The brightest "cosmic fireworks show" to date has been fully recorded and selected as one of the top ten scientific advances of the year. What does this achievement of "Lasso" tell us?

On February 29, 2024, the National Natural Science Foundation of China announced the "Top Ten Advances in Chinese Science" for 2023. The major progress announced by the international collaboration of the High Altitude Cosmic Ray Observatory (LHAASO) led by Academician Cao Zhen of the Institute of High Energy Physics of the Chinese Academy of Sciences - LHAASO discovered the extremely narrow jet and 10 trillion electron volt photons of the brightest gamma-ray burst in history - was selected. The related papers of this achievement were published in Science on June 8, 2023 and Science Advances on November 15, 2023.

When you first see this result, you may recognize the characters but not understand how to put them together. Let me explain it to you.

Lasso discovered the extremely narrow jet and 10-trillion-electron-volt photons of the brightest gamma-ray burst in history (artistic diagram)

1. Gamma-ray bursts, a "cosmic fireworks show" billions of light years away

Gamma-ray bursts (GRBs) are short-lived gamma-ray bursts that occur suddenly in the sky. They are the most violent celestial explosions since the Big Bang. To use a vivid metaphor, a GRB is like a giant firework, except that we can see fireworks with our naked eyes, but we need specialized large-scale scientific equipment to observe GRBs.

Gamma-ray bursts were first discovered in 1967. Their main radiation is in the 100keV-10MeV gamma-ray band. Humans have recorded nearly 10,000 gamma-ray burst events using space satellite detectors. According to the duration of the radiation, gamma-ray bursts are divided into two categories: short bursts with a duration of less than 2 seconds and long bursts with a duration of more than 2 seconds.

So how are gamma-ray bursts produced? At present, the scientific community generally believes that long bursts originate from the collapse of the core of a supermassive star, and short bursts originate from the merger of two neutron stars. Both processes produce jets with speeds close to the speed of light. Regardless of whether they are long or short, they all come from distant extragalactic galaxies, with an average distance of about 10 billion light years.

2. It is not easy to observe high-energy gamma-ray burst radiation completely

The radiation of a gamma-ray burst is divided into two stages: transient radiation and afterglow radiation. Afterglow is the follow-up radiation after the gamma-ray burst, which was first discovered in 1997.

It is generally believed that the central engine of a gamma-ray burst first produces an extremely hot fireball, which expands outward at an extremely relativistic speed. When the faster matter behind catches up with the slower matter in front, a collision occurs, generating an internal shock wave. The internal shock wave heats the electrons to relativistic energy. The relativistic electrons move in the magnetic field and produce keV/MeV gamma rays through synchrotron radiation or inverse Compton scattering. This is the instantaneous radiation of the gamma-ray burst.

Relativistic matter continues to sweep through the interstellar medium, generating an outer shock wave, which accelerates electrons in the interstellar medium and generates relatively slowly changing radiation in the X-ray, optical, radio and other bands through the synchrotron radiation of the electrons, namely afterglow radiation.

The teravolt radiation of the gamma-ray burst was not observed until 2019 by the ground-based atmospheric Cherenkov telescope in the afterglow phase, and only three cases have been observed so far. Through multi-band joint observations, scientists found that the energy spectrum of the afterglow radiation has two spectral components. The low-energy component is believed to be the synchrotron radiation of electrons in the magnetic field, while the high-energy component is believed to be the inverse Compton scattering of synchrotron radiation photons by high-energy electrons.

Since atmospheric Cherenkov telescopes can only observe on clear moonless nights and have a very narrow field of view, it takes some time for them to turn to the direction of the gamma-ray burst after receiving the satellite alarm information. They only saw the descending phase of the afterglow and missed the observation of the transient radiation phase and the rising phase of the afterglow.

3. China's "Lasso" helps humans uncover the secrets of the brightest gamma-ray burst to date

"Laso" is a medium- and long-term planning project for the construction of major national scientific and technological infrastructure during the 12th Five-Year Plan. A number of key core technologies were developed during the research and development process. All construction was completed in 2021 and began stable operation. It has reached the international leading level in ultra-high energy gamma-ray detection sensitivity, very high energy gamma-ray survey sensitivity, and cosmic ray energy coverage range, and passed the national acceptance in May 2023 with performance exceeding the design indicators.

Aerial photo of the High Altitude Cosmic Ray Observatory (LHAASO) located on a plateau 4,410 meters above sea level. Photo courtesy of the Institute of High Energy Physics, Chinese Academy of Sciences

Gamma-ray bursts are one of the important scientific targets of Lasso. Compared with the atmospheric Cherenkov telescope, the Lasso detector has a much larger field of view. It can monitor 1/7 of the entire sky at any time, and has all-weather observation time, so it can observe the transient radiation and early afterglow of gamma-ray bursts.

