Cosmic Ray Hotspot Found

The Telescope Array group has identified a hotspot in the Northern hemisphere where super high energy cosmic rays seem to be clustered.

AsianScientist (Jul 21, 2014) – Scientists have identified a hotspot emitting unusually high energy cosmic rays. The results have been published in The Astrophysical Journal Letters.

Cosmic rays are tiny particles that travel around at high speed through space. Many cosmic rays arrive at Earth in a wide range of energies and extremely isotropically, meaning that they arrive from all directions equally. In other words, in observations of cosmic rays, researchers have so far been unable to detect anything special about any particular direction in the universe.

The Telescope Array (TA), an international research group including the University of Tokyo Institute for Cosmic Ray Research and researchers from Korea, studied the arrival directions of 72 cosmic-ray events with energies above 5.7×1019 electron volts using data collected over a five year period by the surface detector of the Telescope Array in Utah, USA.

They found an indication of a 40-degree-diameter hotspot emitting an excessive number of highest energy cosmic rays, calculated from their arrival directions located in the northern sky. The size of the hotspot is six percent of the northern sky. In this region, only 4.5 high energy events were expected but 19 were observed, about a quarter of the total of 72 events recorded over the five year study. The probability of the cluster of events appearing by chance in an isotropic cosmic-ray sky is extremely low, estimated to be 3.7×10-4.

The highest energy cosmic rays observed by the Telescope Array are an extremely rare phenomenon, with only one observed event expected per year in an area of 100 square kilometers. This time, researchers used a ground based particle detector seven times larger in area than the observation equipment that had formerly been used in the Northern hemisphere. As a result, they were able to obtain several times more statistical events of highest energy cosmic ray phenomena and capture indications of the hotspot.

It is still a mystery as to how cosmic rays obtain energies of about 1020 electron volts. By increasing the number of observed events, the Telescope Array will explore the relation of highest energy cosmic rays to extreme phenomena in the universe, which may be the origin of this cluster of events.

The article can be found at: Abbasi et al. (2014) Indication of Intermediate-Scale Anisotropy of Cosmic Rays with Energy Greater than 57 EeV in the Northern Sky Measured with the Surface Detector of the Telescope Array Experiment.

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Source: University of Tokyo.
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