Astrophys. J., 693, 1261-1274 (2009/March-2)
On the possible association of ultra high energy cosmic rays with nearby active galaxies.
MOSKALENKO I.V., STAWARZ L., PORTER T.A. and CHEUNG C.C.
Abstract (from CDS):
Data collected by the Pierre Auger Observatory (Auger) provide evidence for anisotropy in the arrival directions of cosmic rays (CRs) with energies higher than 57 EeV, which suggests a correlation with the positions of active galactic nuclei (AGNs) located within ∼ 75 Mpc and 3°.2 of the arrival directions. This analysis, however, does not take into account AGN morphology. A detailed study of the sample of AGNs whose positions are located within 3°.2 of the CR events (and extending our analysis out to ∼ 150 Mpc) shows that most of them are classified as Seyfert 2 and low-ionization nuclear emission-line region galaxies whose properties do not differ substantially from other local AGNs of the same type. Therefore, if the production of the highest energy CRs is persistent in nature, i.e., operates in a single object on long (≳ Myr) timescales, the claimed correlation between the CR events observed by Auger and the local active galaxies should be considered as resulting from a chance coincidence. In addition, most of the selected sources do not show significant jet activity, and hence, in most conservative scenarios, there are no reasons for expecting them to accelerate CRs up to the highest energies, ∼1020 eV. If the extragalactic magnetic fields and the sources of these CRs are coupled with matter, it is possible that the deflection angle is larger than expected in the case of a uniform source distribution due to effectively larger fields. A future analysis has to take into account AGN morphology and may yield a correlation with a larger deflection angle and/or more distant sources. We further argue that the nearby radio galaxy NGC 5128 (Cen A) alone could be associated with at least four events due to its large radio extent, and PKS 1343-60 (Cen B), another nearby radio galaxy, can be associated with more than one event due to its proximity to the Galactic plane and, correspondingly, the stronger Galactic magnetic field the ultra-high-energy CRs (UHECRs) encounter during propagation to the Earth. If the UHECRs associated with these events are indeed accelerated by Cen A and Cen B, their deflection angles may provide information on the structure of the magnetic field in the direction of these putative sources. Future γ-ray observations (e.g., Fermi Gamma-ray Space Telescope formerly Gamma-Ray Large Area Space Telescope, and High Energy Stereoscopic System in the Southern hemisphere) may provide additional clues to the nature of the accelerators of the UHECRs in the local universe.
cosmic rays - galaxies: active - galaxies: individual (Cen A, Cen B, PKS 2158-380, PKS 2201+044) - intergalactic medium
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