Big Dipper Cosmic Ray Hotspot Mystery Solved? Page 2

Solar-powered detector at the Telescope Array cosmic ray observatory measures the strength and direction of cosmic rays after they travel through Earth's atmosphere.
John Matthews, University of Utah

The hotspot region of the sky lies near the supergalactic plane, which contains local galaxy clusters such as the Ursa Major cluster, the Coma cluster and the Virgo cluster.

The research, which is an international collaboration of over 100 scientists, was recently accepted for publication in the Astrophysical Journal Letters.

Discovered in 1912, cosmic rays are thought to consist of the bare protons of hydrogen nuclei, or the centers of heavier elements. The powerful particles stream in from various regions of the sky, with energies reaching as high as 300 billion billion electron volts. Cosmic rays are classified as "ultrahigh-energy" if they carry the energy of 1 billion billion electron volts, comparable to a fast-pitch baseball.

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While low-energy cosmic rays come from stars like the sun over the course of their life or explosive deaths, the origins of more energetic rays remain a mystery.

Suggested progenitors for the more powerful cosmic rays include Active Galactic Nuclei (AGN), where material is sucked into supermassive black holes at the center of galaxies, or gamma-ray bursts from the explosive supernova death of massive stars. Other potential causes include shockwaves from noisy radio galaxies and colliding galaxies. More exotic possibilities include the decay of "cosmic strings," hypothetical one-dimensional defects proposed by string theory.

Ultrahigh-energy cosmic rays stem from outside the Milky Way, but are weakened by interactions with the cosmic microwave background radiation — the leftover fingerprint from the Big Bang that kicked off the universe. As a result, 90 percent of the detected ultrahigh-energy cosmic rays originate within 300 million light-years of Earth.

According to Jui, a separate study currently in progress suggests that the distribution of ultrahigh-energy cosmic rays in the northern sky is related to concentrations of large-scale structures like clusters and superclusters of galaxies.

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"It tells us there is at least a good chance these are coming from matter we can see, as opposed to a different class of mechanisms where you are producing these particles with exotic processes," Jui said.

The Telescope Array houses 523 detectors spread over 300 square miles of desert. Physicists hope to make the observatory more sensitive by doubling the number of detectors and quadrupling the area they cover, which should capture more cosmic rays.

"With more events, we are more likely to see structure in that hotspot blob, and that may point us toward the real sources," Jui said.

A preprint of the article may be found online at

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