Even the Science editor helped “cut the queue”, and “Lasso” sent another blockbuster

More than two months ago, at two a.m. on March 29, scientists around the world were partying to celebrate their joint study of the “brightest gamma-ray burst ever” (GRB 221009A) produced by the instant of the death of a massive star. At this time, Cao Zhen, chief scientist of China’s high-altitude cosmic ray observatory (“Lasso”, LHAASO), sat in a quiet corner, silently watching everything.

More than two months later, on June 9, still at two o’clock in the morning, the “Lasso” cooperative group released a “big move” in Science that had been holding back for months, solving the problem that still plagued the world after the global carnival – why this outbreak is “the brightest in history”.

287 scientists signed a joint signature, relying on the massive observation data of “Lasso”, to complete the complete brightness change curve of the star’s death moment. After seeing this result, 3 anonymous reviewers marveled that it was “groundbreaking”, “extraordinary” and “outstanding”. In order to allow more scientists to see the data and results sooner, the editor of Science decided to let the article “cut the queue” for publication.

“Lasso” detection of gamma-ray bursts (GRB 221009A) art. Photo courtesy of Chinese Academy of Sciences

Calm in the

About 1.9 billion years ago, a “super sun” more than 20 times heavier than the sun burned its fusion fuel and instantly collapsed and exploded to form a fireball. A huge “cosmic firework”, a gamma-ray burst, lasted for hundreds of seconds. After the fireball collided with interstellar matter, a large number of trillions of electron volt high-energy gamma photons passed through the vast universe and flew straight to Earth.

About 1.9 billion years later, the Earth-level began. Excited humans used extreme words such as “once in a thousand years”, “once in 10,000 years” and “brightest in history” to describe the brightness of this gamma-ray burst.

While reveling, most scientists did not lose their rationality. They know that some of the most basic questions have not yet been answered. Although scientists around the world worked together to draw the instantaneous and afterglow brightness curves of this outbreak, the afterglow curve did not have a “head” or a “tail”. No one can explain when it lit up, when it went out, and why the gamma-ray burst was so bright.

In the absence of data support, there are two guesses. One speculation is that the energy of the star’s explosion is so great that the light thrown at Earth is extremely bright; Another speculation is that the star used the last of its “life” power to spew huge amounts of energy into either side, one of which happened to hit Earth.

Which guess is right? No one dared to “clapp” until the “lasso” appeared …

At 21:20:50 on October 9, 2022, Lasso, located at an altitude of 4410 meters, received photons from this gamma-ray burst. The star’s explosion lasted 10 minutes, and Lasso received more than 60,000 high-energy photons.

Based on the unprecedented amount of data, the team completed the “head” and “tail” of the brightness change curve.

They found that the “head” part of the curve was very “steep”. Yao Zhiguo, one of the corresponding authors of the paper and a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, said that this shows that the photons reaching the earth become brighter at an extremely fast speed, and in less than 2 seconds, the photons received by the “Lasso” are brightened by more than 100 times, exceeding the expectations of previous theoretical models.

They judged that the rapid increase in brightness was probably because after the first wave of explosions, the “back wave” generated by subsequent explosions instantly pushed the “front wave” to a new height.

What’s more, the “tail” part of the brightness change curve is also “steep”. The Lasso’s team found that less than 10 minutes after the explosion began, the brightness of the photons received by Lasso suddenly and rapidly decreased.

According to this “steep” “tail”, “Lasso” has identified the reason for the “brightest” gamma-ray burst in history. “After the star explodes, the angle of the projectile radiation is only 0.8 degrees, and it is facing the Earth.” Wang Xiangyu, one of the corresponding authors of the paper and a professor at Nanjing University, said that this result proves that massive stars eject extremely fine spray columns to both sides at the moment of death.

“Lasso” completes the “head” and “tail” of the curve. Photo courtesy of Institute of High Energy Physics, Chinese Academy of Sciences

Cosmic-level lucky + earth-level strength

It was not until the initial results that the “Lasso” cooperation group suddenly realized how lucky they were.

