Beijing, June 6 (Zhongxin Net) -- Deep in the universe, about 9.24 billion light-years from Earth, a massive star of a "super sun" more than 20 times heavier than the sun burned its nuclear fusion fuel, instantly collapsed and triggered a huge explosive fireball, sending out a huge "cosmic firework" that lasted for hundreds of seconds - gamma-ray bursts (GRB, also referred to as gamma-ray bursts).

The "cosmic fireworks" fireball and the interstellar matter collision generated a large number of trillions of electron volt high-energy gamma photons through the vast universe, straight to the earth, at 2022:10:9 Beijing time on October 21, 20, arrived at the field of view of the China High-altitude Cosmic Ray Observatory (English abbreviation LHAASO, Chinese nicknamed "Lasso"), more than 50,6 gamma photons were collected by Lasso. After months of continuous analysis and research, scientists from the "Lasso" international cooperation group finally unveiled the mystery of this massive star explosion and produced the complete whole process of "cosmic fireworks" documentary for the first time in the world.

The "cosmic fireworks" achievement paper, completed by the "Lasso" international cooperation group and set a number of world records, was published online in the early morning of June 221009, Beijing time entitled "Trillion electron volts afterglow of extremely bright gamma-ray burst 6A narrow jet".

The eigenspectrum and observation spectrum of GRB221009A measured by "Lasso" over five time periods are between 5 billion and 2 trillion electron volts. Photo courtesy of Institute of High Energy of the Chinese Academy of Sciences

The first accurate measurement of the complete process of high-energy photon bursts

Cao Zhen, chief scientist of the Lasso project, spokesperson of the Lasso International Cooperation Group, and researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences (Institute of High Energy of the Chinese Academy of Sciences), said that the details of the signal collected by Lasso showed that the detected photons came from the "subsequent explosion" after the main explosion. The "main detonation" of a gamma-burst event, also known as instantaneous radiation, is a huge explosion in the initial phase, manifested as intense low-energy gamma-ray radiation; Explosives close to the speed of light collide with surrounding ambient gases to produce "follow-up explosions," also known as afterglow. "For the first time, Lasso accurately observed the complete process of the 'subsequent explosion', recording the entire phase of the trillion-electron volt gamma ray flow enhancement and attenuation."

With observations of tens of thousands of gamma-ray bursts, scientists have built what is hailed as a perfect and convincing theoretical model. "Lasso" realizes textbook complete observation of the high-energy band light change process that has not been achieved by other experiments, and also provides an experimental basis for the accurate test of theoretical models. "Given the millennial scarcity of this outbreak, this observation is expected to remain optimal for tens or even centuries to come." Cao Zhen said.

The light change process and energy spectral index evolution of GRB3A afterglow radiation in the range of 5 billion to 221009 trillion electron volts observed by Lasso. Photo courtesy of Institute of High Energy of the Chinese Academy of Sciences

For the first time, a rapid enhancement process of high-energy photon flow was measured

"During the 'follow-up explosion', the 'Lasso' detected for the first time an extremely rapid increase in photon flow." Yao Zhiguo, one of the corresponding authors of the paper and a researcher at the Institute of High Energy of the Chinese Academy of Sciences, pointed out that the flow rate increased by more than 100 times in less than two seconds, and the slow growth behavior after that was in line with the expected characteristics of the subsequent explosion.

What is the mechanism for such rapid enhancement in the early days beyond the expectations of previous theoretical models? The observations published by Lasso will trigger in-depth discussions in the scientific community on mechanisms such as gamma-ray burst energy injection, photon absorption, and particle acceleration.

Schematic diagram of the array of water Cherenkov detectors of "Lasso". Photo courtesy of Institute of High Energy of the Chinese Academy of Sciences

Discover the brightest secret in the history of 221009A gammaburst

Wang Xiangyu, one of the corresponding authors of the paper and a professor at Nanjing University, believes that the "cable" observation shows that the brightness of high-energy radiation suddenly and rapidly weakens at some point less than 10 minutes after the detonation. "This can be explained by the fact that the projectile after the explosion is a jet-like structure, causing a rapid decrease in brightness when the radiation angle extends to the edge of the jet."

Since this brightness transition occurred very early, the opening angle of the jet was also extremely small, only 0.8 degrees. This is the smallest known jet angle , meaning that what has been observed is actually the brightest core of a typical inner and outer dark jet.

