A colossal star with a mass exceeding 20 times that of the sun exhausted its nuclear fusion fuel about 2 billion years ago, and unleashed a massive explosive fireball at the moment of collapse, resulting in a spectacular “hypernova” that lasted for hundreds of seconds. This cosmic fireworks show produced a gamma-ray burst (hereinafter referred to as GRB), the most powerful type of electromagnetic radiation in the universe. A large number of tera-electron-volt high-energy gamma photons generated by the collision between the fireball and the interstellar matter traversed the vast cosmos, soared towards the earth, and finally reached the field of view of “LHAASO”, the China High Altitude Cosmic Ray Observatory, at 21:20:50 pm on October 9, 2022. In this rare astronomical phenomenon, more than 60,000 gamma photons were detected by “LHAASO”. After months of analysis, scientists finally revealed the mystery of the explosion.
The relevant observational research results were published online in the top international scientific journal Science on June 9, 2023. The Chinese scientists who participated in the study said in an interview with reporters recently that the results of this observational study will be a “textbook-level” achievement and are expected to produce many “revolutionary” discoveries.
1 “Truly very fortunate”
Gamma-ray bursts are recognized by the astrophysics community as the most violent eruption phenomenon in the universe. The energy radiated in a few seconds is equivalent to the sum of the energy radiated by the sun in 10 billion years. After half a century of research, people have realized that GRBs are produced in extremely extreme physical environments, such as extremely high magnetic fields, strong gravitational forces, and extremely fast velocities, etc. GRBs have thus become the extreme physics laboratory favored by the fields of astrophysics and even physics. People expect to use GRBs to study major issues such as the evolutionary history of the universe, the origin of heavy elements, and the validity of the theory of relativity, but prior to this, it is necessary to have a deep understanding of the physical origin of GRBs themselves.
Cao Zhen, the chief scientist of “LHAASO” and a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, recently used “very fortunate” to describe the whole process of the “LHAASO” complete recording of the TEV gamma-ray burst at the moment of death of a massive star in an interview. In Cao Zhen’s view, this is definitely an event with a very low probability. An extremely bright gamma-ray burst that lasted for hundreds of seconds came to the earth after 2 billion years of “flight”. “LHAASO” is the only ground-based detector in the world facing the source of the gamma-ray burst that was able to perfectly capture this astronomical spectacle.
“Professor Gao He from the Department of Astronomy of Beijing Normal University said that tens of thousands of GRBs have been captured by humans so far, and almost every breakthrough in the understanding of GRBs has benefited from the observation of some exceptional cases. On the night of October 9, 2022, this gamma-ray burst, which was later named GRB221009A, was detected. This is also the brightest gamma-ray burst ever recorded. Scientists estimate that the probability of such a bright gamma-ray burst sweeping across the earth is once in ten thousand years. Humans were extremely fortunate this time, because GRB221009A happened to fall within the optimal observation range of “LHAASO”. For the first time, “LHAASO” has provided the complete light curve and energy spectrum of GRBs at TeV energies. This observation has greatly enhanced human understanding of the radiation mechanism of GRBs and the structure of jets.
One of the corresponding authors of the paper, Dai Zigao, a professor at the University of Science and Technology of China, told reporters that the observation of “LHAASO” highlights its special significance. He compared the gamma-ray burst phenomenon to an “embryo”, and the TeV observation adopted by “LHAASO” fully discovered how this “embryo” grew from a “fertilized egg” to a “fetus” process. “TeV observation is equivalent to an ‘ultrasound’ that accurately observes this process. This is the first time we have such a comprehensive understanding of such a process.” Dai Zigao said.
2 This observation yielded a number of “first discoveries”
At present, “LHAASO” has recorded more than 60,000 gamma photons from this gamma-ray burst. Cao Zhen said that this is an astonishing data, which shows that the brightness of the gamma-ray burst observed this time is very remarkable.
It is understood that during this observation, the “LHAASO” accurately measured the complete process of high-energy photon bursts for the first time. Cao Zhen said that with the observation of tens of thousands of gamma-ray bursts, scientists have established a near-perfect theoretical model, and the “LHAASO” has achieved a textbook-like complete observation of the light variation process in the high-energy band that other experiments have not achieved, providing an experimental basis for an exact test of the theoretical model. In view of the rarity of this outbreak, it is expected that this observation result will remain the best in the next few decades or even hundreds of years.
In addition, the “LHAASO” also measured for the first time the process of rapid increase in the flux of high-energy photons. 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 he believes that the published observation results will trigger in-depth discussions in the scientific community on the mechanisms of gamma-ray burst energy injection, photon absorption, and particle acceleration.
In this once-in-a-thousand-year observation, “LHAASO” also discovered the secret that this gamma-ray burst became the brightest in history. Dai Zigao believes, “Because ‘LHAASO’ happens to be facing the brightest core of the jet, it naturally explains why this gamma-ray burst is the brightest in history, and further explains why such an event is extremely rare.”
3 “Luck favors the prepared”
Although many scientists use “extreme fortune” to describe the rarity of this observation, Cao Zhen also pointed out that luck favors the prepared. When this gamma-ray burst that occurred 2 billion years ago came to the earth, the excellent “LHAASO” did not miss this “opportunity”.
Located on Haizi Mountain with an average altitude of 4,410 meters in Daocheng County, Sichuan Province, the “LHAASO” covers an area of about 1.36 square kilometers. It is a ground cluster of 1 square kilometers composed of 5,216 electromagnetic particle detectors and 1,188 muon detectors. It consists of three types of arrays: the particle detector array (KM2A), the 78,000-square-meter Water Cherenkov Detector Array (WCDA) consisting of 3,120 detection units, and the wide-angle Cherenkov telescope array composed of 18 telescopes. The “LHAASO” became fully operational in July 2021.
“The special feature of ‘LHAASO’ is that before it, humans did not have such highly sensitive ground equipment to detect gamma-ray bursts.” Cao Zhen said that the advantage of “LHAASO” is that it can firstly realize 24-hour uninterrupted detection, secondly, it is completely unaffected by weather conditions, and the “LHAASO” has the characteristics of a large field of view, which can observe one-sixth of the sky at a time, while the gamma ray telescope can only observe one point at a time , one star. The combination of the above factors makes “LHAASO” the “lucky one”.