2021-06-18

Decades of gamma-ray burst puzzle solved

By yqqlm yqqlm

Science and Technology Daily, Beijing, June 17 (intern reporter Zhang Jiaxin) The driving factor of star explosions to produce gamma-ray bursts is a long-standing mystery. Recently, an international team of scientists led by the University of Bath in the United Kingdom measured gamma-rays The magnetic field of the storm, for the first time, confirmed the decades-long theoretical prediction: After the ejected material hits and impacts the surrounding medium, the magnetic field in these shock waves will become chaotic. This new study shows that the most powerful gamma-ray bursts can be powered by large-scale ordered magnetic fields. The research results were published in the British “Monthly of the Royal Astronomical Society” on the 16th.

Decades of gamma-ray burst puzzle solved

When a distant galaxy When a massive star (at least 40 times larger than the sun) dies, a shock wave will be generated after the explosion, and it will spin at a speed close to the speed of light to form a black hole. Produce extremely bright gamma-ray bursts.

When a rotating black hole is formed, the magnetic field is twisted into a spiral shape, which can focus and accelerate the movement of the ejected material. We cannot see the magnetic field directly, but their characteristics are reflected in the light generated by charged particles (electrons) that revolve around the magnetic field lines.

This time, the Bass team analyzed the gamma-ray burst 141220A and directly detected the physical properties of the magnetic field driving the explosion by measuring a special property of light-polarization.

Researchers predict that when the expanding shock front collides with surrounding stellar debris, any original magnetic field will eventually be destroyed. According to predictions, when the large-scale original field is still intact and driving the outflow of matter, there will be a high level of polarization (>10%) of light shortly after the explosion. Subsequently, since the light field is disrupted in the collision, the light should not be substantially polarized.

The team of Professor Carol Mundell, head of the Department of Astrophysics at the University of Bath and gamma-ray expert, discovered highly polarized light for the first time a few minutes after the explosion, which confirmed the large-scale structure The existence of the original magnetic field. But facts have proved that the prospects for the expansion of positive shocks are more controversial.

Another team observed at a slower time (from a few hours to a day after the explosion) that the polarization level of the gamma-ray burst is very low, which indicates that the star’s magnetic field has long been destroyed, but it is impossible to say Out when or how to destroy. In contrast, a group of Japanese astronomers claimed that 10% of polarized light was found in gamma-ray bursts. The researchers believe that this can be explained by the polarized light generated before the original magnetic field is destroyed in the shock wave.

In the new paper, the Mundell team reported that only 90 seconds after the explosion of 141220A, extremely low polarization in the pre-shock light was discovered. The research team used the fully automated robot Liverpool telescope and the new RINGO3 polarimeter to achieve ultra-high-speed observations. The RINGO3 polarimeter records the color, brightness, polarization degree and attenuation rate of the gamma-ray burst. Combining these data, the research team was able to prove: the light is emitted by a forward shock; the length of the magnetic field is much smaller than the Japanese team inferred; the explosion is likely to be driven by the collapse of the orderly magnetic field at the initial moment of the formation of the black hole.

“This result solves a long-term problem of these extreme cosmic explosions.” Mundell said, “We now need to detect the earliest moments of these explosions, capture significant explosions, and put research into more In a broad context, real-time multi-messenger tracking of the extreme universe.”