Dai Zigao, a professor at the University of Science and Technology of China and one of the corresponding authors of the paper, said, "It is precisely because the observer happens 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."

Schematic diagram of the composition and layout of the High Altitude Cosmic Ray Observatory ("Lasso"). Photo courtesy of Institute of High Energy of the Chinese Academy of Sciences

It will provide a new perspective for gamma-ray burst research

Professor Peter Meszaros, academician of the American Academy of Arts and Sciences and the founder of the gamma-ray burst fireball model, commented that thanks to the huge observation area of the "Lasso" and advanced detector technology, this is the first time to detect the early afterglow curve of the trillion electron volt (TeV) energy region, the light curve shows a deceleration feature, and the truncation phenomenon of the light curve is also observed, and the relevant calculation results are consistent with other gamma-ray bursts.

Professor Gao He of the Department of Astronomy at Beijing Normal University pointed out that GRB221009A is the brightest gamma-ray burst on record, and scientists deduce that the probability of it being detected by Earth is once in <>,<> years, and this time it is extremely lucky to fall within the best observation range of the "cable". "Lasso" lived up to expectations, giving the complete light curve and energy spectrum of gamma-ray bursts in the TeV energy region for the first time, which greatly improved human understanding of the radiation mechanism of gamma-ray bursts and the structure of jets.

Science reviewers said that the Lasso international cooperation team made a very important and breakthrough observation of the earliest "afterglow" of gamma-ray bursts in the TeV energy region, which is an extraordinary experimental result that deserves rapid publication. Through this "once-in-a-millennium" event, it will provide a new perspective for the study of gamma-ray bursts.

Aerial photography of the large scientific installation "Lasso" panorama. Photo courtesy of Institute of High Energy of the Chinese Academy of Sciences

High-statistics observations in the highest energy band will reveal more mysteries

Cha Min, one of the corresponding authors of the paper and a researcher at the Institute of High Energy of the Chinese Academy of Sciences, said that in the hundreds of seconds that the "cosmic fireworks" lasted, the number of photons recorded by the "cable" exceeded the accumulation of observations of the "standard candlelight" crab nebula in the past few years. If the selection conditions are minimized, the number of photons can reach 10,60. Compared with other experimental devices in the same energy region, and even devices specially designed to track gamma-ray bursts, they measured only the number of photons below a thousand, and all only measured the "afterglow" of the massive star <> seconds after the explosion.

"As of now, the 'cosmic fireworks' triggered by this massive star explosion event and many other new discoveries, scientists are still tirelessly digging into the 'Lasso' data, trying to reveal more mysteries, please wait for the follow-up data analysis results of the 'Lasso'." Cao Zhen gave optimistic expectations for the next stage of "Lasso".

Array of detectors of the large scientific device "Lasso". Photo by Sun Zifa, reporter of China News Agency

China's space-ground joint observation and data analysis are in full swing

The "Lasso" project team of the Institute of High Energy of the Chinese Academy of Sciences said that gamma-ray bursts are the most violent celestial explosion phenomenon after the Big Bang, which refers to the sudden enhancement of gamma rays from a certain direction in the sky. Gamma bursts can be as short as thousandths of a second and as long as several hours. Short-lived gamma-ray bursts are produced by the merger of two nearby compact stars (black holes or neutron stars), while long-duration gamma-ray bursts are produced by massive stars (superstars) collapsing and exploding when they run out of fuel.

Array of detectors of the large scientific device "Lasso". Photo by Sun Zifa, reporter of China News Agency

At 2022:10:9 on October 21, 16 Beijing time, the Fermi satellite (Fermi) received an early warning that it first detected an unusually bright gamma-ray burst, named GRB59A according to international practice, and dozens of subsequent space and ground probes in the world have observed this gamma-ray burst. The gamma burst is a long burst, its brightness is more than dozens of times higher than the brightest gamma burst in the past, and the excessive photon flow makes the detectors of many international experiments saturated and "burst".

China's Lasso, High Energy Burst Explorer satellite (HEBS) and Insight-HXMT simultaneously detected the gamma-ray burst GRB221009A and achieved space-ground coordinated observation across a wide energy range of 11 orders of magnitude. Previously, a preprint of the gamma-ray burst detection by Chinese satellites had been released. The Institute of High Energy of the Chinese Academy of Sciences revealed that China's follow-up joint data analysis of space and earth has been fully carried out, and more observation and research results will be published one after another. (End)