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

TL;DR

A lower bound B ≥ 3 × 10−16 gauss on the strength of intergalactic magnetic fields is reported, which stems from the nonobservation of GeV gamma-ray emission from electromagnetic cascade initiated by tera–electron volt gamma rays inintergalactic medium.

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 $λ_B^{-1/2}$ if magnetic field correlation length, $λ_B$, is much smaller than a megaparsec. This lower bound constrains models for the origin of cosmic magnetic fields.

Figures (3)

• Figure 1: A comparison of models of cascade emission from TeV blazars (thick solid black curves) with Fermi upper limits (grey curves) and HESS data (grey data points). Thin dashed curves show the primary (unabsorbed) source spectra. Dotted curves show the spectra of electromagnetic cascade initiated by pair production on EBL. Vertical lines with arrows show the energies below which the cascade emission should be suppressed.
• Figure 2: Light, medium and dark grey: known observational bounds on the strength and correlation length of EGMF, summarized in the Ref. neronov09. The bound from Big Bang Nucleosynthesis marked "BBN" is from the Ref. grasso01. The black hatched region shows the lower bound on the EGMF derived in this paper. Orange hatched regions show the allowed ranges of $B, \lambda_B$ for magnetic fields generated at the epoch of Inflation (horizontal hatching) the electroweak phase transition (dense vertical hatching), QCD phase transition (medium vertical hatching), epoch of recombination (rear vertical hatching) neronov09. White ellipses show the range of measured magnetic field strengths and correlation lengths in galaxies and galaxy clusters.
• Figure 3: Greyscale: 68 and 95% confidence levels for the cut-off energy $E_{cut}$ and photon index $\Gamma$ found from the fitting of HESS spectra. Crosses mark the best-fit parameter values. Contours show the levels of integral energy flux above 0.1 TeV for the fitted spectra at each $E_{cut}$ and $\Gamma$, with the increments of $2.5\times 10^{-12}$ erg/cm$^2$s. Arrows show the 95% confidence level lower bounds on the cut-off energy for the photon index $\Gamma\ge 1.5$ (upper arrows) and absolute lower bounds on $E_{cut}$ (lower arrows).