Anisotropy at the end of the cosmic ray spectrum?

Abstract

The starburst galaxies M82 and NGC253 have been proposed as the primary sources of cosmic rays with energies above ${10}^{18.7}\mathrm{eV}.$ For energies $\ensuremath{\gtrsim}{10}^{20.3}\mathrm{eV}$ the model predicts strong anisotropies. We calculate the probabilities that the latter can be due to chance occurrence. For the highest energy cosmic ray events in this energy region, we find that the observed directionality has less than 1% probability of occurring due to random fluctuations. Moreover, during the first 5 years of operation at Auger, the observation of even half the predicted anisotropy has a probability of less than ${10}^{\ensuremath{-}5}$ to occur by chance fluctuation. Thus, this model can be subject to test at very small cost to the Auger priors budget and, whatever the outcome of that test, valuable information on the galactic magnetic field will be obtained.