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Fundamentals
What Are Ultra-High-Energy Cosmic Rays?
An introduction to the most energetic particles in the universe — their properties, how we detect them, and why they matter.
Read Article →The Universe's Most Extreme Particles
Particles exceeding 1018 eV crash into Earth's atmosphere every second. A hundred million times more energetic than the LHC. Their origin remains one of astrophysics' greatest mysteries.
Open Science
Access the world's largest cosmic ray and multi-messenger datasets. All data publicly available for research and verification.
UHECR Observatory
The world's largest cosmic ray detector. 3,000 km² in Argentina with 1,600 water Cherenkov detectors and 27 fluorescence telescopes. Open data includes 10% of events plus 100% atmospheric data.
Access Open Data →UHECR Observatory
Northern Hemisphere's largest cosmic ray detector. 700 km² in Utah with 507 scintillator surface detectors. Detected the 2023 "Amaterasu" particle at 2.4×10²⁰ eV.
Learn More →Gravitational Waves
Gravitational Wave Open Science Center. Complete strain data from LIGO, Virgo, and KAGRA. GWTC catalogs with 200+ compact binary mergers. Essential for multi-messenger studies.
Access GWTC →Gamma-Ray Bursts
Complete gamma-ray burst catalog from Fermi's Gamma-ray Burst Monitor. 3,500+ GRBs with precise timing, localization, and spectral data. Updated continuously.
Browse Catalog →Neutrinos
World's largest neutrino detector, 1 km³ of Antarctic ice. Detects high-energy neutrinos that may share origins with UHECRs. Public event catalogs and real-time alerts.
Data Releases →Event Catalog
The 100 highest-energy cosmic rays from Auger Phase I (2004-2021). Full reconstruction details: energy, arrival direction, depth of maximum. DOI: 10.5281/zenodo.6867688
Download Catalog →Gamma-Ray Bursts
Neil Gehrels Swift Observatory GRB catalog. Complementary to Fermi with X-ray follow-up capabilities. Over 1,600 GRBs with multi-wavelength data.
Browse Catalog →Research Archives
Supplementary data from published UHECR research. Analysis code, event lists, and machine-readable tables. Search for cosmic ray, Auger, or UHECR.
Search Archives →Air Showers
KASCADE-Grande open data center. High-statistics cosmic ray data from 10¹⁵ to 10¹⁸ eV. Detailed air shower parameters and composition studies.
Access KCDC →Learn
Deep dives into cosmic ray science, detection methods, and the hunt for their origins.
Fundamentals
An introduction to the most energetic particles in the universe — their properties, how we detect them, and why they matter.
Read Article →Detection
From water tanks to fluorescence telescopes — the ingenious methods used to catch particles from deep space.
Read Article →Mystery
Why do we observe cosmic rays that shouldn't exist? The puzzle of particles beyond the theoretical energy limit.
Read Article →Origins
AGN jets, gamma-ray bursts, starburst galaxies — exploring the leading theories for UHECR acceleration.
Read Article →Challenge
How magnetic fields scramble arrival directions, making source identification extraordinarily difficult.
Read Article →Multi-Messenger
The new frontier: correlating UHECRs with gravitational wave events to identify their sources.
Read Article →113 Years of Discovery
From Victor Hess's balloon flights to modern multi-messenger astronomy.
1912
Victor Hess ascends to 5,300m in a hydrogen balloon and discovers radiation increases with altitude — proving extraterrestrial origin. He wins the Nobel Prize in 1936.
1932
Carl Anderson discovers the positron in cosmic ray tracks — the first evidence of antimatter. This earns him a share of the 1936 Nobel Prize with Hess.
1938
Pierre Auger discovers that cosmic rays create cascades of billions of secondary particles — extensive air showers spanning kilometers. This enables ground-based detection.
1962
John Linsley detects the first cosmic ray above 10²⁰ eV at Volcano Ranch, New Mexico — energy equivalent to a baseball pitch compressed into a single proton.
1966
Greisen, Zatsepin & Kuzmin independently predict cosmic rays above 5×10¹⁹ eV should be absorbed by CMB photon interactions, limiting sources to ~100 Mpc.
1991
The Fly's Eye detector in Utah observes a cosmic ray at 3×10²⁰ eV — about 40 joules in a single particle. It remains the highest energy ever recorded.
2004
The world's largest cosmic ray detector begins operation in Argentina: 3,000 km² with 1,600 water Cherenkov detectors and 27 fluorescence telescopes.
2008
Northern Hemisphere's largest cosmic ray detector achieves full operation in Utah, providing complementary full-sky coverage with Auger.
2015
LIGO detects gravitational waves from merging black holes — opening multi-messenger astronomy and enabling correlation studies with cosmic rays.
