Update on the correlation of the highest energy cosmic rays with nearby extragalactic matter

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, \nobreak{$6\times 10^{19}$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 $12^{\rm th}$ 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.

💡 Research Summary

The Pierre Auger Observatory, the world’s largest detector of ultra‑high‑energy cosmic rays (UHECRs), has accumulated a significantly larger data set since its first anisotropy claim in 2007. The original analysis, based on 27 events with energies above the Greisen‑Zatsepin‑Kuz’min (GZK) threshold (E > 6 × 10¹⁹ eV), reported a striking correlation: 69 % of the events fell within 3.1° of an active galactic nucleus (AGN) listed in the 12th edition of the Véron‑Cetty & Véron (VCV) catalog and located within 75 Mpc. This result suggested that nearby extragalactic objects, particularly AGN, might be the dominant sources of the most energetic particles reaching Earth.

In the present work the authors extend the analysis to 69 events collected up to 31 December 2009, more than doubling the original sample. Using the same angular (3.1°) and distance (75 Mpc) cuts, they find that 38 % (+7 %/–6 %) of the events correlate with VCV AGN, compared with an isotropic expectation of 21 %. Although the correlation fraction has dropped substantially from the early estimate, the excess remains statistically significant (roughly a 2‑σ deviation from isotropy). The reduction is interpreted as a natural consequence of larger statistics: the early high fraction was likely amplified by small‑sample fluctuations.

To test the robustness of the result, the authors repeat the correlation study with two independent catalogs of nearby extragalactic objects. The 2‑Micron All‑Sky Survey (2MASS) galaxy catalog traces the overall distribution of nearby galaxies, while the Swift Burst Alert Telescope (BAT) hard‑X‑ray AGN catalog selects AGN that are bright in the 14–195 keV band, a regime less affected by obscuration. Correlations with 2MASS galaxies are weaker than with VCV AGN, indicating that UHECRs do not simply follow the overall galaxy density. The Swift‑BAT AGN show a correlation level comparable to or slightly below that of VCV, suggesting that not all AGN classes contribute equally to the UHECR flux.

A particularly striking feature of the enlarged data set is an excess of events in the sky region surrounding the radio galaxy Centaurus A (Cen A). Cen A lies at a distance of ≈3.8 Mpc, well within the GZK horizon, and hosts powerful relativistic jets that could accelerate particles to ultra‑high energies. Several events cluster within ≈20° of Cen A, a pattern that is difficult to reconcile with a purely isotropic sky but is compatible with modest magnetic deflections (a few degrees) expected from Galactic and extragalactic magnetic fields at these energies.

The authors also present a two‑point autocorrelation function for the 69‑event sample. The function shows an excess of pairs at angular separations ≤30°, again deviating from the isotropic expectation. This indicates that the events are not uniformly distributed but tend to appear in small‑scale groups, consistent with the presence of a few dominant nearby sources rather than a completely diffuse background.

In summary, the updated analysis confirms that the arrival directions of the highest‑energy cosmic rays are not isotropic. While the correlation strength with VCV AGN is lower than initially reported, it remains significantly above random expectations. The strongest localized excess points toward Cen A, reinforcing the hypothesis that at least a fraction of UHECRs originate from nearby radio‑loud AGN. The weaker correlations with the broader 2MASS galaxy distribution and with Swift‑BAT AGN imply that specific source classes, perhaps those with powerful jets and favorable magnetic environments, dominate the UHECR sky.

Future work will benefit from continued data accumulation, refined models of Galactic and extragalactic magnetic fields to back‑track particle trajectories, and multi‑messenger observations (radio, X‑ray, gamma‑ray) that can pinpoint active acceleration sites. Such efforts are essential for finally identifying the astrophysical accelerators responsible for the most energetic particles in the Universe.