Polarized radio waves originating from a gamma-ray burst (GRB) have been detected for the first time. The discovery was made by an international team of astronomers led by Tanmoy Lasker at the UK’s University of Bath and provides important information about relativistic jets associated with GRBs and the patchwork of magnetic fields between stars.
GRBs are extremely energetic explosions that are believed to occur when a large star collapses to form a black hole or neutron star. They can last for milliseconds to hours and emit vast amounts of electromagnetic radiation – briefly shining billions of times brighter than the Sun. Most of the explosion is thought to be focused in two narrow jets of matter that blast out at nearly the speed of light in opposite directions, but astronomers know very little about the astrophysical processes involved in forming these jets.
One popular theory is that the jets are structurally supported by magnetic fields that permeate interstellar space. These fields exist in mosaic-like arrangements of patches; each of which contains a field pointing in a different direction.
An ideal opportunity to test this theory came in January 2019, when NASA’s Swift Observatory spotted GRB 190114C, which is a GRB located 7 billion light-years away. For several hours following the burst, two radio telescopes (the Atacama Large Millimetre/Sub-Millimetre Array in Chile and the Very Large Array in the US) captured signals from the object.
As the jet blasted out into space it produced a huge radio signal, which should be polarized to a certain extent by the magnetic-field patches encountered by the jet. As the jet moves outward, its radius expands and it encompasses more patches (see video). Because the magnetic patches have random relative orientations, the overall polarization of the radio waves should decrease as the jet expands and covers more patches.
Now, Lasker’s team have combined and analysed the measurements of the two telescopes to reveal for the first time how the linear polarization of the radio waves changes over time as the forward shock of the jet quickly advanced and cooled.
No magnetic doughnuts
In the hours following the burst, the degree to which the signal was polarized decreased from around 0.8% to 0.6%, while the angle of polarization rotated through around 50⁰. These values allowed Lasker and colleagues to set new boundaries on the extent of each magnetic field patch; concluding that each one is similar in size to the solar system. In addition, the observed rotation ruled out the possibility of large-scale, doughnut-shaped axisymmetric fields being responsible for GRB jets, as some previous studies have predicted.
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In future studies, the researchers hope to further explore the magnetic structures supporting GRB jets by combining their results with X-ray and visible light observations of GRBs. They will also aim to use these insights to study the properties of the shockwaves at the fronts of GRB jets.
The research is described in The Astrophysical Journal Letters.