What are UHECRs?

Ultra-high-energy cosmic rays (UHECRs) are charged particles — protons and heavier atomic nuclei — that arrive at Earth with kinetic energies above about 10¹⁸ eV (1 EeV). The most energetic events ever recorded exceed 10²⁰ eV: a single subatomic particle carrying roughly the kinetic energy of a well-hit tennis ball, some ten million times more energy than the beams of the Large Hadron Collider.

How rare are they?

The cosmic-ray flux falls steeply with energy. At 10²⁰ eV, fewer than one particle per square kilometre per century reaches Earth. This extreme rarity is why UHECR observatories must instrument enormous areas — thousands of square kilometres — to collect a meaningful number of events.

The energy spectrum

The cosmic-ray energy spectrum spans more than ten decades in energy and falls roughly as a power law. Several features stand out at the highest energies:

• The ankle (~5×10¹⁸ eV) — a hardening of the spectrum, commonly interpreted as the transition to an extragalactic population or a signature of proton pair-production on cosmic photon backgrounds.
• The instep (~10¹⁹ eV) — a softening feature revealed by high-statistics measurements of the Pierre Auger Observatory.
• The suppression (~5×10¹⁹ eV) — a strong steepening of the flux, consistent with the GZK effect (interaction of protons with the cosmic microwave background) and/or with the maximum energy attainable at the sources.

Why extragalactic?

Above a few EeV, the gyroradius of a proton in the Galactic magnetic field exceeds the thickness of the Milky Way disk — the Galaxy simply cannot confine such particles. Together with the observed large-scale dipole anisotropy above 8 EeV, which points well away from the Galactic centre, this establishes that UHECRs at the highest energies come from outside our Galaxy.

What accelerates them?

That is one of the central open questions of astroparticle physics. Candidate sources must satisfy the Hillas criterion — they must be large enough and magnetized enough to confine particles during acceleration. Leading candidates include active galactic nuclei and their jets, radio galaxies, starburst galaxies, gamma-ray bursts, and tidal disruption events.