On May 20, the "China Sky Eye" captured mysterious signals 3 billion light years away for the fourth time in a row.

Text | Gan Xiao, reporter of China Science News

In the research group of Li Di, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences (hereinafter referred to as the National Astronomical Observatory), Niu Chenhui, a postdoctoral fellow born in the 1990s, is nicknamed "Superstar". He got the name simply because he looked particularly tall and burly in a group photo.

In the journey of searching for fast radio bursts (FRBs), Niu Chenhui is striving to become a true "superstar".

On June 9, Beijing time, an international team led by Li Di published an article in Nature, reporting that they had discovered the only case of a continuously active repeating fast radio burst so far with the help of the "China Sky Eye" FAST (500-meter Aperture Spherical Radio Telescope), and named it "FRB 20190520B". Subsequent coordinated observations by multiple international equipment in the sky and the earth locked its "home" in a metal-poor dwarf galaxy 3 billion light-years away from Earth. Niu Chenhui is the first author of the paper.

As the world's largest single-aperture telescope, FAST, with the support of the major priority project "Multiple Science Targets Simultaneous Sky Survey (CRAFTS)", has carried out searches for fast radio bursts and has discovered at least six new FRBs, making unique contributions to revealing the mechanism of this mysterious phenomenon in the universe and advancing research in this new field of astronomy.

Panoramic view of the "China Sky Eye" (drone photo). Photo by Xinhua News Agency reporter Ou Dongqu

At a press conference held by the Chinese Academy of Sciences on June 7, Jiang Peng, executive deputy director and chief engineer of the FAST Operation and Development Center of the National Astronomical Observatory, was very pleased to look back on the 20-year construction history of FAST. He joked: "The hardships at that time once made us confused about the future. If we had known that FAST would have so many achievements, we might have worked harder!"

No more "blink"

Fast radio bursts are the brightest radio bursts in the universe. They were first discovered in 2007. They can be divided into three parts: "fast", "radio" and "burst". Among them, "fast" means that the burst lasts very short. "It takes about 300 milliseconds for a person to blink, and FRB lasts for 1 millisecond, which is much faster than the blink of an eye." Niu Chenhui introduced.

"Radio" refers to the electromagnetic wave band in which it is located; "burst" refers to the violent explosion phenomenon, which can release the energy radiated by the sun in about an entire year in less than 1 millisecond.

From the dispersion-redshift relationship, it is clear that FRB 20190520B has the largest host galaxy electron density. (Photo courtesy of the National Astronomical Observatory of the Chinese Academy of Sciences)

So the question is, what on earth can burst out such high energy in a moment shorter than the blink of an eye? Is it a signal sent by aliens? In this regard, Li Di's view is that "the current scientific evidence does not support the 'supernatural' phenomenon that FRBs originate from aliens. Scientists believe that FRBs may originate from neutron stars or dwarf galaxies."

For a long time, the same FRB can often only be detected once, just like every "secret glance" from the depths of the universe that is fleeting and never appears again. Naturally, it is impossible to see or find the "winking person" - "Goodbye, never see you again."

It was not until 2016 that a repetitive "blink" was detected by the Arecibo telescope in the United States. It was named FRB 20121102A. With the continuous upgrading of observation equipment, nearly 500 FRBs have been announced worldwide, and less than 10 of them are active. But even repetitive FRBs have their window period. Once the window period is over, they hide in the darkness again. The continuously active repetitive fast radio bursts that scientists expect to "blink" have not appeared for a long time.

4 times in 20 seconds, a glance for a thousand years

The turning point came on the night of May 20, 2019. Standing in a corner of Pingtang, Guizhou, FAST looked up at the sky as usual. The large aperture of 500 meters gives it high sensitivity in detecting electromagnetic waves, trying its best to listen to the voice of the universe. "Beep, beep, beep..." Within 10 seconds of a beam sweeping, FAST saw three "blinks". 20 seconds later, another "blink" was detected when another beam swept to an adjacent position.

Niu Chenhui found four pulse signals within 20 seconds in the complex data. He was surprised and delighted: another repeating FRB?! Then, the rationality of a scientific researcher made him calm down. After eliminating pulsar and radio interference, he determined that the pulse was indeed from a new FRB. According to the convention of naming by the detection date, he named it FRB 20190520B.

In subsequent observations, he and his colleagues found new surprises: FRB 20190520B is not only a repeating fast radio burst, but also a continuously active fast radio burst without an outburst window. In this latest paper, they confirmed that this is the world's first continuously active repeating fast radio burst.

"20190520, still 520." Niu Chenhui, who is usually very rigorous in scientific research, feels a wonderful fate that cannot be expressed in words. "Its continued activity seems to be a permanent love story with FAST after that 'one glance, one love' encounter."

"Searching for Home" 3 Billion Light Years Away

The scientific research did not stop at romantic imagination. Researchers quickly launched a full-scale exploration of this special FRB.

In order to accurately locate its "home", Li Di's research team initiated international cooperation. Through cooperation with the Very Large Array Telescope in the United States, the Palomar 200-inch Telescope and the Keck Telescope in the United States, the Canada-France-Hawaii Telescope and the Subaru Near-Infrared Optical Telescope in Japan, the team determined that its "host galaxy" is a metal-poor dwarf galaxy 3 billion light-years away from Earth.

Li Qi emphasized, "Although telescopes around the world are invested and built by different countries, we are facing the same sky. Astronomy is the common pursuit of all mankind."

At the same time, the characteristic data of "dispersion value" shows that the electron density near the source of FRB 20190520B is much higher than that of other FRBs, indicating that it has passed through a complex electron density distribution in its host galaxy propagation path. Simply put, the environment of its host galaxy is so complex that the violent radio burst experienced great difficulties when leaving its "home", but its journey to the earth was relatively smooth afterwards.

Duncan Lorimer, founder of the FRB field, commented: "Based on the above understanding, I think fast radio bursts may have different classifications. As the fast radio burst sample continues to grow, it is expected that in the next few years we will be able to unveil the mystery of fast radio bursts."

In addition, researchers also found that FRB 20190520B and FRB 20121102A have similarities like "twin brothers", including the presence of persistent radio source counterparts nearby, being extremely active and having complex electromagnetic environments, etc. "They may represent the early stages of the evolution of fast radio bursts," Niu Chenhui told China Science Daily.

Today, the Arecibo telescope that discovered FRB 20121102A has collapsed due to disrepair, and the baton has been passed to FAST. Li Qi expects that with the support of the "Fast Radio Burst Survey" priority project, FAST's continued observations will hopefully establish a new picture of FRB evolution.

Of course, Niu Chenhui also took over the "baton". He will always remember the words of the late Mr. Nan Rendong, who built FAST himself: "The best astronomical equipment is actually built for the next generation of astronomers, for the young people who are currently studying and will be enrolled."

"I am the 'young man' that the gentleman mentioned back then. I feel lucky to be able to stand on the shoulders of giants," said Niu Chenhui.

Related paper information:

https://doi.org/10.1038/s41586-022-04755-5

China Science Daily (2022-06-10 Page 1 News original title: "FAST and 520's encounter that will last a lifetime")

Editor | Zhao Lu

Typesetting | Zhihai