Tracing protons through the Galactic magnetic field: a clue for charge composition of ultra-high-energy cosmic rays

We reconstruct the trajectories of ultra-high-energy cosmic rays (UHECR)––observed by the AGASA experiment––in the Galactic magnetic field assuming that all particles have the same charge. We then study correlations between the reconstructed events and BL Lacertae (BL Lacs). The correlations have significance below 10−3 in the case of particles with charge +1. In the case of charge −1 the correlations are absent. We interpret this as evidence that protons are present in the flux of UHECR. Observed correlation provides an independent evidence that BL Lacs emit UHECR.