Lower limit on the strength and filling factor of extragalactic magnetic fields

TL;DR

This paper uses gamma-ray observations from blazars to set lower limits on the strength and filling factor of extragalactic magnetic fields, constraining their origin and distribution in the universe.

Contribution

It introduces a Monte Carlo simulation approach to analyze gamma-ray cascades and derives new lower bounds on EGMF strength and filling factor based on Fermi-LAT data.

Findings

EGMF fills at least 60% of space with fields > 10^{-16} G

Non-observation of GeV extensions constrains EGMF origin

EGMF strength and filling factor relate to cosmic magnetic field origins

Abstract

High energy photons from blazars can initiate electromagnetic pair cascades interacting with the extragalactic photon background. The charged component of such cascades is deflected and delayed by extragalactic magnetic fields (EGMF), reducing thereby the observed point-like flux and leading potentially to multi degree images in the GeV energy range. We calculate the fluence of 1ES 0229+200 as seen by Fermi-LAT for different EGMF profiles using a Monte Carlo simulation for the cascade development. The non-observation of 1ES 0229+200 by Fermi-LAT suggests that the EGMF fills at least 60% of space with fields stronger than {\cal O}(10^{-16}-10^{-15})G for life times of TeV activity of {\cal O}(10^2-10^4)yr. Thus the (non-) observation of GeV extensions around TeV blazars probes the EGMF in voids and puts strong constraints on the origin of EGMFs: Either EGMFs were generated in a space filling manner (e.g. primordially) or EGMFs produced locally (e.g. by galaxies) have to be efficiently transported to fill a significant volume fraction, as e.g. by galactic outflows.