A giant "bubble" is hidden in the Cygnus. How far is Lasso from solving the "mystery of the century" of the origin of cosmic rays?

Recently, scientists at the LHAASO cosmic ray observatory located at an altitude of 4,410 meters in Daocheng County, Sichuan Province, used LHAASO to discover a giant ultra-high-energy gamma-ray bubble structure with a diameter of 1,000 light years in the star-forming region of Cygnus, and found the origin of the celestial body with an energy of more than 100 billion electron volts, certifying the first super cosmic ray acceleration source. This is an important step towards solving the mystery of the origin of cosmic rays. This achievement was published as a cover article in the academic journal Science Bulletin on February 26.

Cosmic rays, also known as cosmic rays, are high-speed charged particles from outer space. They are mainly composed of protons, but also include various atomic nuclei, electrons, and some antimatter particles. These particles were first discovered by Austrian physicist Victor Hess through high-altitude balloon experiments in 1912. Subsequently, scientists discovered a large number of elementary particles through the study of cosmic rays, which promoted the development of particle physics.

The energy distribution of cosmic ray particles is very wide, with the highest energy reaching 3×1020 electron volts, which is 10 million times higher than the highest energy of particles that can be accelerated by the most powerful artificial particle accelerator. How cosmic rays are produced and why they carry such high energy has always puzzled scientists. Which celestial bodies in the Milky Way produce the energy that ultimately forms cosmic rays remains an unsolved mystery and has been a hot topic of research in the past 10 years.

Starting in 2019, Lasso adopted a "construction and operation" mode, and the detector array was gradually put into scientific operation in batches. In 2020, Lasso detected 12 ultra-high-energy gamma-ray sources in 11 months, and detected that the highest-energy photons at that time came from the constellation Cygnus, up to 1.4 quadrillion electron volts, opening a window for ultra-high-energy gamma astronomy. After completing the construction of the entire array in July 2021, with its superior detector sensitivity, Lasso increased the number of ultra-high-energy gamma-ray sources detected to 43 within a year, demonstrating its superb gamma-ray source detection capability.

The star-forming region in Cygnus is the brightest region in the northern sky of the Milky Way and has many massive star clusters. The life span of massive stars is only a few million years, so the clusters are full of violently active stars, with a complex strong shock wave environment, which is an ideal place for cosmic ray acceleration. This highly-watched region has become the best area in the sky for searching for ultra-high-energy cosmic ray sources and a breakthrough in solving the "mystery of the century".

As observation time and data accumulated, the research team discovered a giant ultra-high-energy gamma-ray bubble structure with a diameter of more than 1,000 light-years in the direction of Cygnus, 5,000 light-years away from the Earth. After more than three years of observation, "Lasso" recorded 66 photons exceeding 40 trillion electron volts in this direction, of which 8 photons had energies exceeding 1 quadrillion electron volts. The highest energy reached 2.5 quadrillion electron volts, setting a new record for the highest energy photon.

In the central area of ​​the gamma-ray bubble structure, the distribution of photons is relatively concentrated, significantly exceeding the average photon density in the bubble structure. This indicates that there must be a cosmic ray acceleration source in the center of the bubble structure, which continuously injects cosmic rays into the surrounding area, and this giant "bubble" is produced by a super cosmic ray acceleration source.

What kind of celestial body can accelerate the energy of cosmic rays to such a high level? Scientists believe that the massive star cluster in the center of the star-forming region of Cygnus is the most likely corresponding celestial body for super cosmic ray accelerators.

Most of the stars in the cluster are young and hot, with some having surface temperatures of tens of thousands of degrees Celsius. The radiation intensity of these stars is hundreds to millions of times that of the sun. The huge radiation pressure blows out the surface material of the star, forming a strong stellar wind with a speed of thousands of kilometers per second. The collision of the stellar wind with the surrounding interstellar medium and the violent collision between the stellar winds produce an extreme environment of strong shock waves and strong turbulence, becoming a powerful particle accelerator. When the particles gain high enough energy, they will escape from the acceleration zone, and cosmic rays will spread to the wider interstellar space.

The discovery by Lasso not only locates for the first time the peta-electron-volt cosmic ray acceleration source that high-energy astrophysicists have been looking for for decades, but also shows that the particle acceleration ability of celestial bodies in the Milky Way is likely to break through the traditional peta-electron-volt limit, which will have an important impact on the study of the origin of cosmic rays. As the observation time increases, Lasso may detect more sources of cosmic ray acceleration of peta-electron-volt or even higher energy cosmic rays, which is expected to solve the mystery of the origin of cosmic rays in the Milky Way.

(Zhang Chongyang is an engineer at the Institute of High Energy Physics, Chinese Academy of Sciences, and Liu Ruoyu is a researcher at Nanjing University)