At 100 Trillion Electron Volts, a Pulsar's Halo Reveals How Cosmic Rays Move Through the Galaxy
The question of where cosmic rays come from - and how they travel through the galaxy without scattering wildly in every direction - has occupied physicists for more than a century. A set of new measurements from a high-altitude detector in Tibet's Yangbajing basin is adding a crucial piece to that puzzle.
The Tibet ASgamma Experiment, operating at 4,300 meters above sea level since 1990, has now made the first direct measurement of magnetohydrodynamic turbulence at scales below one parsec - roughly 3.3 light-years - by analyzing the gamma-ray halo surrounding a pulsar called Geminga. The findings, published March 4 in Science Advances, push fragmentome-style thinking about cosmic environments into genuinely new territory.
Why Geminga is an ideal target
Geminga is an ancient, nearby pulsar sitting about 800 light-years from Earth. Pulsars are the collapsed remnants of massive stars that exploded as supernovae, spinning rapidly and flinging energetic particles into the surrounding space through what is called a pulsar wind nebula (PWN). Electrons and positrons from Geminga's PWN collide with background photons and boost them up to gamma-ray energies, creating a detectable glow around the pulsar - a gamma-ray halo.
That halo is scientifically interesting because its size and energy profile encodes information about how the particles are moving. If particles diffuse rapidly, the halo spreads wide. If something is inhibiting their movement, the halo stays compact. Geminga's halo is conspicuously compact - which suggests something is suppressing particle diffusion in that region.
Diffusion 100 times slower than the galactic average
The Tibet team measured the spatial extent of the Geminga halo across an energy range from approximately 16 TeV to 250 TeV. What they found was striking: the diffusion coefficient near Geminga was only about 1 percent of the average value in the Milky Way's galactic disk. Particles there are moving through space at a tiny fraction of the rate typical for the galaxy at large.
They also detected a cutoff in the energy spectrum of electrons and positrons injected by the Geminga PWN at around 100 TeV. That constitutes the first direct evidence pinning the acceleration limit for electrons in this system - confirming that the pulsar can push particles to that energy but not significantly beyond.
Kolmogorov turbulence, verified at the smallest scales ever measured
The most theoretically significant finding concerns the character of the magnetic turbulence itself. The turbulence spectrum inferred from the halo measurements follows what is known as a Kolmogorov-type scaling law - the same mathematical relationship that describes how energy cascades through turbulent fluids, from large eddies down to small ones.
Kolmogorov turbulence has been inferred from cosmic-ray propagation data on scales of hundreds to thousands of parsecs - the broad swaths of the galactic disk and halo. The Tibet experiment's measurements now show that the same law holds at scales below one parsec, in the immediate vicinity of a single pulsar. That is an extrapolation of roughly eight to nine orders of magnitude in scale, and it holds up.
This is the first experimental determination of MHD turbulence characteristics at sub-parsec scales in astrophysical settings. The turbulent properties derived from the Geminga halo are consistent with predictions extrapolated from much larger-scale observations, but no one had verified the connection with actual data before.
What this means for understanding cosmic rays
Cosmic rays - high-energy charged particles that rain down on Earth from space - are accelerated somewhere in the galaxy and must travel enormous distances before reaching us. How they propagate through interstellar magnetic fields determines what we ultimately detect. The extreme suppression of diffusion around Geminga suggests that the galactic disk has stronger magnetic turbulence than the galactic halo, and that turbulence near pulsars might be partly an environmental effect tied to the pulsar's influence on its surroundings.
The measurement does not immediately settle all of the debates about cosmic-ray origins, but it provides an unusually clean experimental constraint on the physical conditions that govern particle transport. The China-Japan collaboration running the Tibet array, which uses underground muon detectors to suppress 99.92 percent of cosmic-ray background noise, has demonstrated that sub-parsec turbulence physics is measurable from the ground at the highest energies.
Future multi-messenger observations - combining gamma rays with neutrino and gravitational wave data - may use Geminga and other nearby pulsars as precision probes of magnetic structure across the galaxy. The turbulence laws that the Tibet team has now anchored at small scales could serve as a calibration point for those efforts.