First detection of polarized radio waves from gamma ray burst sheds light on collapsing stars
The first polarized light from a gamma ray burst seven billion light years away is helping astronomers to gain a better understanding of collapsing stars and other high-energy phenomena. Originating from gamma-ray burst GRB 190114C in the constellation of Fornax, the light brighter than a billion suns came from massive jets of plasma passing through powerful magnetic fields.
Gamma ray bursts are extremely rare events that have, so far, only been seen in galaxies billions of light years away. They are the brightest electromagnetic phenomena known, outshining entire galaxies during their brief lives that range from milliseconds to a few hours. They are usually caused when a supermassive star collapses into a neutron star or a black hole, producing a tremendous nuclear explosion. As they do so, they form huge jets at their axis of rotation and produce the titanic gamma ray bursts that travel along them.
According to an international team of scientists led by Tanmoy Laskar, a postdoctoral researcher at the University of Bath, very little is known about these jets, so GRB 190114C triggered much fascination. It was discovered on January 19, 2019 by NASA's Swift satellite as the gamma ray burst sprang into life. Subsequent examination by the Atacama Large Millimeter/Submillimeter Array (ALMA) telescope in Chile and the Very Large Array (VLA) radio telescope in New Mexico confirmed its location, nature, and the remarkable fact that it was transmitting polarized radio signals.
This is important because astronomers have hypothesized that the jets associated with the bursts are held together by cosmic magnetic fields, much like the plasma inside a fusion reactor. Finding out more about these magnetic fields would reveal a lot about the gamma ray bursts, and one lucky property of magnetic fields is that they polarize light along a linear axis. Since the size of portions of a magnetic field affects the amount of polarization, it is, therefore, possible to measure the strength and structure of the magnetic field.
The team found that the radio signals from GRB 190114C were polarized by 0.8 percent, meaning that the magnetic field was made up of patches about the size of our solar system. This was supported by cross-checking with similar data from the X-ray and visible light spectra, showing that the polarization was from the jet and not the surrounding environment.
"This measurement opens a new window into gamma-ray burst science and the studies of energetic astrophysical jets," says Laskar. "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 research was published in The Astrophysical Journal Letters.
The animation below explains the polarization process.
Source: Northwestern University