A scientific collaboration of the Pierre Auger Observatory, including researchers from the University of Delaware, has found a “kink” in the cosmic ray energy spectrum that is shining more light on the possible origins of these subatomic space travelers.
The team’s findings are based on the analysis of 215,030 cosmic ray events with energies above 2.5*10^18 eV, recorded over the past decade by the Pierre Auger Observatory in Argentina. It is the largest observatory in the world for studying cosmic rays.
The new spectral feature, a kink in the cosmic-ray energy spectrum at about 13*10^18 eV, represents more than points plotted on a graph. It brings humanity a step closer to solving the mysteries of the most energetic particles in nature. Previously, scientists thought these ultra-high-energy cosmic ray particles were mostly protons of hydrogen, but this latest analysis confirms that the particles have a mix of nuclei—some heavier than oxygen or helium, such as silicon and iron, for example.
Plotted on the curving graph representing the cosmic-ray energy spectrum, you can see the kink—a steep, flattened section—between the area referred to by scientists as “the ankle,” and the beginning point of the graph, called “the toe.”
Active galactic nuclei (AGN) and starburst galaxies are now in the running as potential sources. While their typical distance is some 100 million light years away, a few candidates are within 20 million light years.
“If we learned what the sources were, we could look into new details about what is going on,” Coleman said. “What’s happening that allows these incredibly high energies? These particles may be coming from something we don’t even know.”