Seeking the Sources of Ultra-High-Energy Cosmic Rays

Where do the highest-energy particles in the universe come from? New research suggests that the sources of ultra-high-energy cosmic rays aren’t necessarily the sources of ultra-high-energy photons as well.

Cosmic Rays Across the Universe

Across the universe, extremely energetic charged particles called cosmic rays zoom through space. These particles are usually protons or the bare nuclei of helium atoms, but they can also be electrons, the nuclei of atoms heavier than helium, or other particles like positrons.

Exactly where these particles are accelerated to nearly the speed of light is an open question. One clue to the origin of the most energetic cosmic rays — ultra-high-energy cosmic rays — is that these particles are not distributed evenly across the sky. Any proposed source of ultra-high-energy cosmic rays, like supernovae, gamma-ray bursts, or other highly energetic cosmic beacons, must be able to explain this distribution.

Distribution of resolved extragalactic gamma-ray sources used in this work

Distribution of resolved extragalactic gamma-ray sources used in this work. The color bar indicates the gamma-ray flux of each source. [Partenheimer et al. 2024]

From Energetic Photons to Energetic Particles

Angelina Partenheimer (University of Wisconsin) and collaborators investigated the possibility that the highest-energy cosmic rays and the highest-energy photons have the same source. To test this hypothesis, the team constructed a sample of resolved gamma-ray sources with energies between 50 megaelectronvolts and 1 teraelectronvolt. They then modeled the distribution of ultra-high-energy cosmic rays that might be emitted by this collection of gamma-ray sources.

They used two scenarios to model the cosmic-ray distribution. In the first scenario, the cosmic-ray flux scales with the gamma-ray flux, meaning that sources that are brighter in gamma rays also produce more cosmic rays. In the second scenario, each gamma-ray source appears equally bright in cosmic rays from our vantage point. While this doesn’t reflect reality — it would imply that more distant gamma-ray sources produce more cosmic rays — this treatment may help correct for the fact that catalogs of gamma-ray sources are increasingly incomplete at larger distances.

projection of the modeled cosmic-ray distribution

The modeled cosmic-ray flux for the scenario in which the sources are uniformly bright. The resulting dipole for this scenario is roughly five times larger than what has been observed. [Partenheimer et al. 2024]

Cosmic-Ray Bright, Gamma-Ray Dim?

In both scenarios, the modeled cosmic-ray distribution is far more uneven than what has been observed. Much of the unevenness comes from the extremely bright gamma-ray source Markarian 421, which helps to produce a dipole in the cosmic-ray distribution 5–10 times larger than what has been observed. This suggests that resolved gamma-ray sources alone cannot be the sources of ultra-high-energy cosmic rays; many more sources are needed to balance out the few extremely luminous objects.

Partenheimer’s team found that roughly 80,000 “missing” sources are needed to match the observed distribution of ultra-high-energy cosmic rays. This is far larger than the known population of resolved gamma-ray sources, which could mean that the producers of the highest-energy cosmic rays are unresolved gamma-ray sources. Alternatively — and perhaps surprisingly — the sources of ultra-high-energy cosmic rays might not produce gamma rays at all!


“Ultra-High-Energy Cosmic-Ray Sources Can Be Gamma-Ray Dim,” Angelina Partenheimer et al 2024 ApJL 967 L15. doi:10.3847/2041-8213/ad4359