China News Service, Beijing, February 26 (Reporter Sun Zifa) In the Cygnus star formation region of the Milky Way, there is a giant ultra-high-energy gamma ray bubble structure - a universe nearly ten million times the size of the solar system and with an energy exceeding 1 billion electron volts. The "bubble" was recently discovered by China's large scientific facility High Altitude Cosmic Ray Observatory (English abbreviation LHAASO, Chinese nickname "Lasuo"), and research confirmed that it is an object that originates cosmic rays.
Academician Cao Zhen explains the latest major discoveries and research progress of "Raso".
Photo by China News Network reporter Sun Zifa
This important high-energy astrophysics research result, which is the first time in the world and history to discover the origin of cosmic rays with energy higher than 1 billion electron volts and certify the first super cosmic ray acceleration source, was conducted by the Institute of High Energy Physics, Chinese Academy of Sciences (Institute of High Energy Physics). The "Raso" international cooperation group led by the team was completed, and the related paper was published as a cover in the well-known academic journal "Science Bulletin" on February 26.
The cover of Science Bulletin, which published this important result.
Institute of High Energy, Chinese Academy of Sciences/Photo provided
Why did the latest discovery research come about?
On the eve of the official publication of the results, Cao Zhen, an academician of the Chinese Academy of Sciences, a researcher at the Institute of High Energy, Chinese Academy of Sciences, and the chief scientist of "Raso" and the main author of the paper, explained the major discovery and research progress of "Raso" to the media.
Cosmic rays are charged particles from outer space, whose main components are protons.
The origin of cosmic rays is one of the most significant cutting-edge scientific issues in contemporary astrophysics.
Measurements found that the energy spectrum of cosmic rays (that is, the distribution of the number of cosmic rays in particle energy) presents an inflection structure near 1 quadrillion electron volts, which is called the cosmic ray energy spectrum because its shape resembles a knee joint. "knee".
Display a poster introducing this important achievement.
Institute of High Energy, Chinese Academy of Sciences/Photo provided
Scientists believe that cosmic rays with energy lower than the "knee" originate from celestial bodies in the Milky Way, and the existence of the "knee" also shows that the energy limit of most cosmic ray sources in the Milky Way to accelerate protons is around 1 quadrillion electron volts.
However, exactly what kind of celestial body can accelerate cosmic ray energy to such a high energy and form a "knee" energy spectrum structure is still an unsolved mystery, and it is also one of the most attractive topics in cosmic ray research in recent years.
In this study, "Raso" discovered a giant ultra-high-energy gamma ray bubble structure in the Cygnus star formation region. Within this cosmic "bubble", which is conservatively estimated to be about 1,000 light-years in diameter and about 5,000 light-years away from the Earth, there are Multiple photons with energy exceeding 1 quadrillion electron volts are distributed, with the highest reaching 2 quadrillion electron volts.
How to identify the celestial body that originates cosmic rays?
The "Lasso" team pointed out that generally speaking, producing gamma photons with an energy of 2 quadrillion electron volts requires cosmic ray particles with at least 10 times higher energy.
Therefore, this indicates that there is a super cosmic ray accelerator inside the giant ultra-high-energy cosmic "bubble", which continuously produces high-energy cosmic ray particles with an energy of at least 2 billion electron volts and injects them into interstellar space.
Aerial photo of "Raso".
Institute of High Energy, Chinese Academy of Sciences/Photo provided
These high-energy cosmic rays collide with the gaseous material in interstellar space to produce gamma photons. The number of photons is clearly related to the distribution of the surrounding gas. The massive star clusters located near the center of the "bubble" of the universe are the super universe. The most likely corresponding celestial body for linear accelerators.
Massive star clusters are dense clusters of young, hot, massive stars with surface temperatures exceeding about 35,000 degrees (O-type stars) and stars with surface temperatures exceeding about 15,000 degrees (B-type stars).
The radiation intensity of these stars is a hundred to a million times that of the sun. The huge radiation pressure blows out the material on the surface of the stars, forming a strong stellar wind with a speed of up to thousands of kilometers per second.
Subsequently, the collision of the stellar wind with the surrounding interstellar medium and the violent collision between the stellar winds produced an extreme environment of strong shock waves and strong turbulence, becoming a powerful particle accelerator.
The "Raso" team believes that this powerful particle accelerator is the first super cosmic ray acceleration source that scientists have been able to verify so far.
It is believed that with the increase of observation time, "Raso" will be able to detect more acceleration sources of quadrillion electron volts or even higher energy cosmic rays in the future, and is expected to solve the mystery of the origin of cosmic rays in the Milky Way.
What other important discoveries have been made about the "bubbles" in the universe?
"Three feet of ice does not freeze in one day."
The "Lasso" team emphasized that the first discovery of a cosmic ray originating object with an energy higher than 100 billion electron volts and the certification of the first super cosmic ray acceleration source were not achieved overnight by one or two observations, but were achieved by "Lasso" itself. It has been obtained through continuous research based on the accumulation of long-term observation data since the trial operation in 2019.
While discovering and revealing the mystery of the origin of ultra-high-energy cosmic rays in the Milky Way, Lasso also inferred based on observations that the super cosmic ray accelerator in the cosmic "bubble" makes the density of cosmic rays in the surrounding interstellar space significantly higher than that in the Milky Way. The average level of cosmic rays affects the spatial range even far beyond the currently observed "bubble" scale, thus providing a possible explanation for the excess of diffuse gamma-ray radiation in the Milky Way previously detected by Lasso.
The spatial distribution of high-energy gamma rays observed by "Raso". Each dot in the figure represents an ultra-high-energy gamma photon detected by "Raso".
Institute of High Energy, Chinese Academy of Sciences/Photo provided
According to reports, as the most sensitive ultra-high-energy gamma ray detection device in the world and a major national scientific and technological infrastructure in China with cosmic ray observation and research as its core, the "Raso" is constructed and operated by the Institute of High Energy, Chinese Academy of Sciences, and uses universal The international cooperation model enables the open sharing of facility platforms and observation data.
"Raso" is located in Haizi Mountain at an altitude of 4,410 meters in Daocheng County, Sichuan Province. It consists of a 1 square kilometer ground shower particle detector array composed of 5,216 electromagnetic particle detectors and 1,188 muon detectors, and a 78,000 square meter water cerenko The Cherenkov detector array and a composite array composed of 18 wide-angle Cherenkov telescopes will be completed in July 2021 and will begin high-quality and stable operation.
Academician Cao Zhen explains the latest major discoveries and research progress of "Raso".
Photo by China News Network reporter Sun Zifa
At present, the "Raso" international cooperation group has 32 Chinese and foreign astrophysics research institutions as member units and about 280 scientist members.
In addition to Academician Cao Zhen, the co-corresponding authors of the latest published paper include doctoral candidates Gao Chuandong and Associate Researcher Li Cong of the Institute of High Energy, Chinese Academy of Sciences, Researcher Liu Ruoyu of Nanjing University, and Professor Yang Ruizhi of the University of Science and Technology of China.
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