&Bullet; physics 14, p77

Observations by the LHAASO gamma-ray observatory show large differences in the speed with which charged particles propagate through the Milky Way.

Our galaxy is a turbulent place for cosmic rays. Instead of going straight from their origin to where they ultimately release their energy, these charged particles – a mixture of atomic nuclei, electrons and positrons – are hit by interstellar magnetic fields. The effect of this turbulence on the diffusion rate of cosmic rays was previously estimated by analyzing the gamma-ray “halos” around two pulsars. The Large High Altitude Air Shower Observatory (LHAASO) in China has now discovered such a halo around a third pulsar [1] . Analysis of the properties of this halo suggests that cosmic rays propagate particularly slowly near these stellar remains.

LHAASO made the observations over about ten months in 2019 and 2020. When analyzing the data, the LHAASO researchers found a fuzzy source of very high-energy gamma rays in a part of the sky that coincides with a previously discovered pulsar. Similar features observed around the “Geminga” and “Monogem” pulsars were attributed to interactions between interstellar photons of the environment and the high-energy electrons and positrons emitted by the pulsar. These interactions occur in a diffuse sphere around the pulsar and cause the energy of the photons to be increased to the tenths of TeV. It is these amplified photons that form the gamma-ray halo that LHAASO discovered.

If this model is correct, the size of the new halo, the energy of the gamma rays, and the estimated age of this pulsar suggest that the rate of diffusion of cosmic rays around the pulsar is hundreds or thousands of times less than it is in the rest of the world Milky Way. The reason for this difference remains unknown, but the collaboration suggests that it could be caused by the complex magnetic environments around pulsars.

–Marric Stephens

Marric Stephens is the corresponding editor for physics based in Bristol, UK.


  1. F. Aharonian et al., “Extended very high-energy gamma-ray emission around PSR J0622 + 3749, observed by LHAASO-KM2A”, Phys. Rev. Lett.126, 241103 (2021).

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