On the evening of October 9, 2022 Beijing time, space satellites recorded the brightest known gamma-ray burst in human history (named GRB 221009A), which was produced by the collapse and explosion of a massive star more than 20 times heavier than the sun when its fuel was exhausted. Based on existing observations, it is estimated that such bright gamma-ray bursts occur once every thousand years.

Thanks to the large field of view and high sensitivity of Lasso, humans finally saw for the first time the complete evolution of the GRB's tera-electron-volt afterglow radiation from rising to falling. Based on the peak brightness time detected by Lasso, the Lorentz factor of the GRB's initial jet velocity was calculated to be about 440, which is much faster than that of an ordinary GRB jet.

Lasso observed that the tera-electron-volt radiation had an extremely fast rise in the early stage. This new phenomenon has never been observed in afterglows of other wavelengths. It may be due to the fact that the central engine continuously injected a large amount of energy into the afterglow in the early stage. Lasso also observed that the brightness dropped rapidly at around 700 seconds. According to the measurement time of Lasso, it can be inferred that the half-angle of the jet is only 0.8°, which is the narrowest gamma-ray burst jet discovered so far, thus revealing the secret of the brightest gamma-ray burst in history.

4. The discovery of " Laso " brings new insights to the theoretical study of gamma-ray bursts

Lasso detected the highest photon energy from GRB 221009A, reaching 13 trillion electron volts, opening the 10 trillion electron volt observation window for gamma-ray bursts for the first time, which is a milestone in the 60-year history of gamma-ray burst research. Based on the low-energy observations of nearly 10,000 gamma-ray bursts, scientists have established a standard theoretical model for afterglows. The trillion electron volt afterglow radiation originates from the synchronous self-Compton radiation of relativistic electrons. In theory, the brightness of photons with higher energy should decrease significantly, but Lasso's measurements found that the radiation of GRB 221009A extends to more than 10 trillion electron volts, without the expected dimming phenomenon, which challenges the standard radiation model of gamma-ray burst afterglows and indicates that photons of around 10 trillion electron volts may be generated by more complex particle acceleration processes or new radiation mechanisms.

When high-energy gamma photons propagate through space, they will be absorbed by the background light that permeates the universe. The higher the energy of the gamma photons, the stronger the absorption.

The cosmic background light is the sum of all galaxy radiation products at different distances in the universe, and is closely related to the formation and evolution of galaxies. GRB 221009A is about 2.4 billion light-years away from the Earth. Under the existing cosmic background light model, it is expected that Lasso will have difficulty detecting its photons above 10 trillion electron volts. The measurement results of Lasso show that the universe is more transparent than originally expected, requiring the intensity of the cosmic background light in the infrared band to be only about 40% of that expected by the existing cosmological model, which is of great value for studying the formation and evolution of galaxies in the universe.

On the other hand, if the absorption intensity of the existing cosmic background light is considered correct, then some new physical mechanism beyond the current standard model of particle physics is required to explain the observations. For example, if the "Lorentz symmetry" that is the basis of Einstein's special theory of relativity is very slightly violated in the high-energy region, this effect will be magnified into an observable phenomenon during the long-distance flight of gamma photons of 2.4 billion light years, thus explaining the observations of "Lasso". In addition, axions are a new particle outside the standard model. If there is oscillation between high-energy photons and axions, it can also explain the weak absorption of high-energy gamma photons observed by "Lasso".

Conclusion

"Laso" is a major national scientific and technological infrastructure, located at Haizi Mountain at an altitude of 4,410 meters in Daocheng County, Sichuan Province. It is a one-square-kilometer ground-based shower particle detector array consisting of 5,216 electromagnetic particle detectors and 1,188 muon detectors, a 78,000-square-meter water Cherenkov detector array, and a composite array consisting of 18 wide-angle Cherenkov telescopes. "Laso" was completed in July 2021 and began high-quality and stable operation. It is an internationally leading high-energy gamma-ray detection device with a large field of view and all-weather characteristics. It can monitor 2/3 of the sky area every day. "Laso" is expected to operate for more than 20 years. In the future, it is expected to capture more gamma-ray burst events and make more breakthroughs in exploring this most violent celestial explosion phenomenon after the Big Bang.

Produced by: Science Popularization China

Author: Chen Songzhan, Researcher at the Institute of High Energy Physics, Chinese Academy of Sciences

Producer: China Science and Technology Digital Media Co., Ltd.