Figure 1. Distribution of the ultra-high-energy gamma rays (yellow dots) detected by the Tibet ASγ experiment in the galactic coordinate system. It is, of course, concentrated along the galactic disk. The gray shadow area indicates what is outside the field of view. The background color shows atomic hydrogen distribution in the galactic coordinates. Credit: NASA
The Tibet ASγ experiment, a joint research project between China and Japan on cosmic ray observation, extracted ultra-high-energy diffuse gamma rays from the Milky Way galaxy. The highest energy detected is estimated to be unprecedentedly high, almost 1 Peta-electron volt (PeV, or one million billion eV).
Surprisingly, these gamma rays do not point to known high-energy gamma ray sources, but are scattered across the Milky Way (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 interstellar gas in the Milky Way galaxy. This observational evidence is an important milestone in the revelation of the origin of cosmic rays, which has surprised mankind for more than a century.
Cosmic rays are high-energy particles from outer space that consist mainly of protons and nuclei, as well as small numbers of electrons / positrons and gamma rays. It is believed that cosmic rays are produced under some PeV in our Milky Way system, and a source that can accelerate cosmic rays to PeV energy is called a PeVatron. Although supernova remnants, star-forming regions and the supermassive black hole in the galactic center it is suggested that they are candidate PeVatrons, no observations have been identified yet, mainly because the majority of cosmic rays have an electric charge and will lose their original direction if they propagate in the Milky Way as well as by the magnetic field. . .
However, cosmic rays can interact with the interstellar medium near their point of acceleration and produce gamma rays with about 10% of the energy of their older cosmic rays. Since the direction of electric neutral gamma rays cannot be changed by the magnetic field, ultra-high-energy gamma rays (0.1-1 PeV) can tell us where the PeVatrons are in the Milky Way.
Figure 2. The cooperation between China and Japan in 2014 placed new water-Cherenkov-type muzzle detectors under the existing air shower arrangement. Credit: Institute of High Energy Physics
The Tibet ASγ experiment was started in 1990. After a number of expansions, the current air dew collection consists of more than 500 radiation detectors spread over approximately 65,000 square meters. In order to improve the sensitivity of gamma ray observations, new water-Cherenkov-type muzzle detectors with a total effective area of 3400 m2 was added in 2014 under the existing surface cosmic ray detectors (see Figure 2).
Since gamma radiation events are muonarm and the dominant proton / nuclear events are muon rich, this function can be used to suppress the background caused by the proton / nuclear events. Using this technique, the Tibet ASγ experiment successfully reduced proton / nuclear background events to one millionth, the most efficient one ever achieved in this type of experiment. We can therefore detect ultra-high-energy gamma rays that are almost free of cosmic radiation background events.
Scientists from the Tibet ASγ experiment observed gamma rays with energy between about 0.1 and 1 PeV from the galactic disk regions. Specifically, they found 23 ultra-high-energy cosmic gamma rays with energy above 398 TeV along the Milky Way. Of these, the highest observed energy was almost 1 PeV, which is a new world record for gamma ray photons that can be detected anywhere.
Figure 3. The Tibetan air shower setup located 4300 m above sea level in Tibet, China. Credit: Institute of High Energy Physics
Surprisingly, these gamma rays do not point to the most powerful high-energy gamma sources, but are scattered along the Milky Way! Scientists soon noticed that these gamma rays probably originated from the interaction of PeV cosmic rays and the interstellar medium after escaping from the accelerating sources (PeVatrons). This process, known as’ hadronic origin ‘, produces gamma rays with energies that are about one-tenth of their parents’ cosmic rays, through the production and subsequent decay of neutral peonies.
These diffuse gamma rays indicate the ubiquitous existence of powerful cosmic particle accelerators (PeVatrons) in the Milky Way. In other words, if PeVatrons exists, the cosmic rays they emit will penetrate into the galaxy and cause a diffuse glow of gamma rays of extreme energies. This is just what scientists have found with the Tibet ASγ experiment. This is a long-awaited discovery for decades, which provides unequivocal evidence for the existence of PeVatrons in the past and / or now in our Milky Way system.
Two years ago, scientists from the Tibet ASγ experiment found extremely energetic gamma rays from the nebula, a pulsar wind mist in the Milky Way. These gamma rays are probably produced in another way, such as by high-energy electrons / positrons in the nebula, a process called ‘leptonic origin’.