Anisotropy studies with the Pierre Auger Observatory

Abstract

Recent results from the Pierre Auger Observatory about anisotropies in the arrival directions of ultra-high energy cosmic rays are reported. In the EeV energy range, we present the results of a search for a large scale dipolar anisotropy pattern. In the same energy range, we show the results of a search for localized excesses of cosmic ray neutrons. For cosmic rays with energies above 20EeV, we discuss the results of the search for multiplets of aligned events. Finally, we present an update on the search for correlations between events with energy above 55EeV and the positions of active galactic nuclei from the Veron-Cetty and Veron catalog. The results of a study of the region around Centaurus A are also shown. In this article, we summarize recent results of the Pierre Auger Observatory on anisotropy searches. Anisotropies in the distribution of ultra-high energy cosmic rays (UHECR) are a key observable towards understanding their origin and identifying the transition region from galactic to extragalactic cosmic rays. In the energy range where galactic sources dominate the flux, models predict both a dipolar and a quadrupolar anisotropy, determined by the galactic magnetic fields and the source distribution of cosmic rays in the galaxy. If such an anisotropy can be detected, the transition to extragalactic cosmic rays should reveal itself by a change in the anisotropy pattern towards isotropy. Results on large-scale anisotropies are presented in section 2. Galactic neutrons in the EeV range live long enough to reach the Earth from the galactic center before decaying. If such neutrons are present they would point to their sources in the galactic disk. A search for neutron point sources is presented in section 3. At energies of tens of EeV, where cosmic rays are suspected to be of extragalactic origin, cosmic rays may be only weakly deflected by the extragalactic magnetic fields. This opens the possibility of identifying the extragalactic sources of UHECR from their arrival directions. In section 4 we present the 2011 update on the correlation of the highest energy events with the Veron-Cetty and Veron catalog of Active Galactic Nuclei (AGN). Centaurus A is a particularly promising candidate for being an UHECR source and is discussed separately. Finally, in section 5, we review an analysis searching for multiplets of particles from a common source, deflected in a coherent galactic or extragalactic magnetic field.