X Marks the Spot: A Treasure Map for High-Energy Cosmic Rays

Researchers have explored the best places to search for ultra-high-energy cosmic rays: charged particles that can travel at nearly the speed of light. New cosmic ray “treasure maps” bring us one step closer to tracking down the origins of these rare particles.

Cosmic Particle Accelerators

diagram of the particle shower created when a cosmic ray enters Earth's atmosphere

A diagram illustrating the shower of particles created when a cosmic ray enters Earth’s atmosphere. Click to enlarge. [CERN]

Across the universe, cosmic rays are accelerated to extraordinary velocities. These speedy particles are mostly protons or the nuclei of helium atoms, with electrons and the nuclei of atoms heavier than helium rounding out the population. When these particles reach Earth, they can make quite a splash — the highest-energy cosmic ray ever detected had an energy of 320 exaelectronvolts (that’s 3.2×1020 electronvolts!) and earned the moniker the “Oh-My-God particle.”

Where in the universe cosmic rays reach their extreme speeds is still up for debate, though supernovae, accreting supermassive black holes, highly magnetized stellar remnants called magnetars, and gamma-ray bursts are all possible sites of cosmic-ray acceleration. Complicating the hunt for these sites is the fact that after cosmic rays are shot into space, they’re buffeted and misdirected by a tangled web of magnetic fields. As a result, where we see a cosmic ray come from might not be where it actually came from.

plot showing simulation results for the detectability of iron nuclei and nitrogen nuclei for sources of varying distance.

The loss-of-number density, aGZK, as a function of the distance of the cosmic-ray source, shown for nitrogen nuclei (dot-dashed green line) and iron nuclei (dashed orange line) with energies greater than 150 exaelectronvolts. The source distance at which 95% of particles fail to reach Earth is marked for each species with an arrow. In this simulation, the detector is sensitive to particles with masses greater than 12 atomic mass units. [Adapted from Globus et al. 2023]

Looking for Paired Particles

In a recent research article, a team led by Noémie Globus (University of California, Santa Cruz, and the Institute of Physical and Chemical Research, Japan) suggested that we may be able to identify the sites of cosmic-ray acceleration by detecting two cosmic rays from the same source arriving from the same direction at the same time. Magnetic fields between the cosmic-ray source and Earth make this kind of coordination unlikely. Globus and collaborators proposed that these paired particles might arrive more often from some parts of the sky than others.

The team focused on cosmic rays with energies in excess of 150 exaelectronvolts, about two of which are caught by our cosmic ray detectors each year. In addition to considering how magnetic fields affect the passage of these particles, the team also considered which cosmic rays are most likely to survive the journey to Earth. As cosmic rays zip through space, they interact with photons from the cosmic microwave background that suffuses the universe. This interaction strips protons and neutrons away from the cosmic ray, reducing its mass and energy. This means that the distance a cosmic ray can travel before being disassembled — and therefore, the distance of the cosmic-ray sources we can detect — depends on its initial mass and energy.

Treasure Maps for Rare Cosmic Rays

An example treasure map for a nitrogen nucleus with an energy of 150 exaelectronvolts. The projection is centered on the galactic anticenter (GA), while the galactic center (GC) appears at the right and left sides. [Adapted from Globus et al. 2023]

By modeling how cosmic rays of different masses, energies, and source locations navigate the magnetic fields within the Milky Way and beyond it, Globus and coauthors determined the likeliest places on the sky to search for paired cosmic rays. These “treasure maps” show that the detection probability depends on the location of the cosmic-ray observatory, the location on the sky, and the mass of the cosmic-ray particle.

In addition to suggesting where to look, the team notes that their work can be used to guide how to look. Because a cosmic ray’s mass determines how far it can travel before being destroyed, building detectors that can measure the mass of cosmic rays reaching Earth can help us pinpoint where the particles originated.


“Treasure Maps for Detections of Extreme Energy Cosmic Rays,” Noémie Globus et al 2023 ApJ 945 12. doi:10.3847/1538-4357/acaf5f