "But we have not known how they manage to launch these outflows with such extreme properties, and we don't know why only a few stars do this."

"This measurement opens a new window into gamma-ray burst science and the studies of energetic astrophysical jets," said Tanmoy Laskar, a postdoctoral researcher at the University of Bath in the U.K. and lead author of the study. "We would like to understand whether the low level of polarization measured in this event is characteristic of all gamma-ray bursts and, if so, what this could tell us about the magnetic structures in gamma-ray burst jets and the role of magnetic fields in powering jets throughout the universe."

The paper was published last week in the Astrophysical Journal Letters.

The international team included three astrophysicists from Northwestern's Weinberg College of Arts and Sciences: Kate Alexander, Wen-fai Fong and Margutti. All are members of Northwestern's Center for Interdisciplinary and Exploratory Research in Astrophysics (CIERA).

Astronomers have hypothesized that cosmic magnetic fields might flow through the jets, helping them form and providing structural support. The physical extent of these magnetic fields, which have implications for the jet launching mechanism, however, had never before been measured.

To obtain these measurements, the international team employed a novel trick. They observed the jets in linearly polarized light, which is sensitive to the size of magnetic field patches. Larger magnetic field patches, for example, produce more polarized light.

On January 14, 2019, a flash of gamma rays triggered NASA's Swift satellite, which alerted astronomers of the burst's location in the direction of the constellation Fornax. The astronomers then used the Atacama Large Millimeter/Submillimeter Array (ALMA) telescope in Chile to search for radio waves from the explosion, which occurred more than 4.5 billion years ago in a galaxy 7 billion light-years away.

"Magnetic fields are ubiquitous but notoriously difficult to constrain in our universe," said Fong, an assistant professor of astrophysics. "The fact that we have been able to detect their presence - let alone in the fastest jets we know of - is an incredible and storied feat of observation."

The team detected a subtle, but revealing, polarization signal of 0.8%, implying magnetic field patches about the size of our solar system. Next, the researchers will combine this new information with data from X-ray and visible light telescopes.

"The lower frequency data from the Very Large Array (VLA) in New Mexico helped confirm that we were seeing the light from the jet itself rather than from the interaction of the jet with its environment," said Alexander, a NASA Einstein Fellow who led the VLA observations.

"This is a truly remarkable measurement," said Margutti, "both from the technical side and for its deep scientific implications on the nature of magnetic fields in the most relativistic sources known in our universe."

Research Report: "ALMA detection of linearly polarized reverse shock in GRB 190114C"

Related Links
University of Bath
Stellar Chemistry, The Universe And All Within It

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