Chinese scientists achieve first-ever complete recording of a massive star's death

Chinese researchers have unveiled the mystery behind a massive explosion some 2billion years ago by analyzing over 60,000 gamma photons captured by China’s Large High Altitude Air Shower Observatory (LHAASO). The findings would be a "textbook-level" achievement, with the potential to yield many revolutionary discoveries, scientists involved in this research told the Global Times on Friday.

In this observation, LHAASO has achieved the first precise measurement of the complete process of high-energy photon eruption, accomplishing a comprehensive observation of the light variation process in high-energy bands that other experiments had not achieved. This provides an experimental basis for the precise testing of theoretical models, the scientists noted.

About 2 billion years ago, a "super sun" star with a mass over 20 times that of the Sun exhausted its nuclear fusion fuel. In an instant collapse, it triggered a massive explosion, creating a prolonged burst of gamma-ray radiation known as a gamma-ray burst (GRB) that lasted for hundreds of seconds.

Trillions of electron-volt gamma photons generated from the collision of the fireball and interstellar matter traveled through the vast universe, heading straight toward Earth. On October 9, 2022, these high-energy gamma photons reached the field of view of China's LHAASO, and approximately over 60,000 gamma photons were collected by the facility.

After months of analysis, the research findings related to these observations were published online on Friday in the prestigious international scientific journal Science.

GRBs are recognized as the most powerful explosive phenomena in the universe, with energy emitted within a few seconds equivalent to the total radiation energy of the Sun over 10 billion years. Over the past half-century, researches have revealed that GRBs originate in extreme physical environments characterized by intense magnetic fields, strong gravitational forces, and ultra-high speeds.

GRBs have become a main subject for studying the fundamental physics in astrophysics and basic physics. Scientists hope to utilize GRBs to investigate the cosmic evolution, the origin of heavy elements, and the validity of relativity, among other significant questions.

Cao Zhen, the chief scientist of LHAASO and research fellow at the Institute of High Energy Physics under the Chinese Academy of Sciences, described the complete recording of the trillion-electron-volt GRB as “extremely lucky” in an interview with the Global Times.

Cao said this event was an extremely rare occurrence – when a bright GRB lasting several hundred seconds that traveled to Earth after 2 billion years of travel, LHAASO was the world's only ground-based detector directly facing the source, allowing for the perfect capture of this astronomical spectacle.

Gao He, a professor from the Department of Astronomy at Beijing Normal University, explained that on the night of October 9, 2022, the burst detected by China was the brightest GRB ever recorded, and scientists estimate that the probability of such a bright GRB sweeping across Earth is “once in a millennium.”

For the first time, LHAASO provided a complete light curve and energy spectrum of the GRB in the Teraelectronvolt (TeV) energy range, greatly enhancing our understanding of the radiation mechanism and jet structure of GRB, Gao added.

The observation by LHAASO is of special significance, experts said. Dai Zigao, one of the corresponding authors of the paper and a professor at the University of Science and Technology of China, told the Global Times that he likened the phenomenon of GRB to an embryo, and LHAASO's TeV observations revealed the process of how this "embryo" developed from a "fertilized egg" into a "fetus." The observations can be seen as an "ultrasound," accurately observing this process, allowing humans to gain such a comprehensive understanding of it for the first time.

Currently, LHAASO has recorded over 60,000 gamma photons from this event. Cao said this is an astonishing data set, demonstrating the remarkable brightness of the burst.

“LHAASO has accomplished a textbook-like complete observation of the light variation process in high-energy bands, which other experiments had not achieved. This provides an experimental basis for the precise testing of theoretical models. Given the rarity of such an event, the results of this observation are expected to remain optimal for the next several decades or even centuries,” Cao noted.

Additionally, LHAASO has made the first measurement of the rapid enhancement process of high-energy photon flux. Yao Zhiguo, one of the corresponding authors of the paper, said he believed the results will trigger in-depth discussion throughout the scientific community on mechanisms such as energy injection, photon absorption, and particle acceleration in GRBs.

LHAASO also discovered the secret behind this GRB becoming the brightest in history. Dai noted that it is precisely because LHAASO happened to be directly facing the brightest core of the burst that it naturally explains why this GRB is the brightest ever recorded and further explains why such events are extremely rare.