On November 16, the High Altitude Cosmic Ray Observatory (hereinafter referred to as “Lasso”) officially released the precise energy spectrum of high-energy gamma radiation of GRB 221009A, the brightest gamma-ray burst to date, in Science Advances.
Cao Zhen, chief scientist of “Lasso” and researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, introduced that the energy spectrum challenges the traditional standard radiation model of gamma burst afterglow, revealing that the intensity of cosmic background light in the infrared band is lower than expected, and also provides important information for testing the scope of application of Einstein’s theory of relativity and exploring new physics research such as dark matter candidate particles – axons.
Schematic diagram of the results. Courtesy of the Institute of High Energy Physics, Chinese Academy of Sciences
Challenging the standard model of gamma bursts
Gamma bursts, which are flashing flashes of gamma rays from a certain direction in the sky, are the most violent celestial explosions since the Big Bang and were first discovered in 1967.
From 2019 to 2022, a total of 3 gamma bursts were discovered, with photons with a maximum energy of 1 trillion electron volts.
On October 9, 2022, Lasso recorded gamma photons from gamma burst GRB 221009A with more than 10 trillion electron volts, marking a milestone in the 60-year history of gamma burst research.
As the brightest gamma burst to date, GRB 221009A was born from a massive star more than 20 times heavier than the Sun. The massive star collapsed and exploded when it ran out of fuel, releasing intense radiation.
Previously, Lasso meticulously measured the complete change behavior of this radiation, determined that the radiation originated from afterglow radiation, and revealed the reason why this gamma burst is the brightest in history, and the results were published in the journal Science in June 2023.
In the Standard Model of Gamma Bursts, explosives traveling at nearly the speed of light collide with gaseous substances in the surrounding environment, resulting in high-speed shock waves that accelerate electrons to very high energies, which further impact surrounding photons to form high-energy gamma radiation, or afterglow radiation.
Theoretically, the higher the photon energy, the faster the radiation intensity decays, however, the accurate measurement of the radiation spectrum by Lasso found that although the photon energy is more than 10 trillion electron volts, the radiation intensity of gamma bursts not only does not decay, but extends to 13 trillion electron volts.
“This spectrum challenges the standard model of gamma burst afterglow. Chen Songzhan, the physics coordinator of the “Lasso” international cooperation group and a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, said.
They conclude that this violation of the Standard Model indicates that the 10 trillion electronvolts or so photons in the afterglow of gamma bursts may be generated by more complex particle acceleration processes or the existence of new radiation mechanisms.
Challenge the model of cosmic evolution
High-energy gamma photons are absorbed by the background light that permeates the universe as they fly. Cosmic background light is the sum of all galaxial radiation products at different distances in the universe, which is closely related to the evolution of the universe.
The higher the gamma photon energy, the more intensely it is absorbed. Therefore, researchers have gone the opposite way, studying the intensity and properties of the background light in the universe according to the degree of gamma ray absorption, and then understanding the evolution of the universe.
“The extremely high brightness of GRB 221009A gives us the opportunity to detect high-energy gamma photons generated from the depths of the universe 2.4 billion light-years away, and how well they are absorbed by the cosmic background light. Chen Songzhan said.
According to the current model of cosmic evolution, a 1 trillion electron volt gamma photon flies 2.4 billion light-years and has a probability of being absorbed by background light of about 80%, while a 10 trillion electron volt gamma photon is absorbed with a probability of more than 99.5%.
However, the researchers found that the absorption of high-energy gamma light by the cosmic background light was lower than expected, and the intensity of the cosmic background light in the infrared band was only about 40% of what the existing cosmological models expected.
Of course, there is also the possibility that the standard model of the evolution of the universe is still correct. “The lower-than-theoretical absorption of high-energy gamma photons by the cosmic background light, then, may imply some new physical mechanism beyond the current Standard Model of particle physics. Cao Zhen said.
He introduced that there are many possibilities for this new physical mechanism beyond the Standard Model.
One possibility is that there is a very small disruption of the Lorentz symmetry, which underlies Einstein’s special theory of relativity. “This effect is amplified into an observable phenomenon during the long flight of 2.4 billion light-years from gamma photons, which can explain the high-energy gamma spectrum observed by Lasso. Cao Zhen said.
Another possibility is that the “axes” are at work. “The axon is a new particle outside of the Standard Model and one of the widely discussed candidates for dark matter, and the presence of the axion may also explain the weak absorption of high-energy gamma photons observed by Lasso. Cao Zhen said.
Open the door to a new physics
“This result will prompt a reconsideration of the formation and evolution of galaxies in the universe. Cao Zhen said.
In Cao Zhen’s view, now, “Lasso” has released the precise gamma photon energy spectrum of the brightest gamma burst, opening the door to new physics exploration, which is expected to lead to more related physical research.
“The radiation spectrum shows some new phenomena, and as for how to explain them, there is no mainstream theory, which will stimulate scientists around the world to think together. Cao Zhen said.
Located in Haizi Mountain, Daocheng County, Sichuan Province, at an altitude of 4,410 meters, “Lasso” is a one-square-kilometer ground cluster particle detector array composed of 5,216 electromagnetic particle detectors and 1,188 muzi detectors, a 78,000-square-meter water Cherenkov detector array and a composite array composed of 18 wide-angle Cherenkov telescopes.
In July 2021, “Lasso” was completed and began high-quality and stable operation, which is the most sensitive ultra-high-energy gamma-ray detection device in the world, with a large field of view and all-weather characteristics, and can monitor 2/3 of the sky area every day.
Cao Zhen introduced that in the future, “Lasso” will continue to wait for more gamma burst phenomena and try to find answers to unknown puzzles. (Source: China Science News Ni Sijie)
Related Paper Information:https://www.doi.org/10.1126/sciadv.adj2778
