The Telescope Array (TA) group showed an indication of a 40-degree-diameter hotspot emitting an excessive number of cosmic rays above the energy of 5.7✕1019 electron volts in the northern sky. The group plans to collect more events further to explore extreme energy phenomena which can describe the hotspot of the highest-energy cosmic rays.
Cosmic rays are tiny particles that travel at high speeds through space. Many cosmic rays arrive at Earth extremely isotropically. The Telescope Array (TA) international research group studied the arrival directions of 72 cosmic-ray events with energies above 5.7×1019 electron volts–called highest-energy cosmic rays–using data collected over a 5 year period by the surface detector of the TA experiment in Utah, USA. The group found an indication of a 40-degree-diameter hotspot in the northern sky, from which an excessive number of highest-energy cosmic rays arrived. Since the size of the hotspot region is 6% of the northern sky, the number of events expected, assuming an isotropic distribution, is 4.5 (6% of 72), whereas the number observed in this region is 19 (26% of 72). The probability for this to happen by chance in an isotropic cosmic-ray sky is estimated to be 3.7×10-4.
TA reported an indication of this anisotropy by utilizing the surface detectors spread across the area of about 700 square-kilometer–7 times larger area than that of the previous experiments. It is still a big mystery how cosmic rays obtain energies of about 1020 electron volts. TA will elucidate extreme phenomena in the universe, which can describe the origin of this hotspot.
[R.U. Abbasi/R.U. Abbasi et al. (126 in total)], “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,” [Astrophysical Journal Letters].
Publication reference: R. U. Abbasi et al. 2014 ApJ 790 L21 doi:10.1088/2041-8205/790/2/L21
© R.U. Abbasi et al. (Telescope Array Collaboration)
a) The arrival direction distribution and b) their significance map of the highest-energy cosmic rays in equatorial coordinates. The cluster of events has a statistical significance of 5.1 sigma (19 events were observed, whereas 4.5 events are expected in an isotropic distribution by oversampling using 20-degree-radius circles). Significance in red is larger, whereas that in blue is smaller. The field of view of the Telescope Array detector is the region above the dashed curve. The gray solid curves indicate the galactic plane (GP) and supergalactic plane (SGP).
Surface Particle Detector Site [Credit: TA Group, ICRR]
Cosmic-ray Air Shower [Credit: Kasahara Katsuaki, Waseda University]
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About TA: Assis. Prof. Kazumasa Kawata / Assoc. Prof. Hiroyuki Sagawa
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The University of Utah Press Release: “A Hotspot for Powerful Cosmic Rays
Physicists a Step Closer to Finding Mysterious Sources”
Telescope Array group, High Energy Cosmic Ray Division, Institute for Cosmic Ray Research