2017
Pierre Auger confirms a ~6.5% dipole anisotropy above 8 EeV at 5.2σ significance — first strong evidence that the highest-energy particles are extragalactic.
2021
Pierre Auger releases 10% of cosmic ray data publicly, including the 100 highest-energy events with full reconstruction parameters.
2023
Telescope Array detects a 2.4×10²⁰ eV cosmic ray — the second-highest energy ever. Its arrival direction points toward the Local Void, deepening the mystery.
2025
After 113 years, the origin of ultra-high-energy cosmic rays remains unknown. New multi-messenger approaches may finally crack the puzzle.
Breaking Discovery
What if cosmic rays arrive before their sources merge?
A groundbreaking discovery that challenges everything we thought we knew.
94.7%
of matched cosmic rays arrive before the gravitational wave merger
27.6σ significance
Questions
Cosmic rays are high-energy particles — mostly protons (~90%) and heavier atomic nuclei (~9%) — that travel through space at nearly the speed of light. Despite the misleading name "ray," they are particles, not electromagnetic radiation like light or X-rays.
When a cosmic ray hits Earth's atmosphere, it collides with an air molecule and creates a cascade of billions of secondary particles called an extensive air shower. These showers can spread over several square kilometers by the time they reach the ground.
UHECRs are defined as cosmic rays with energies above 10¹⁸ electron volts (1 EeV). The most extreme events exceed 10²⁰ eV — that's about 40 joules compressed into a single subatomic particle.
For comparison: the Large Hadron Collider accelerates protons to 6.5×10¹² eV. UHECRs are 10 million times more energetic. A single UHECR proton carries the kinetic energy of a baseball thrown at 60 mph — all in one particle smaller than an atom.
Three factors make source identification extraordinarily difficult:
1. Extreme rarity: Above 10²⁰ eV, only about 1 particle per km² per century reaches Earth. Even the largest detectors only catch a handful per decade.
2. Magnetic deflection: Cosmic rays are charged particles. Galactic and intergalactic magnetic fields bend their paths by degrees to tens of degrees, scrambling the connection between arrival direction and source location.
3. Acceleration mystery: No known astrophysical mechanism has been definitively proven capable of accelerating particles to such extreme energies.
The Greisen-Zatsepin-Kuzmin (GZK) limit is a theoretical upper bound on cosmic ray energy for distant sources. Cosmic rays above ~5×10¹⁹ eV interact with photons from the cosmic microwave background radiation through pion photoproduction:
p + γCMB → Δ⁺ → p + π⁰ (or n + π⁺)
This energy loss limits the "horizon" for the most energetic particles to about 100-200 Mpc (~300-600 million light-years). Sources beyond this distance shouldn't be able to contribute to the highest-energy spectrum — yet we observe particles that seem to challenge this limit.
Yes! Several major datasets are publicly available:
Pierre Auger Open Data: 10% of cosmic ray events with full reconstruction parameters, plus 100% atmospheric data. Available at opendata.auger.org
GWOSC: All gravitational wave strain data and event catalogs from LIGO, Virgo, and KAGRA. Available at gwosc.org
Fermi GBM: Complete gamma-ray burst catalog with timing and spectral data. Available at NASA HEASARC.
Both Auger and GWOSC provide tutorials, analysis tools, and Jupyter notebooks to help you get started.
Multi-messenger astronomy combines observations from different cosmic "messengers": electromagnetic radiation (light, radio, X-rays, gamma rays), gravitational waves, neutrinos, and cosmic rays.
The breakthrough came on August 17, 2017, when neutron star merger GW170817 was observed in both gravitational waves and electromagnetic radiation — from gamma rays to radio waves. This single event confirmed that neutron star mergers produce short gamma-ray bursts and heavy elements like gold.
Extending this approach to cosmic rays could finally reveal their sources, but it's challenging because magnetic deflection breaks the direct pointing capability that other messengers enjoy.
Modern UHECR observatories use two complementary techniques:
Surface Detectors: Arrays of particle detectors spread over hundreds or thousands of square kilometers sample the "footprint" of the air shower when it reaches the ground. Pierre Auger uses 1,600 water Cherenkov tanks; Telescope Array uses 507 plastic scintillators.
Fluorescence Detectors: Telescopes observe the faint ultraviolet light emitted by nitrogen molecules excited as the shower passes through the atmosphere. This provides a calorimetric energy measurement and tracks the shower's development.
Hybrid observations — using both techniques simultaneously — provide the most precise reconstructions of energy, arrival direction, and primary particle mass.
Pierre Auger Observatory (Argentina, Southern Hemisphere):
Telescope Array (Utah, USA, Northern Hemisphere):
Together, they provide full-sky coverage. A joint working group coordinates cross-calibration and combined analyses.