Extragalactic magnetic field constraints from ultra-high-energy cosmic rays from local galaxies

TL;DR

This study constrains extragalactic magnetic fields and source densities by analyzing correlations between local star-forming galaxies and ultra-high-energy cosmic rays, accounting for magnetic deflections and energy losses.

Contribution

It provides new constraints on extragalactic magnetic field strength and source density using cosmic ray anisotropy data and galaxy correlations.

Findings

Extragalactic magnetic fields are constrained to be less than 22 nG.

Source density is limited to below 8.4 x 10^-2 Mpc^-3.

Consistency found between cosmic ray anisotropy and galaxy distribution for strong magnetic fields.

Abstract

We interpret the correlation between local star-forming galaxy positions and ultra-high-energy cosmic ray (UHECR) directions, recently detected by the Pierre Auger Observatory (PAO), in terms of physical parameters: the local density of sources and the magnetic fields governing the UHECR propagation. We include a Galactic magnetic field model on top of a random extragalactic magnetic field description to determine the level of UHECR deflections expected from an ensemble of source positions. Besides deflections in magnetic fields, we also take into account energy losses with background photon fields as well as spectrum and composition measurements by the PAO. We find consistency between the PAO anisotropy measurement and the local star-forming galaxy density for large extragalactic magnetic field strengths with $B > 0.2 \ \rm nG$ (for a coherence length of $1 \ \rm Mpc$) at the $5\sigma$ confidence level. Larger source densities lead to more isotropic background and consequently allow for weaker extragalactic magnetic fields. However, the acceleration of UHECR by such abundant sources is more challenging to motivate. Too large source densities and extragalactic magnetic field strengths, on the other hand, are also disfavored as that decreases the expected level of anisotropy. This leads to upper limits of $B < 22 \ \rm nG$ and $\rho_0 < 8.4 \cdot 10^{-2} \ \rm Mpc^{-3}$ at the 90\% confidence level.