{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"f0f6b65dbe19e39dbf48c774e81ba40c941e5b6d1c26ebc73f37b0da78af8c1b","created":"2026-10-03T07:17:54Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"6196fee46794b06db468e197877b7411e9b56451bd61fc53329b2ba15f7e49cd","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"astro.cosmic-ray-origins.knee-origin","kind":"phenomenon","literature_status":"partially-resolved","n":"1","parents":[],"plain":"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.","posed_since":"1958","precise":"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.","problem_ref":null,"references":"","settled_by":"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_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.","title":"What produces the cosmic-ray knee near 3 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