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

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 yr 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 degrees radius circles. The hotspot has a Li-Ma statistical significance of 5.1 sigma, and is centered at R. A. = 146 degrees 7, decl. = 43 degrees 2. The position of the hotspot is about 19 degrees 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 x 10(-4) (3.4 sigma).