The Tibet ASgamma experiment, a China-Japan joint research project, has discovered gamma rays beyond 100 TeV from G106.3+2.7, a supernova remnant (SNR) 2600 lightyears from Earth (see Fig.1). These gamma rays are of the highest energy ever observed from SNRs, and are probably produced in collisions between cosmic rays (protons) accelerated in G106.3+2.7 and a nearby molecular cloud. SNR G106.3+2.7 is thus the first candidate object in the Milky Way that can accelerate cosmic rays (protons) up to 1 PeV. It will open an important window in the search for the 'PeVatron'. The study was published online in Nature Astronomy.

Cosmic rays (protons and other atomic nuclei arriving from space) have been detected in the 10⁹-10²⁰ eV energy range. Astrophysical sources that can accelerate cosmic rays up to PeV energies (100 times more energetic than the highest energy achieved in any man-made accelerator on Earth) are called 'PeVatrons'. They are believed to exist in our galaxy, but none have been detected yet, making it a long-standing mystery in the universe. Since cosmic rays can be deflected by the galactic magnetic field due to their electric charge, their arrival directions observed on Earth do not point back to their place of origin. Therefore, it is impossible to find a 'PeVatron' by using the direction of cosmic rays.

Fortunately, cosmic rays, after being accelerated at their sources, can collide with nearby molecular clouds and produce gamma rays with energies roughly one-tenth that of their parent cosmic rays via the production and subsequent decay of neutral pions. Gamma rays, with no electric charge, can travel straight from their sources to Earth, and their observed arrival directions point back to their sources.

To identify an astrophysical source as a 'PeVatron', therefore, the following three points must be demonstrated: 1) The source emits gamma rays up to and beyond 100 TeV (one-tenth of 1 PeV). 2) The gamma-ray emission region coincides with the location of a molecular cloud near the cosmic-ray accelerator. 3) It can be excluded that ultra-high energy gamma rays come from the high energy electrons of pulsars, that is, 'leptonic origin'.

There is no astrophysical source for which all the above three points have been established so far. SNR G106.3+2.7 has been detected by the VERITAS Imaging Air Cherenkov Telescope at TeV energies and the Fermi Gamma-ray Space Telescope at GeV energies. However, neither of the two experiments is sensitive enough to 100 TeV gamma rays. Recently the HAWC experiment observed gamma rays in the 40-100 TeV energy range from this SNR, but its gamma-ray emission region overlaps with PSR J2229+6114, the pulsar born in the supernova explosion of SNR G106.3+2.7 as shown in Fig.1.

The Tibet ASgamma experiment, located at an altitude of 4300 m above sea level in the town of Yangbajing in Tibet, has been operated jointly by China and Japan since 1990. The China-Japan collaboration added new water-Cherenkov-type muon detectors under the existing cosmic-ray detectors in 2014 (see Fig. 2). These underground muon detectors suppress 99.92% of the cosmicray background noise and thus improve sensitivity significantly. Using data taken over a period of about two years, they observed ultrahigh-energy gamma rays up to and beyond 100 TeV from the supernova remnant (SNR) G106.3+2.7, and found that the gamma-ray emission region is far away from the pulsar at the northeast corner of G106.3+2.7 and in good agreement with the location of a nearby molecular cloud. These observational facts suggest that cosmic-ray nuclei are accelerated up to PeV energy in this SNR and then collide with the molecular cloud, thus producing gammaray photons via the production and subsequent decay of neutral pions. The important work shows that SNR G106.3+2.7 is the first potential 'PeVatron' in our galaxy, which is a big step in the attempt to reveal the mysterious origin of cosmic rays.

Scientific Contact:

Prof. HUANG Jing , co-spokesperson for the Tibet ASgamma Experiment , huangjing@ihep.ac.cn