Correlation of the highest energy cosmic rays with nearby extragalactic objects (0711.2256v1)

Abstract: Using data collected at the Pierre Auger Observatory during the past 3.7 years, we demonstrated a correlation between the arrival directions of cosmic rays with energy above ~ 6x10^{19} electron volts and the positions of active galactic nuclei (AGN) lying within ~ 75 megaparsecs. We rejected the hypothesis of an isotropic distribution of these cosmic rays with at least a 99% confidence level from a prescribed a priori test. The correlation we observed is compatible with the hypothesis that the highest energy particles originate from nearby extragalactic sources whose flux has not been substantially reduced by interaction with the cosmic background radiation. AGN or objects having a similar spatial distribution are possible sources.

• The paper presents evidence from the Pierre Auger Observatory showing a significant correlation between ultrahigh-energy cosmic rays above 56 EeV and the positions of nearby active galactic nuclei.
• Statistical analysis of collected data demonstrates that a significant fraction of observed events align with potential source locations, strongly challenging the assumption of cosmic ray isotropy.
• These findings support the idea that the highest energy cosmic rays originate from relatively nearby extragalactic sources, consistent with energy loss predicted by the GZK effect.

Correlation of the Highest Energy Cosmic Rays with Nearby Extragalactic Objects

This paper, authored by the Pierre Auger Collaboration, investigates the correlation between ultrahigh-energy cosmic rays (UHECRs) and extragalactic sources, specifically focusing on active galactic nuclei (AGN) as potential origins, leveraging observations from the Pierre Auger Observatory. The paper analyses the cosmic rays arriving at Earth with energies exceeding approximately $6 \times 10^{19}$ electron volts (eV) and correlates these with AGN located within 75 megaparsecs.

Observational Methodology and Data Analysis

Data was collected over a 3.7-year period utilizing the Pierre Auger Observatory in Argentina, which employs a hybrid detection system combining surface detectors (SD) and fluorescence detectors (FD). The SD array is comprised of 1600 detectors positioned in a triangular grid covering 3000 km$^2$, while the FDs are deployed at four peripheral sites equipped with optical telescopes. These instruments collectively facilitate accurate measurements of the energy and arrival directions of cosmic rays via air showers, corroborating the existence of anisotropic distribution patterns.

To quantify and scrutinize the degree of cosmic ray correlation with AGN, the research adopts a binomial distribution protocol in a multi-parameter space, scanning variables including maximum angular separations ($\psi$), maximum redshifts ($z_{max}$), and energy thresholds ($E_{th}$). A noteworthy discovery was achieved with $\psi = 3.1^\circ$, $z_{max} = 0.018$, and $E_{th} = 56$ EeV, depicting a significant correlation among recorded events and AGN positions.

Results and Statistical Significance

The paper reveals that, among 27 high-energy cosmic events, a significant fraction aligns with the positions of nearby AGN, implying a non-random, anisotropic distribution of these cosmic rays. Specifically, an exploratory analysis identified 12 out of 15 events correlating with AGN, against an isotropy-based chance expectation of 3.2. Subsequent data, independent of the initial exploratory set, demonstrated 8 out of 13 high-energy events falling within the pre-specified correlation distance, a result statistically challenging the isotropy hypothesis with a confidence level exceeding 99%.

Implications and Theoretical Context

These findings reinforce the assumption that the most energetic cosmic rays could be primarily protons emanating from relatively nearby extragalactic objects. The correlation suggests compatibility with the Greisen, Zatsepin, and Kuzmin (GZK) effect, which postulates a flux attenuation for protons exceeding 60 EeV due to cosmic microwave background interactions. Furthermore, the paper posits that the observed patterns may provide insights into intervening magnetic field characteristics and cosmic ray source distributions, although comprehensive source identification remains inconclusive.

Prospects for Future Research

The continued operation of the Pierre Auger Observatory, with its extensive sky coverage and detection capabilities, promises enhanced cosmic ray event cataloging in ensuing years. As more data accumulates, it could potentially elucidate the specific origins and cosmic transit mechanisms of UHECRs, offering more definitive source identifications. Follow-up investigations could refine the understanding of both the astrophysical sources and fundamental physics influencing cosmic ray behavior, contributing substantively to astroparticle physics.