Signatures of galactic magnetic lensing upon ultra high energy cosmic rays

We analyse several implications of lensing by the regular component of the galactic magnetic field upon the observed properties of ultra high energy c osmic rays. Magnetic fields deflect cosmic ray trajectories, causing flux ( de)magnification, formation of multiple images of a single source, and time delays. We derive the energy dependence of these effects near the caustics at which the flux amplification of a point source diverges. We show that t he large magnification of images around caustics leads to an amplification bias, which can make them dominate the flux in some energy ranges. We argue that clustering in the arrival directions of UHECRs of comparable energy m ay be due to magnetic lensing around caustics. We show that magnetic lensin g can also significantly alter the observed composition of cosmic rays at t he highest energies. We also show that the time delay between events from a single image may monotonically decrease with decreasing energy in the neig hborhood of a caustic, opposite to its behaviour in normal regions. Lensing effects in the magnetic field model considered are significant for cosmic rays with ratio between energy and electric charge E/Z between 10(18) and 5 10(19) eV approximately. Similar effects may also occur for higher E/Z val ues if the galactic magnetic field is stronger or more extended than what i s assumed here, or in the magnetic field of source galaxies, or even in int ergalactic fields.