ASTRO In the literature: contested

Is the cosmic-ray flux cutoff above $4e19\,\mathrm{eV}$ a GZK effect?

In plain words

Very few cosmic rays arrive above about 4e19 eV, either because they lose energy on the leftover light of the Big Bang (the GZK effect) or because their sources cannot push them higher.

Precise statement

Determine whether the suppression of the ultra-high-energy cosmic-ray flux at $E > \sim 4e19\,\mathrm{eV}$ is mainly due to propagation losses (photopion production on the cosmic microwave background for protons, photodisintegration for nuclei) or to source maximum rigidity of a few $1e18\,\mathrm{V}$ with composition getting heavier at higher energy. Answer: yes or no for a dominant GZK origin, with the proton fraction above $5e19\,\mathrm{eV}$ and the implied cosmogenic neutrino flux.

What would settle it

A composition measurement above $5\mathrm{e}19\,\mathrm{eV}$ with per-event mass resolution, and detection or exclusion of the cosmogenic neutrino flux at $1\mathrm{e}17 \text{ to } 1\mathrm{e}19\,\mathrm{eV}$.

Status in the literature

Auger shower-depth data favor a mixed composition that becomes heavier with energy and a source cutoff, while Telescope Array reports spectral differences from Auger at the highest energies.