Hotspot Update, and a new Excess of Events on the Sky Seen by the Telescope Array Experiment

Abstract

The Telescope Array (TA) experiment, the largest observatory studying ultrahigh energy cosmic rays in the northern hemisphere, has reported an excess in the arrival direction distribution for events with energies above 5.7 × 1019 eV, called the hotspot. We report here the latest results of the TA hotspot using the most recent data measured by the TA surface detector array, which is more than doubled exposure since the first publication; the hotspot still exists with 3 sigma post-trial significance. By using an oversampling search with a 20◦-circle, similar to the study of the hotspot, we find an additional excess of events at slightly lower energies. The Perseus-Pisces supercluster lies at the location of the new excess. Assuming this structure is responsible for the excess, we conducted a statistical analysis to verify the correlation between observed events and the members of the Perseus-Pisces supercluster.