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

TL;DR

This study leverages TeV gamma-ray cascades from the blazar 1ES 0229+200 to constrain extragalactic magnetic fields (EGMF). Using a Monte Carlo cascade simulation with realistic EBL attenuation and magnetic deflections, the authors translate the non-detection of GeV halos by Fermi-LAT into a lower limit on the field strength ($B \gtrsim 5\times10^{-15}$ G) and the required volume filling factor ($f \gtrsim 0.6$–0.8) for plausible TeV activity lifetimes. They test both uniform/top-hat and cosmological MHD-based EGMF models, finding that either space-filling primordial fields or efficient transport of local fields into filaments/voids are needed to suppress the point-like GeV flux, with time delays up to $10^{5}$–$10^{6}$ years for certain parameter choices. Overall, the work provides strong constraints on the origin and distribution of EGMFs and highlights the role of high-energy observations in probing cosmic magnetism.

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.

Figures (5)

• Figure 1: Fluence contained inside the 95% confidence contour of the PSF of Fermi-LAT as function of energy together with Fermi-LAT upper limits and HESS observations for a uniform EGMF with strengths varying from $B=10^{-16}$ G to $B=10^{-13}$ G with $E_{\rm max}=20$ TeV (solid) and 100 TeV (dashed). The direct component for $B=10^{-14}$ G is also shown.
• Figure 2: Fluence contained inside the 95% confidence contour of the PSF of Fermi-LAT as function of energy for a EGMF with top-hat profile and filling factor $f$ varying from $f=0.1$ to $f=0.9$ with $E_{\rm max}=20$ TeV (solid) and 100 TeV (dashed).
• Figure 3: The EGMF component perpendicular to the line-of-sight towards 1ES 0229+200 from four different MHD simulations.
• Figure 4: Cumulative volume filling factor $C(B)$ for the four different EGMF models found in MHD simulations.
• Figure 5: Fluence contained inside the 95% confidence contour of the PSF of Fermi-LAT as function of energy for EGMFs from four different MHD simulations with $E_{\rm max}=20$ TeV (solid) and 100 TeV (dashed).