Gamma-ray bursts (GRBs) are the most energetic explosions in the Universe, beaming out mighty jets which travel through space at 0.99 times 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. Scientists have struggled to understand how these powerful jets are formed and why they seem to appear only in GRBs. Because these jets are extremely bright at radio wavelengths, the detection of polarized radio signals will offer clues to help solve this mystery. Now, using NASA’s Neil Gehrels Swift Observatory and the Atacama Large Millimeter/Submillimeter Array (ALMA), astronomers have captured the first-ever polarized radio waves from a GRB.
An artist’s impression of the GRB 190114C jet over time, and the small patches of magnetic fields. Image credit: Laskar et al.
“We know that only a very tiny fraction (less than 1%) of massive stars form jets when they collapse,” said Dr. Raffaella Margutti, an astronomer at Northwestern University.
“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.”
Dr. Margutti and colleagues 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 from the GRB 190114C event triggered the Swift satellite, which alerted astronomers of the burst’s location in the direction of the constellation Fornax.
The astronomers then used the ALMA telescope to search for radio waves from the explosion, which occurred in a galaxy 7 billion light-years away.
“Magnetic fields are ubiquitous but notoriously difficult to constrain in our Universe,” said Dr. Wen-fai Fong, also from Northwestern University.
“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 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 polarization 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,” said University of Bath’s Professor Carole Mundell.
The scientists detected a subtle, but revealing, polarization signal of 0.8%, implying magnetic field patches about the size of our Solar System.
Next, they will combine this new information with data from X-ray and visible light telescopes.
“The lower frequency data from NRAO’s Karl G. Jansky Very Large Array helped confirm that we were seeing the light from the jet itself rather than from the interaction of the jet with its environment,” said Dr. Kate Alexander, from Northwestern University.
“This is a truly remarkable measurement, 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.”
“This measurement opens a new window into GRB science and the studies of energetic astrophysical jets,” said Dr. Tanmoy Laskar, an astrophysicist at the University of Bath.
“We would like to understand whether the low level of polarization 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.”
The team’s paper was published in the Astrophysical Journal Letters.
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Tanmoy Laskar et al. 2019. ALMA Detection of a Linearly Polarized Reverse Shock in GRB 190114C. ApJL 878, L26; doi: 10.3847/2041-8213/ab2247
