Strange, high-energy cosmic rays: Did something outside our galaxy send them here?

Cosmic rays are high-speed particles that move through space and crash into our atmosphere from every direction. These energetic particles, mostly atomic nuclei stripped of their electrons, have puzzled scientists for decades.

They hit Earth from different places with a wide range of energies. Some arrive at moderate levels, while others carry so much punch that researchers use special detectors stretching over vast areas to understand their true nature.

Cosmic rays above 5 EeV

Particles carrying more than about five exa-electronvolts (EeV) of energy stand out because they mark a turning point.

At lower energies, cosmic rays often show a smoother distribution, and are strongly influenced by magnetic fields in our own galaxy.

Once they cross the 5 EeV threshold, however, their behavior and properties change. They begin to show patterns that cannot be explained by local sources alone.

Instead of originating mostly from within the Milky Way, they appear to come from distant corners of the cosmos that lie far beyond our familiar galactic neighborhood.

Studies indicate that at these energies, called the ankle region, the usual mixture of mostly light nuclei, like protons, gradually gives way to heavier particles that are less scrambled by galactic magnetic fields.

According to Jonathan Biteau, from the Academic Institute of France’s Pierre Auger Collaboration, this energy range represents a step into a more revealing territory.

Observing signs in the data

Changes in the large-scale patterns of cosmic rays seen across the sky start to stand out and offer clues to their sources.

This is where the cosmic ray story moves beyond a random spray of particles and hints at something else: extragalactic origins with distinct characteristics.

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

Measurements gathered over many years at facilities like the Pierre Auger Observatory show that once energies climb past this point, a pattern emerges in the shape of the energy spectrum.

Distinct features, known as the ankle near 5 EeV, the instep at about 15 EeV, and the toe around 45 EeV, mark noticeable shifts in the composition of the flux.

Clues to sources of cosmic rays

Observations indicate that heavier nuclei start dominating the picture at higher energies. These changes tell researchers that different acceleration processes and astrophysical environments might be at play.

As energies increase, the sky coverage reveals that some directions supply more cosmic rays than others. This nonuniformity, known as anisotropy, has become more apparent.

Over the past decade, scientists have reported that the pattern of cosmic-ray arrival directions changes in a way that strongly suggests extragalactic sources.

Research shows a large-scale anisotropy above a few EeV rising to a significance of about 7 σ above 8 EeV.

One step closer

Pinpointing the birthplaces of these high-energy particles has proved challenging, but scientists continue to gather clues.

As cosmic rays push into these higher energy realms, they are less scattered by magnetic fields, making it easier to identify their original directions.

Emerging evidence supports the idea that many of these ultra-high-energy rays are launched by extragalactic sources like star-forming galaxies or other intense astrophysical sites.

The data hint that active regions far beyond our galaxy could be connected to the observed arrival patterns.

Researchers also notice that certain sky regions, like those associated with clusters of nearby galaxies, line up better with the arrival directions than a purely uniform spread would allow.

Shifts in cosmic rays’ composition

Stepping above 5 EeV not only shines a light on extragalactic origins but also shows that the “stuff” hitting us at these energies is different.

Instead of mostly light nuclei like hydrogen and helium, many cosmic rays are heavier, involving atoms like carbon or oxygen.

This suggests that whatever “engines” are producing them, they must either selectively accelerate heavier particles or conditions in their environment alter the mix by the time they reach Earth.

The surprising scarcity of lighter elements runs counter to earlier guesses and requires fresh thinking from theorists.

What holds the key to their birthplaces?

As these rays travel huge distances, their paths bend slightly in magnetic fields along the way. Even tiny bends can shuffle their directions enough to complicate efforts to pinpoint sources.

To address this, scientists model how nuclei of different masses respond to magnetic fields. If the rays still show a pattern that points to certain neighborhoods in space, that strengthens the argument that those regions contain the engines fueling these incredible energies.

No single culprit yet

Many questions remain. Some experts suggest star-forming galaxies as prime suspects since these environments can host energetic events that might whip particles up to extreme energies.

Others consider whether active galactic nuclei, powered by supermassive black holes, might do the trick.

Current evidence favors the idea that star-forming galaxies provide a better match to the observed patterns than other candidates, but that does not slam the door on other possibilities.

Bridging theory and observations

The unusual composition and the changing patterns seen above 5 EeV push theorists to re-examine old ideas.

Traditional models that relied on smooth distributions of particle energies and the presence of light nuclei are under pressure.

New models must explain the sharp transitions, the heavy nuclei, and the hints of extragalactic origins. As measurements improve, scientists hope to confirm or refute certain types of sources.

If future data link cosmic rays more closely to certain types of galaxies or astrophysical conditions, it would be a major step forward in understanding how the universe’s most energetic particles are made.

Ongoing upgrades and the future

As detectors improve and more data are collected, the community expects to refine the energy calibration, measure more showers, and achieve better accuracy. Each improvement adds detail to this cosmic jigsaw puzzle.

If a tighter correlation with certain galaxy groups emerges, that could finally close in on the actual sources. Until then, each new measurement, each new analysis, and each new insight keeps the hunt alive.

The research paper was published in the Physical Review Journals.

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