Testing a reported correlation between arrival directions of ultrahigh-energy cosmic rays and a flux pattern from nearby starburst galaxies using Telescope Array data

TL;DR

This study tests the correlation between ultrahigh-energy cosmic ray arrival directions and nearby starburst galaxies using Telescope Array data, finding results consistent with isotropy and previous Auger findings, thus not confirming the correlation.

Contribution

It provides an independent test of the correlation hypothesis using TA data with fixed parameters from Auger, without parameter optimization.

Findings

TA data is compatible with isotropy within 1.1σ.

TA data is compatible with Auger results within 1.4σ.

No significant correlation detected in TA data.

Abstract

The Pierre Auger Collaboration (Auger) recently reported a correlation between the arrival directions of cosmic rays with energies above 39 EeV and the flux pattern of 23 nearby starburst galaxies (SBGs). In this Letter, we tested the same hypothesis using cosmic rays detected by the Telescope Array experiment (TA) in the 9-year period from May 2008 to May 2017. Unlike the Auger analysis, we did not optimize the parameter values but kept them fixed to the best-fit values found by Auger, namely 9.7% for the anisotropic fraction of cosmic rays assumed to originate from the SBGs in the list and 12.9{\deg} for the angular scale of the correlations. The energy threshold we adopted is 43 EeV, corresponding to 39 EeV in Auger when taking into account the energy-scale difference between two experiments. We find that the TA data is compatible with isotropy to within 1.1{\sigma} and with the Auger result to within 1.4{\sigma}, meaning that it is not capable to discriminate between these two hypotheses.