The intergalactic magnetic field constrained by Fermi/LAT observations of the TeV blazar 1ES 0229+200

TL;DR

This paper uses Fermi/LAT observations of the blazar 1ES 0229+200 to set lower limits on the intergalactic magnetic field strength by analyzing the reprocessed GeV emission resulting from TeV photon interactions.

Contribution

It introduces a simple relation for the reprocessed spectrum from steady sources and constrains the intergalactic magnetic field based on observational upper limits.

Findings

Intergalactic magnetic field B > 5×10^{-15} Gauss

Reprocessed emission constrains magnetic field strength

Method applicable to other steady TeV sources

Abstract

TeV photons from blazars at relatively large distances, interacting with the optical-IR cosmic background, are efficiently converted into electron-positron pairs. The produced pairs are extremely relativistic (Lorentz factors of the order of 1e6 1e7 and promptly loose their energy through inverse Compton scatterings with the photons of the microwave cosmic background, producing emission in the GeV band. The spectrum and the flux level of this reprocessed emission is critically dependent on the intensity of the intergalactic magnetic field, B, that can deflect the pairs diluting the intrinsic emission over a large solid angle. We derive a simple relation for the reprocessed spectrum expected from a steady source. We apply this treatment to the blazar 1ES 0229+200, whose intrinsic very hard TeV spectrum is expected to be approximately steady. Comparing the predicted reprocessed emission with the upper limits measured by the Fermi/Large Area Telescope, we constrain the value of the intergalactic magnetic field to be larger than $B \simeq 5\times 10^{-15}$ Gauss, depending on the model of extragalactic background light.