Constraining the astrophysical origin of intergalactic magnetic fields

TL;DR

This paper uses gamma-ray observations to constrain the origin and properties of intergalactic magnetic fields, ruling out many astrophysical models and setting lower bounds on primordial field strength.

Contribution

It provides new constraints on the cosmological origin of intergalactic magnetic fields using observational data and MHD simulations, excluding models without primordial components.

Findings

Models without primordial fields are excluded at 95% confidence.

The space-filling fraction of strong IGMF must be at least 0.67.

Lower limits on primordial IGMF strength are set at 5.1×10⁻¹⁵ G and 1.0×10⁻¹⁴ G.

Abstract

High-energy photons can produce electron-positron pairs upon interacting with the extragalactic background light (EBL). These pairs will in turn be deflected by the intergalactic magnetic field (IGMF), before possibly up-scattering photons of the cosmic microwave background (CMB), thereby initiating an electromagnetic cascade. The non-observation of an excess of GeV photons and an extended halo around individual blazars due to this electromagnetic cascade can be used to constrain the properties of the IGMF. In this work, we use publicly available data of 1ES 0229+200 by Fermi LAT and H.E.S.S. to constrain cosmological MHD simulations of various magnetogenesis scenarios, and find that all models without a strong space-filling primordial component or over-optimistic dynamo amplifications can be excluded at the 95% confidence level. In fact, we find that the fraction of space filled by a strong IGMF has to be at least $f\gtrsim 0.67$, thus excluding most astrophysical production scenarios. Moreover, we set the lower limits $B_0>5.1\times 10^{-15}$ G ($B_0>1.0\times 10^{-14}$ G) of a space-filling primordial IGMF for a blazar activity time of $\Delta t = 10^4$ yr ($\Delta t = 10^7$ yr).