What produces the cosmic-ray knee near 3 PeV?
In plain words
The number of cosmic rays falls off faster above about $3e15\,\mathrm{eV}$, a feature called the knee, which may mark the highest energy the Milky Way's particle accelerators reach.
Precise statement
The all-particle spectrum steepens from dN/dE ~ E^-2.7 to ~E^-3.1 near $E \sim 3-4\,\mathrm{PeV}$, and LHAASO finds that the proton spectrum hardens and then breaks sharply near 3 PeV. Determine which Galactic source class (supernova remnants, young massive star clusters, microquasars, the Galactic center) accelerates protons to at least 3 PeV, and whether the knee is a source maximum-rigidity feature ($E_{\mathrm{max}}$ proportional to charge $Z$) or a propagation feature. Answer: an identified PeVatron class with an acceleration model reproducing the proton and helium knee positions.
What would settle it
Composition-resolved spectra of protons, helium and heavier groups up to 100 PeV showing rigidity-ordered breaks, plus spatial identification of PeV proton sources through hadronic gamma rays above 100 TeV or neutrinos.
Status in the literature
Unverified note
2025: LHAASO measured a proton-only knee at about 3 PeV after a spectral hardening, supporting a rigidity-dependent source cutoff; the source class is unidentified.