Data collected by the Pierre Auger Observatory through 31 August 2007 showed
evidence for anisotropy in the arrival directions of cosmic rays above the
Greisen-Zatsepin-Kuz’min energy threshold,
obreak{$6 imes 10^{19}$~eV}. The
anisotropy was measured by the fraction of arrival directions that are less
than $3.1^circ$ from the position of an active galactic nucleus within 75 Mpc
(using the V’eron-Cetty and V’eron $12^{
m th}$ catalog). An updated
measurement of this fraction is reported here using the arrival directions of
cosmic rays recorded above the same energy threshold through 31 December 2009.
The number of arrival directions has increased from 27 to 69, allowing a more
precise measurement. The correlating fraction is $(38^{+7}_{-6})\%$, compared
with $21\%$ expected for isotropic cosmic rays. This is down from the early
estimate of $(69^{+11}_{-13})\%$. The enlarged set of arrival directions is
examined also in relation to other populations of nearby extragalactic objects:
galaxies in the 2 Microns All Sky Survey and active galactic nuclei detected in
hard X-rays by the Swift Burst Alert Telescope. A celestial region around the
position of the radiogalaxy Cen A has the largest excess of arrival directions
relative to isotropic expectations. The 2-point autocorrelation function is
shown for the enlarged set of arrival directions and compared to the isotropic
expectation.
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