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Imagination of GRB 221009A gamma ray burst detected by Lasso.

Courtesy of Chinese Academy of Sciences

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The China High Altitude Cosmic Ray Observatory, Lasso, has recorded for the first time the entire process of the trillion electron volt gamma ray burst, which is the brightest gamma burst to date. The Lasso international cooperation group has achieved a series of observation and research results on this basis, and the related results were published online in the journal Science on June 9th. The results of this observation are of great significance, enhancing human understanding of the radiation mechanism and jet structure of gamma storms.

On June 9th, the journal Science published online the latest observation and research results of China's high altitude cosmic ray observation station, Lasso, on a gamma ray burst named GRB 221009A: Lasso fully recorded the entire process of a trillion electron volts gamma ray burst at the moment of death of a massive star for the first time. This achievement was completed by the Lasso International Cooperation Group.

Complete record of the entire process of high-energy radiation from the brightest gamma burst to date

About 2 billion years ago, a massive "super sun" star that was more than 20 times heavier than the sun burned its nuclear fusion fuel, collapsed instantly, and triggered a huge explosive fireball, emitting a massive "cosmic fireworks" gamma ray burst that lasted for hundreds of seconds. The collision between fireballs and interstellar matter generates a large number of trillions of electron volts of high-energy gamma photons, which pass through the vast universe and fly straight towards Earth. At 21:20:50 on October 9, 2022, these gamma photons reached the field of view of Lasso, and more than 60000 gamma photons were collected by Lasso. After several months of analysis, scientists have finally uncovered the veil of this explosion event.

Gamma ray burst is the most severe celestial explosion phenomenon after the Big Bang, which refers to the flickering phenomenon of gamma rays suddenly increasing from a certain direction in the sky. Gamma bursts can be as short as one thousandth of a second and as long as several hours. Short term gamma bursts are generated by the merger of two adjacent dense celestial bodies, while long term gamma bursts are generated by the collapse and explosion of massive stars when fuel is depleted.

The signal details collected by Lasso this time indicate that the detected photons originate from the "follow-up explosion" after the "main explosion" of the gamma ray burst event.

Cao Zhen, the chief scientist of the "Lasso" project and a researcher of the Institute of High Energy Physics of the Chinese Academy of Sciences, said: "The 'main explosion' is also known as the instantaneous radiation, which is a huge explosion in the initial stage, and is manifested in strong low-energy gamma ray radiation. The collision of explosives close to the speed of light with the surrounding gas will produce a 'trailing explosion', also known as afterglow. 'Lasso' has accurately observed the complete process of 'trailing explosion' for the first time, recording the entire stage of the enhancement and attenuation of trillion electron volts gamma ray flow, realizing a textbook style complete observation of the light change process in the high-energy band that other experiments have not achieved, and providing an experimental basis for the accurate inspection of the theoretical model."

The GRB 221009A gamma burst is the brightest gamma burst ever detected by humans on record. "Given the scarcity of this outbreak, this observation result is expected to remain optimal for the next few decades or even hundreds of years," Cao Zhen said.

Rapid enhancement of photon flow detected for the first time

"In the process of 'following explosion', 'Lasso' detected the extremely rapid increase of photon flow for the first time," said 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.

According to previous theoretical models, the growth of high-energy photon flux during the "post explosion" process should have been relatively slow and stable. But this time, Lasso observed that in less than two seconds during the early stages of the trailing explosion, the flow of high-energy photons increased by more than 100 times.

Yao Zhiguo said, "The early rapid enhancement phenomenon exceeded the expectations of previous theoretical models. The published observation results will trigger in-depth exploration of mechanisms such as gamma ray burst energy injection, photon absorption, and particle acceleration in the scientific community."

The observation results of this "Lasso" also indicate that the brightness of high-energy radiation suddenly and rapidly decreases less than 10 minutes after the detonation.

One of the corresponding authors of the paper, Professor Wang Xiangyu from Nanjing University, said, "This can be explained by the fact that the projectile after the explosion has a jet like structure, and when the radiation angle extends to the edge of the jet, it causes a rapid decrease in brightness."

Due to the early occurrence of this brightness transition, it means that what is actually observed is the brightest core of a typical inner bright and outer dark jet. "It is precisely because the observer happened to be facing the brightest core of the jet that naturally explains why this gamma ray burst is the brightest in history and why such events are extremely rare," said Dai Zigao, a professor at the University of Science and Technology of China and one of the corresponding authors of the paper.

In addition, during the 10 minute duration of this gamma ray burst event, the number of photons recorded by Lasso exceeded the accumulated observations of the Standard Candlelight Crab Nebula over the past few years.

"If the selection conditions are minimized, the number of photons can reach 100000," said Zha Min, one of the corresponding authors of the paper and a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences. Compared to other experimental devices in the same energy zone, and even devices specifically designed to track gamma bursts, they only measured the number of photons below a thousand, and only measured the afterglow 60 seconds after the explosion.

Comprehensive deployment of joint data analysis between heaven and earth

At 21:16:59.59 Beijing time on October 9, 2022, the US Gamma Ray Space Telescope satellite first detected an unusually bright gamma storm, named GRB 221009A according to international convention, which was subsequently observed by dozens of space and ground probes. Our country's high altitude cosmic ray observation station, high-energy burst explorer satellite, and Huiyan satellite simultaneously detected this gamma storm, achieving a wide energy range of 11 magnitudes for coordinated observation between heaven and earth.

Cao Zhen said, "This gamma burst is a long burst, and its brightness is dozens of times higher than the brightest gamma burst in the past. The excessive photon flow has caused saturation of multiple international experimental detectors."

The observation results of this time are mainly provided by the water Cherenkov detector array of Lasso. The array's observation energy range for gamma rays spans two orders of magnitude across domains, ranging from 100 billion electron volts to 1 trillion electron volts, and has the characteristics of a wide field of view and all-weather conditions. It has outstanding advantages in capturing and observing sudden celestial phenomena such as gamma bursts.

After the international release of the gamma storm GRB 221009A warning, Cao Zhen quickly organized multiple teams to carry out data analysis and theoretical interpretation work. The subsequent joint data analysis between heaven and earth has been fully carried out, and more observational research results will be published one after another.

The results of this observation are of great significance. Professor Gao He from the Department of Astronomy at Beijing Normal University said, "This observation greatly enhances human understanding of the radiation mechanism and jet structure of gamma storms."

"Up to now, scientists are tirelessly delving into the data of 'Lasso' in an attempt to reveal more mysteries," Cao Zhen said.