In the universe, a 0.8-degree cone is as thin as a needle. The earth is rotating and revolving around the sun, and this “needle” flew right at the Chinese land, “poking” on the “cable” with an area of about 1.3 square kilometers on the Haizi Mountain in Daocheng, Sichuan, which was built in less than two years.

“It’s not fair, God is too good for you, we have been working for 20 years and have not seen anything.” After an international colleague initially understood the observation status of the “Lasso”, he couldn’t help but pull Cao Zhen to complain.

Cosmic luck made “Lasso” the only ground-based detector in the world to fully detect this gamma-ray burst, and the “Lasso” cooperative group accurately measured the complete process of high-energy photon bursts for the first time in the world.

“We have a complete record of the entire process of the trillion electron volt gamma ray flow enhancement and attenuation.” Cao Zhen said.

Another colleague who is compiling a textbook on gamma-ray bursts told Cao Zhen that he plans to include the new discovery of Lasso. Cao Zhen responded: “Rest assured, this observation is expected to remain the best in the next few decades or even hundreds of years.” ”

Gao He, a professor in the Department of Astronomy at Beijing Normal University, believes that people have been expecting to use gamma-ray bursts to study the evolution history of the universe, the origin of heavy elements, and the correctness of relativity, and this observation of “Lasso” has greatly improved human understanding of the radiation mechanism of gamma-ray bursts and the structure of jets.

According to Peter Meszaros, chair professor at Pennsylvania State University in the United States, and other international colleagues, the “Lasso” can make this major discovery, not only thanks to cosmic luck, but also thanks to “the huge observation area and advanced detector technology of the ‘Lasso’, as well as the strength of the observation device.”

Completed and fully operational in July 2021, Lasso consists of three detector arrays, including a 1 square kilometer ground-based cluster particle detector array, a 78,000-square-meter aquatic Cherenkov detector array, and a wide-angle Cherenkov telescope array composed of 18 telescopes.

The observations were provided by the Aqua Cherenkov Detector Array, the most sensitive ultra-high-energy gamma ray source survey observation device in the world. It uses 360,000 tons of purified water as a medium to detect the signal left by gamma photons through 6,240 photoprobes of different sizes placed underwater.

287 collaborators freely combined around the “cable”

During the international carnival in March, Cao Zhen was accompanied by another 286 scientists from around the world. They are members of the Lasso collaborative group and co-authors of the paper.

These scientists come from 67 research teams from 32 astrophysical research institutions and universities around the world. “After everyone collaborates to do the analysis and write the article, after about a month of internal review, everyone in the cooperative group signs and agrees before the paper is submitted.” Cao Zhen said.

In terms of authorship, except for the first author of the chief scientist and spokesperson of the International Cooperation Group, Cao Zhenjian, the other scientists are sorted in alphabetical order. There are 6 corresponding authors from the Institute of High Energy Physics, Chinese Academy of Sciences, Nanjing University, and University of Science and Technology of China, who undertook the main data analysis and theoretical interpretation work.

Cao Zhen said that such a cooperative research method has become the usual practice of the “Lasso” cooperation group. Since the work of the 67 research teams overlaps, new combinations of research teams are formed around the “cable” data when studying specific problems.

“When regrouping, people don’t care which unit they come from, because the work they do is determined entirely by interests and strengths.” Cao Zhen said.

In this study, 6 corresponding authors and Cao Zhen formed a 7-person core group. Dai Zigao, one of the corresponding authors and a professor at the University of Science and Technology of China, recalled that from seeing gamma-ray bursts on October 9, 2022, until the submission of the paper, the seven-person group discussed the data of “Lasso” online and conducted theoretical analysis at a frequency of several times a week.

On February 1, 2023, they submitted their paper to Science. In general, according to the practice of journal publishing, papers need to be queued for publication after they are accepted. “But the editor took special care of us, helped us skip the queue, and cut the queue to the front.” Cao Zhen said.

After the publication of the paper, the work of the “Lasso” cooperative group continued. “So far, there are many other new discoveries from this explosion, and scientists will continue to dig deeper into the ‘Lasso’ data to try to uncover more mysteries.” Cao Zhen said. (Source: Ni Sijie, China Science News)

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