What Geminga’s 100 TeV cutoff may mean for cosmic-ray acceleration in the Milky Way

For the first time, the Tibet ASγ Experiment has successfully measured magnetohydrodynamic (MHD) turbulence on scales below one parsec (approximately 3.3 light-years) within the gamma-ray halo surrounding the Geminga pulsar wind nebula (PWN). This observation extends to the highest energies, above 100 tera-electron volts (TeV), providing new insights into the behavior of cosmic rays and magnetic fields within the Milky Way.

The findings are published in Science Advances . The study was conducted by the Tibet ASγ Experiment, including the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS) and the National Astronomical Observatories of CAS.

Geminga’s role in cosmic-ray physics

Geminga, a nearby ancient pulsar located about 250 parsecs (approximately 800 light-years) from Earth, serves as an ideal laboratory for simultaneously exploring the acceleration and propagation mechanisms of cosmic rays. The researchers observed the energy spectrum of electrons/positrons injected by the Geminga PWN and discovered a cutoff around 100 TeV. This marks the first direct evidence indicating that the acceleration limit for electrons in this system is approximately 100 TeV.

Furthermore, the researchers measured the spatial extent of the gamma-ray halo across an energy range from approximately 16 TeV to 250 TeV. They found that the diffusion coefficient near Geminga was only about 1% of the average value in the Milky Way’s galactic disk, indicating that particle diffusion is strongly suppressed in this region.

Remarkably, the turbulence spectrum inferred from these measurements follows a Kolmogorov-type scaling law, inferred from measurements on much larger scales in the galactic halo, and persists even at these minuscule scales. The turbulent properties derived from the Geminga halo are consistent with extrapolations from larger-scale galactic disk measurements, providing for the first time an experimental determination that the MHD turbulence in the region surrounding the Geminga PWN follows Kolmogorov-type turbulence.

This is the first experimental determination of MHD turbulence characteristics on scales below one parsec.

Implications for galactic turbulence and cosmic rays

These findings are significant for understanding the acceleration and propagation of cosmic rays (electrons and positrons) in the Milky Way.

They suggest that the galactic disk exhibits stronger magnetic turbulence compared to the halo. They also suggest that the origin of the strong turbulence around the Geminga PWN might be an environmental effect, opening new avenues for understanding the complexity of cosmic ray propagation.

Tibet ASγ Experiment and future prospects

The Tibet ASγ Experiment is located in Yangbajing Town, in China’s Xizang Autonomous Region, at an altitude of 4,300 meters above sea level. It has been operated jointly by China and Japan since 1990. The underground muon detectors suppress 99.92% of the cosmic-ray background noise, significantly improving sensitivity. The experiment can detect gamma rays above 100 TeV from the Geminga PWN with high precision under low background noise.

This work marks a significant step toward understanding cosmic-ray propagation and magnetic-field dynamics in the universe. The findings are expected to have far-reaching implications for future multi-messenger and high-energy gamma-ray studies.