Cosmic rays, those elusive particles from outer space, continuously bombard Earth. They arrive from various directions with different energies, some gentle and others packing a powerful punch. Scientists use specialized detectors to study these high-energy rays, hoping to unravel their origins and understand their nature.

Cosmic Rays Above 5 EeV

At lower energy levels, cosmic rays exhibit a smoother distribution, often influenced by the Milky Way’s magnetic fields. However, once they surpass the 5 EeV threshold, their behavior changes dramatically.

Originating from distant regions outside our galaxy, these high-energy cosmic rays show patterns that defied explanations from our cosmic neighborhood alone. According to Jonathan Biteau of the Academic Institute of France’s Pierre Auger Collaboration, this energy range is critical for gaining deeper insights into their sources.

Observing Signs in the Data

Analyses of cosmic ray patterns across the sky reveal distinct features that become evident at higher energies. The energy spectrum shows notable shifts known as the ankle near 5 EeV, the instep around 15 EeV, and the toe around 45 EeV, indicating changes in their composition and origins.

“Cosmic rays begin to reveal their secrets at energies above 5 EeV,” commented Biteau.

Clues to Sources of Cosmic Rays

Studies suggest that heavier nuclei dominate at higher energies, implying different acceleration processes and astrophysical environments. As energy increases, nonuniformities in the sky coverage, known as anisotropy, become more apparent.

Research indicates a large-scale anisotropy above a few EeV, peaking around 7 σ above 8 EeV, strongly suggesting extragalactic origins.

One Step Closer

Higher energy cosmic rays are less scattered by magnetic fields, making their original directions easier to trace. Emerging evidence points to extragalactic sources such as star-forming galaxies or other intense astrophysical sites.

Data suggest that active regions far beyond our galaxy may be linked to the arrival patterns of these high-energy rays.

Shifts in Cosmic Rays’ Composition

The composition of cosmic rays changes significantly above 5 EeV. Instead of predominantly light nuclei like hydrogen and helium, they are often heavier, involving carbon or oxygen.

This unusual distribution of elements contradicts earlier assumptions and challenges researchers to rethink their models.

What Holds the Key to Their Birthplaces?

Even though these rays travel vast distances, their paths are slightly bent by magnetic fields along the way. Scientists model how different atomic nuclei respond to these magnetic fields to understand the origins of cosmic rays.

If patterns persist, pointing to specific regions of space, this would support those areas as potential cosmic ray generators.

No Single Culprit Yet

Active galactic nuclei, fueled by supermassive black holes, are considered potential candidates. However, current evidence suggests that star-forming galaxies align better with the observed patterns, though other possibilities remain.

Bridging Theory and Observations

The shifts in composition and patterns above 5 EeV challenge existing theories. New models must explain these phenomena, offering a clearer understanding of what generates these cosmic rays.

Ongoing Upgrades and the Future

As detectors improve and data accumulate, researchers hope to refine energy calibration, measure more showers, and achieve higher accuracy. Each discovery adds a piece to the cosmic puzzle.

Identifying correlations with specific galaxy groups could pinpoint the actual sources. Until then, each measurement keeps the quest for answers alive.

The research was published in the Physical Review Journals.