An international team of scientists led by astrophysicists at the University of Bath in the United Kingdom measured the magnetic field in a remote gamma-ray burst (GRB), confirming for the first time a decades-long theoretical prediction that the ejected matter hits and impacts After the surrounding medium, the magnetic field in these shock waves becomes chaotic.

When a massive star (at least 40 times larger than our sun) dies in a catastrophic explosion, a black hole will form, and this explosion will power the shock wave. These extremely energetic events expel material at close to the speed of light and produce bright, short-lived flashes of gamma rays that can be detected by satellites orbiting the earth—hence their names as gamma-ray bursts.

The magnetic field may pass through the ejected material, and as the rotating black hole is formed, these magnetic fields will be twisted into a spiral, which is thought to focus and accelerate the ejected material.

Magnetic fields cannot be seen directly, but their characteristics are encoded in the light produced by charged particles (electrons) that whizz around the magnetic field lines. Telescopes on Earth have captured this kind of light that has spread in the universe for millions of years.

Carole Mundell, Bass Astrophysical Leader and Gamma Ray Expert Professor, said: "We measured a special property of light-polarization-to directly detect the physical properties of the magnetic field that drives the explosion. This is a good result. , Solved a problem. These long-term puzzles of extreme cosmic explosions-puzzles that I have studied for a long time."

The challenge is to capture the light and decode the physics of the explosion as soon as possible after the explosion. The prediction is that as the expanding shock front collides with surrounding stellar debris, any original magnetic field will eventually be destroyed.

When the large-scale original field is still intact and drives outflow, the model predicts light with a high polarization level (> 10%) shortly after the burst. Later, since the field is disrupted in the collision, the light should be mostly unpolarized.

Mundell's team first discovered highly polarized light a few minutes after the explosion, which confirmed the existence of the original field with a large-scale structure. But facts have proved that the picture of expanding forward shocks is more controversial.

In a slower time (hours to a day after the outbreak), the team that observed GRB found low polarization and concluded that these fields had been destroyed long ago, but could not tell when or how they were destroyed. In contrast, a group of Japanese astronomers announced that 10% of polarized light was found in GRB, which they interpreted as a polarized forward shock with a persistent, ordered magnetic field.

The lead author of the new study and a Bath PhD student Nuria Jordana-Mitjans said: "These rare observations are difficult to compare because they explore very different time scales and physics. There is no way to reconcile them in the Standard Model."

For more than ten years, this mystery has not been solved until the Bath team analyzed GRB 141220A.

In a new paper published in the Monthly Notices of the Royal Astronomical Society, Mundell's team reported the discovery of the extremely low polarization of forward impact light detected only 90 seconds after the explosion of GRB 141220A. The team's intelligent software on the fully autonomous robotic Liverpool telescope and the new RINGO3 polarimeter makes ultra-fast observations possible, which records the color, brightness, polarization, and fading rate of GRB. Putting this data together, the team was able to prove:

Jordana-Mitjans said: "This new research builds on our research, which shows that the most powerful GRBs can be powered by large-scale ordered magnetic fields, but only the fastest telescopes can glimpse them before they are lost. Characteristic polarization signal. Explosion."

Mundell added: "We now need to push the technological frontier to detect the earliest moments of these explosions, capture a statistically significant number of explosions for polarization research, and place our research on a wider real-time multi-messenger follow-up. Track the extreme universe in the background." 

-This press release was originally published on the University of Bath website

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