The “Tibet ASγ Experiment”, a joint research project between China and Japan on cosmic ray observation, discovered ultra-high energy diffuse gamma rays. Milky Way galaxy. It is estimated that the highest energy detected has reached an unprecedented high level, close to 1 pitta electron volt (PeV, or one trillion eV).
Surprisingly, these gamma rays are not directed at known high-energy gamma-ray sources, but are scattered throughout the Milky Way galaxy (see Figure 1).
Scientists believe that these gamma rays are produced by the nuclear interaction between cosmic rays escaping from the most powerful galactic sources (“PeVatrons”) and the interstellar gas in the Milky Way. This observational evidence marks an important milestone in revealing the origin of cosmic rays, and cosmic rays have puzzled mankind for more than a century.
Cosmic rays are high-energy particles from outer space, mainly composed of protons and atomic nuclei, as well as a small amount of electrons/positrons and gamma rays. Cosmic rays below a few PeV are considered to be produced in our galaxy. A source that can accelerate cosmic rays to PeV energy is called PeVatron.Although there are supernova remnants, star-forming regions and supermassive stars Black hole It has been suggested as a PeVatron candidate at the center of the Milky Way. It has not yet been discovered, mainly because most cosmic rays are charged and will lose their original direction when propagating in the Milky Way, and will be bent by the magnetic field.
However, cosmic rays can interact with the interstellar medium near the acceleration location and generate gamma rays with approximately 10% of the energy of their parent cosmic rays. Since the direction of electrically neutral gamma rays cannot be changed by a magnetic field, ultra-high energy gamma rays (0.1-1 PeV) may tell us the position of PeVatron in the Milky Way.
The Tibet ASγ experiment began in 1990. After several expansions, the current air shower array consists of more than 500 radiation detectors distributed on an area of approximately 65,000 square meters. In order to improve its sensitivity to gamma-ray observations, the total effective area of the new water Cerenkov muon detector is 3400 m2 pcs In 2014, a detector was added under the existing surface cosmic ray detector (see Figure 2).
Since the muons of gamma-ray events are weak, and the dominant proton/nucleon events are rich in muons, this function can be used to suppress the background caused by proton/nucleon events. Using this technology, the Tibet ASγ experiment successfully reduced the proton/nuclear background event to one part per million, which is the most efficient in the history of this type of experiment. Therefore, we can detect ultra-high energy gamma rays with almost no cosmic ray background events.
Scientists from the Tibet ASγ experiment observed that the energy of gamma rays is between 0.1 and 1 PeV, and these energies come from the galactic disk area. Specifically, they discovered 23 ultra-high-energy cosmic gamma rays with energies exceeding 398 TeV in the Milky Way. Among them, the highest observed energy is close to 1 PeV, which is a new world record for gamma-ray photons detected anywhere.
Surprisingly, these gamma rays are not directed at the most powerful high-energy gamma-ray source known, but spread along the Milky Way! Scientists quickly noticed that these gamma rays may be produced by the interaction of PeV cosmic rays and interstellar medium after escaping from accelerated sources (PeVatrons). This process is called the “hadron origin”, which produces gamma rays through the production of neutron mesons and subsequent decay, the energy of which is approximately one-tenth of that of its parent cosmic rays.
These diffuse gamma rays suggest that powerful cosmic particle accelerators (PeVatrons) are widespread in the Milky Way. In other words, if PeVatrons exist, the cosmic rays they emit will penetrate into the Milky Way, creating diffused light with extremely high-energy gamma rays. This is what the scientists of the Tibet ASγ experiment discovered. This is a long-awaited discovery for decades and provides clear evidence for the existence of PeVatron in our galaxy in the past and/or present.
Two years ago, scientists from the Tibet ASγ experiment discovered high-energy gamma rays from the Crab Nebula. Pulsar Nebula in the Milky Way. Those gamma rays may be produced in different ways, such as high-energy electrons/positrons in the nebula. This process is called “lepton origin.”