Filaments of Galaxies as a Clue to the Origin of Ultra-High-Energy Cosmic Rays

TL;DR

This study finds a significant correlation between ultra-high-energy cosmic ray hotspots and galaxy filaments connected to the Virgo Cluster, suggesting cosmic rays originate there and propagate along filaments influenced by magnetic fields.

Contribution

It presents the first evidence linking UHECR hotspots to galaxy filaments and proposes a model for their origin involving propagation along these filaments.

Findings

Significant correlation (6.1σ) between UHECR hotspots and galaxy filaments.

Probability of chance correlation is approximately 2.0 × 10⁻⁸.

Proposed magnetic field strength in filaments is ≥20 nG.

Abstract

Ultra-high-energy cosmic rays (UHECRs) are known to come from outside of our Galaxy, but their origin still remains unknown. The Telescope Array (TA) experiment recently identified a high concentration in the arrival directions of UHECRs with energies above $5.7 \times 10^{19} eV$, called hotspot. We here report the presence of filaments of galaxies, connected to the Virgo Cluster, on the sky around the hotspot, and a statistically significant correlation between hotspot events and the filaments. With 5-year TA data, the maximum significance of binomial statistics for the correlation is estimated to be 6.1 $\sigma$ at correlation angle 3.4 degree. The probability that the above significance appears by chance is $\sim 2.0 \times 10^{-8}$ (5.6 $\sigma$). Based on this finding, we suggest a model for the origin of TA hotspot UHECRs; they are produced at sources in the Virgo Cluster, and escape to and propagate along filaments, before they are scattered toward us. This picture requires the filament magnetic fields of strength $\gtrsim 20$ nG, which need to be confirmed in future observations.