Indications 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

TL;DR

This study reports the discovery of a statistically significant hotspot of ultrahigh-energy cosmic rays above 57 EeV in the northern sky, indicating potential anisotropy in their arrival directions based on 5 years of Telescope Array data.

Contribution

First detection of a 5.1σ significant intermediate-scale anisotropy hotspot in ultrahigh-energy cosmic rays using Telescope Array surface detector data.

Findings

Hotspot centered at R.A.=146.7°, Dec.=43.2°

Hotspot significance of 5.1σ with a chance probability of 3.7×10⁻⁴

Hotspot located about 19° off the supergalactic plane

Abstract

We have searched for intermediate-scale anisotropy in the arrival directions of ultrahigh-energy cosmic rays with energies above 57~EeV in the northern sky using data collected over a 5 year period by the surface detector of the Telescope Array experiment. We report on a cluster of events that we call the hotspot, found by oversampling using 20$^\circ$-radius circles. The hotspot has a Li-Ma statistical significance of 5.1$\sigma$, and is centered at R.A.=146.7$^{\circ}$, Dec.=43.2$^{\circ}$. The position of the hotspot is about 19$^{\circ}$ off of the supergalactic plane. The probability of a cluster of events of 5.1$\sigma$ significance, appearing by chance in an isotropic cosmic-ray sky, is estimated to be 3.7$\times$10$^{-4}$ (3.4$\sigma$).