Milky Way passes the baton: seamless transition of cosmic rays ✦
The spectra of cosmic rays (streams of atomic nuclei from space) have characteristic breaks: two sharp declines ('knees') and three flattenings ('ankles'). Their energies depend on the nuclear charge, not always linearly: for example, the second knee occurs at the same energy for different elements. Precise measurements by the DAMPE and LHAASO facilities have for the first time clarified the picture: all features are well described by the sum of three components—two galactic and one extragalactic. Moreover, no additional sources are needed between 10 PeV and 1 EeV. Surprisingly, the complex structure of the spectrum reduces to a simple sum.
The mystery of the main break in the spectrum of galactic cosmic rays—the knee at 3 PeV—has persisted for nearly a century. Back in the 1930s, Fritz Zwicky suggested that particles are accelerated by shock waves from supernovae, but only precise modern data have allowed us to peek behind the curtain.
Imagine a symphony orchestra. The first movement is a chamber introduction: unhurried proton and helium motifs from old supernova remnants. Their spectrum is curved by a double log-parabola with an exponential cutoff, just like a soloist on an antique instrument embellishing the melody with virtuosic fiorituras. Then the orchestra gains power—the full tutti of the second galactic component enters, and with it the tension builds right up to the knee. Here the conductor-accelerator reaches its limit, and the baton falls from its hands. Without pause, attacca, the third theme bursts in—extragalactic jets from active nuclei and cosmic structures. Protons above 100 PeV no longer belong to our orchestra; this is music from distant worlds.
The new analysis draws on data from the DAMPE satellite, ground-based LHAASO and IceTop, as well as high-resolution spectroscopy. Three components—F1, F2, and F3—describe the spectra from 40 GeV to 500 PeV with astonishing precision. The 'ankles' (breaks toward flattening at 500×Z GeV and ~150 TeV for protons) are no mysticism at all, but simply the intersection of populations. No magic, pure superposition. Helium deviates slightly: it may need a slightly stronger cutoff of the first component, but the uncertainties still leave room. What is captivating is that nature avoided a sharp break, creating an almost watercolor-like transition between galactic and extragalactic rays. Importantly, no third galactic source was needed—the baton is passed immediately after the PeV knee.
This seamless handoff forces us to reconsider the Milky Way's role as an accelerator. It was once thought that galactic rays reached the famous 'ankle' at 4 EeV. Now it's clear: the proton conveyor stops at about 100 PeV. Future measurements of carbon and oxygen at LHAASO and the SWGO observatory will serve as a test. Then we will understand how precisely tuned the three-part symphony is. And if anisotropy in the 0.1–10 PeV range points to specific nearby sources, the theory of non-linear shock acceleration will receive long-awaited confirmation. Cosmic rays will cease to be mysterious noise—they will fall onto a score where every note has its own source.
🎯 A proton with an energy of 1 EeV in a typical galactic magnetic field (3 μG) has a Larmor radius of about 300 parsecs—almost half the disk thickness, so the Galaxy cannot contain such particles, and they almost certainly come from beyond its borders.