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Title: Which sources accelerate cosmic rays above $1e19\,\mathrm{eV}$?
Plain: The most energetic cosmic rays come from outside our galaxy, but magnetic fields bend their paths, so their sources have not been identified.
Precise: For $E > 8e18\ \mathrm{eV}$ (Auger large-scale dipole, amplitude $\sim 6.5\ \text{percent}$, pointing away from the Galactic center) and $E > 4e19\ \mathrm{eV}$ (intermediate-scale excesses near Centaurus A and starburst galaxies for Auger, the hotspot for Telescope Array), identify the source population (AGN jets, starburst galaxies, gamma-ray bursts, tidal disruption events, magnetars). Answer: a source class whose emissivity, maximum rigidity and composition reproduce the spectrum, the shower-depth ($X_{\mathrm{max}}$) distributions and the arrival-direction anisotropies after propagation through Galactic and extragalactic magnetic fields.
Settled by: Per-event mass estimates above $4e19\,\mathrm{eV}$ (AugerPrime) allowing light nuclei to be traced back, together with a $5\sigma$ intermediate-scale anisotropy associated with a source catalog.
Kind: phenomenon
Answer type: identification
References: (empty)
Literature status: open
Status note: 2023: Telescope Array reported a $2.4e20\ \mathrm{eV}$ event (Amaterasu) whose arrival direction points toward the Local Void with no plausible nearby source.
Posed since: (empty)