Evidence for strong extragalactic magnetic fields from Fermi observations of TeV blazars

TL;DR

This paper establishes a lower limit on intergalactic magnetic field strength using Fermi observations of TeV blazars, providing insights into cosmic magnetic field origins.

Contribution

It presents the first lower bound on intergalactic magnetic fields based on gamma-ray observations, constraining seed magnetic field models.

Findings

Lower bound B ≥ 3×10^{-16} G on intergalactic magnetic fields

Bound improves with smaller magnetic field correlation length

Constraints on models for cosmic magnetic field origins

Abstract

Magnetic fields in galaxies are produced via the amplification of seed magnetic fields of unknown nature. The seed fields, which might exist in their initial form in the intergalactic medium, were never detected. We report a lower bound $B\ge 3\times 10^{-16}$~gauss on the strength of intergalactic magnetic fields, which stems from the nonobservation of GeV gamma-ray emission from electromagnetic cascade initiated by tera-electron volt gamma-ray in intergalactic medium. The bound improves as $\lambda_B^{-1/2}$ if magnetic field correlation length, $\lambda_B$, is much smaller than a megaparsec. This lower bound constrains models for the origin of cosmic magnetic fields.