Constraining the magnetohydrodynamic turbulence around Geminga by observing the γ-ray halo beyond 100 TeV

The extended γ-ray halo surrounding an old pulsar wind nebula (PWN) is an ideal place to investigate both the acceleration and diffusion processes of cosmic rays. In this work, we report the γ-ray halo observed with high precision by the Tibet ASγ experiment at the world's highest-energy above 100 tera-electron volt (TeV). We determined the acceleration limit at about 100 TeV, beyond which the electron/positron flux suppresses exponentially, while we measured the morphology of the γ-ray halo at three different energies. We also found that the energy dependence of the diffusion coefficient is consistent with the Kolmogorov magnetohydrodynamic (MHD) turbulence and determined the turbulence property at scales smaller than 1 parsec. The measurements indicate that the energy of electrons/positrons is insufficient to amplify the strong MHD turbulence around itself, thereby suppressing the diffusion coefficient.