Searching for a Correlation Between Cosmic-Ray Sources Above 10^{19} eV and Large-Scale Structure

TL;DR

This study investigates the anisotropy of ultra high energy cosmic rays and finds evidence linking their sources to large-scale cosmic structures, using a novel correlation statistic applied to Auger data.

Contribution

The paper introduces a new correlation statistic that is more sensitive to anisotropy signatures in UHECRs and demonstrates its effectiveness with observational data.

Findings

Auger data are inconsistent with isotropy at ~98% CL

UHECR sources likely trace large-scale structure

Increased exposure could improve anisotropy detection significance

Abstract

We study the anisotropy signature which is expected if the sources of ultra high energy, >10^{19} eV, cosmic-rays (UHECRs) are extragalactic and trace the large scale distribution of luminous matter. Using the PSCz galaxy catalog as a tracer of the large scale structure (LSS), we derive the expected all sky angular distribution of the UHECR intensity. We define a statistic, that measures the correlation between the predicted and observed UHECR arrival direction distributions, and show that it is more sensitive to the expected anisotropy signature than the power spectrum and the two point correlation function. The distribution of the correlation statistic is not sensitive to the unknown redshift evolution of UHECR source density and to the unknown strength and structure of inter-galactic magnetic fields. We show, using this statistic, that recently published >5.7x10^{19} eV Auger data are inconsistent with isotropy at ~98% CL, and consistent with a source distribution that traces LSS, with some preference to a source distribution that is biased with respect to the galaxy distribution. The anisotropy signature should be detectable also at lower energy, >4x10^{19} eV. A few fold increase of the Auger exposure is likely to increase the significance to >99% CL, but not to >99.9% CL (unless the UHECR source density is comparable or larger than that of galaxies). In order to distinguish between different bias models, the systematic uncertainty in the absolute energy calibration of the experiments should be reduced to well below the current ~25%.