Many international firsts! China's Sky Eye has made another achievement!

In the vast universe, there is a burst of radio waves that flashes for only a few milliseconds. Who sends out these radio bursts? What information do they contain? Over the past decade, astronomers have been trying to find the truth.

Relying on the "China Sky Eye" (FAST), Chinese researchers observed nearly 2,000 bursts of the repeating fast radio burst FRB 20201124A, and obtained the largest fast radio burst polarization observation sample to date.

They found that this fast radio burst was in a very complex, dynamically evolving strong magnetic field environment, and for the first time detected magnetic field changes within 1 astronomical unit (the distance from the sun to the earth) around the fast radio burst.

These findings show that FRB 20201124A is very active and may come from a binary star system. The research has taken a key step towards determining the generation mechanism of fast radio bursts. The relevant results were published online in the journals Nature and Nature Communications on September 21.

Panoramic view of the "China Sky Eye" in the morning light (drone photo, taken during maintenance on July 25, 2022). Photo by Xinhua News Agency reporter Ou Dongqu

Fast radio bursts are a type of violent radio burst in the vast universe. They last for a very short time, usually only a few milliseconds.

Don't underestimate this few milliseconds of flash. Although it exists for a very short time, its energy is extremely high. In these few milliseconds, it can release the electricity generated on Earth for tens of billions of years in the form of radio waves.

Since the first fast radio burst was discovered in 2007, astronomers have discovered hundreds of fast flashes from the vast universe. However, "the origin and energy mechanism of fast radio bursts is one of the biggest mysteries in astronomy today, and scientists have been arguing about it," said Li Kejia, a researcher at Peking University and the National Astronomical Observatories of the Chinese Academy of Sciences.

Initially, the fast radio bursts detected by astronomers mainly came from outside the Milky Way. In 2020, astronomers detected fast radio bursts from magnetars in the Milky Way (neutron stars with particularly strong magnetic fields). "This shows that some fast radio bursts originate from magnetars, but the origins of those fast radio bursts from outside the Milky Way remain unknown. At the same time, although there is a large amount of observational data on fast radio bursts in the radio band, direct observations of their core areas by the astronomical community have been rare for a long time," Li Kejia said.

Using FAST, the research team conducted long-term monitoring of FRB 20201124A. "During 54 days and 82 hours of observation, we detected 1,863 burst pulse signals from this fast radio burst. Its high burst rate makes it one of the most active repeating bursts," said Li Kejia.

Image courtesy of the National Astronomical Observatories of the Chinese Academy of Sciences. Illustrations by Yu Jingchuan and Fu Hai

Through deep observations of this fast radio burst, the research team made several important discoveries, all of which are international firsts.

They "filmed" the dynamic evolution of the Faraday rotation of the fast radio burst. "The Faraday rotation can help us measure the magnetic field strength in the cosmic environment," Li Kejia explained. They discovered for the first time the strange evolutionary behavior of the Faraday rotation, that is, the Faraday rotation showed irregular short-term changes in the first 36 days, and remained almost unchanged in the following 18 days.

At the same time, the research team discovered for the first time the quenching phenomenon of fast radio bursts, that is, FRB 20201124A maintained a high burst rate in the early stage, and then suddenly extinguished within 74 hours. In this regard, Li Kejia explained: "We have been observing this fast radio burst at a frequency of one or two hours every other day or two days. Suddenly one day, this fast radio burst could not be seen. After that, we continued to observe for 15 days and still did not find its signal. It suddenly disappeared exactly 74 hours after the last observation."

"We also measured for the first time the phenomenon that the polarization degree of fast radio bursts oscillates with the wavelength of electromagnetic waves. These phenomena show that the environment within the distance between the sun and the earth around this fast radio burst is very complex and is dynamically evolving," said Li Kejia.

In addition, through international cooperation, the research team used the 10-meter Keck optical telescope in the United States to conduct in-depth observations of the host galaxy of this fast radio burst. Dong Subo, a co-corresponding author and professor at Peking University, said that they found that its host galaxy is a metal-rich barred spiral galaxy about the size of the Milky Way.

More importantly, the researchers also found that the region where this fast radio burst occurred has a low star density and is neither too far nor too close to the center of the galaxy.

"We believe that this environment is inconsistent with the model of superluminous supernovae caused by extreme explosions of massive stars, nor is it consistent with the model of young magnetars formed after gamma-ray bursts." Li Kejia said that this provides new insights into the study of the origin of fast radio bursts.

Where did this fast radio burst come from? Wang Fayin, the first author and corresponding author of the Nature Communications article and a professor at Nanjing University, said that the team he led was the first to discover that the magnetic field around FRB 20201124A had changed direction. "This phenomenon is similar to the PSR B1259-63/LS 2883 binary star in the Milky Way, so we think that FRB 20201124A may be produced in a binary star system consisting of a magnetar and a Be star (a star that is hotter, larger, and rotates faster than the sun)."

In the model constructed by Wang Fayin's team, the evolution of the Faraday rotation is caused by the fast radio burst radiation passing through the Be star disk. When the magnetar moves between the Be star and the observer, the magnetic field around FRB 20201124A changes direction.

"This study provides important clues to explain the origin of fast radio bursts, that is, some fast radio bursts may originate from binary star systems," said Wang Fayin.