Mapping the Most Energetic Cosmic Rays

A.M. Hillas

The Pierre Auger Collaboration has shown that the cosmic rays detected to August 2007, with estimated energies above 57 EeV, were mostly very close to the direction of a catalogued AGN within ∼75 Mpc.The closeness of the sources to us, and their association with the locality of moderate Seyfert galaxies rather than the most striking radio galaxies, were surprising, leading some authors to question the reality of the apparent associations. Here, three further techniques for examining the correlation of cosmic ray arrival directions with directions of AGNs are introduced to confirm and extend this correlation.These include the uniform-exposure polar plot to examine large-scale associations, and a sensitive “right ascension resonance” test to show the rapidity of the decoherence when the two patterns are displaced.The latter test avoids the choice of a correlation window radius, which makes it possible to see an AGN correlation in other data at a lower energy, and to seek it in HiRes data. On the basis of the closeness of correspondence of the cosmic ray and AGN maps, and the equally significant correspondence with directions of extended radio galaxies listed by Nagar and Matulich but not used in the Auger group's investigation, it is argued that the association with these two sets of objects is by no means accidental, although the efficacy of the57 EeV “cut” in selecting this revelatory sample may have been accidental.The arrival directions of these cosmic rays (avera e energy 75 EeV) can be well described if most of the sources are in or around rather typical Seyfert galaxies, in clusters typically at ∼50 Mpc, with the cosmic rays being scattered by 3-4 on their way to us.Because of close clustering of AGNs, it cannot usually be ascertained which objectwithin 2–3 Mpc is the actual source, but more than a third of the cosmic rays appear to come from FRI or similar radio galaxies in the clusters.It thus seems likely that these and also weaker jet-forming Seyfert galaxies are indeed causing acceleration to 10 eV.Neither the brightest (nearby) radio galaxies nor the Virgo cluster dominate the cosmic-ray sky as had been expected, and Cen A is probably one of the currently inactive 10 eV accelerators, as much more distant FRI galaxies play such a large role.The deflections cannot be much more than 3-4 without destroying the coherence.Intergalactic magnetic fields∼1 nG could be responsible, but in one part of the sky a displaced “resonance” suggests a possible 4 deflection by the B component of our local Galactic magnetic field.A source region limited to <120 Mpc for the Auger cosmic rays is supported.This is compatible with a GZK survival horizon but only if (a) the sudden fall in the energy spectrum is not simply a GZK effect but essentially reflects the energy cut-off in the accelerators,and (b) the Auger energies are underestimated by ∼25%.

Mots clés: Cosmic ray origin; AGNs; Equi-exposure skymap

A.M. Hillas. Mapping the Most Energetic Cosmic Rays. Astroparticle Physics, 2009, 32 (3-4), pp.160. ⟨10.1016/j.astropartphys.2009.07.005⟩. ⟨hal-00498976⟩