Lower Limit on the Strength and Filling Factor of Extragalactic Magnetic Fields

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 O(10 −16 − 10 −15 ) G for life times of TeV activity of 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.

Extragalactic magnetic fields constraints from simultaneous GeV–TeV observations of blazars

“…The detection (non-detection) of the cascade emission signal from known TeV γ-ray emitting blazars could result in the measurement of (lower bound on) the strength of the magnetic field in intergalactic space along the line of sight toward these blazars. The first application of this method for deriving lower bounds on the EGMF have been carried out (Neronov & Vovk 2010;Tavecchio 2011;Dolag et al 2011;Dermer et al 2010), suggesting that a measure of the EGMF may finally soon be within reach.…”

“…In the simplest settings, the lower bounds on EGMF at the level of 10 −17 to 10 −15 G (Neronov & Vovk 2010;Tavecchio 2011;Dolag et al 2011) (depending on assumptions about A&A 529, A144 (2011) intrinsic blazar spectra) were derived. These bounds adopt a simplifying assumption that the measured blazar fluxes provide correct estimates of the time-averaged fluxes in the GeV and TeV energy bands.…”

Radio Continuum Surveys with Square Kilometre Array Pathfinders

“…A lower limit for the intergalactic magnetic field has been determined (10 −15 G; Dolag et al 2011).…”

Non-variable cosmologically distant gamma-ray emitters as a propagation imprint of ultra-high-energy protons

“…The current measurements yield only an upper limit of B ∼ 10 −9 G for the component parallel to the line of sight (Ryu et al 1998;Blasi & Olinto 1999).The lower bound is estimated from the non-observations of GeV gamma-ray counterparts from distant TeV blazars. Depending on the assumptions concerning the cascade development, its suppression, and the upper limit on the variability timescale of the TeV gamma-ray emission, the values of the lower limit inferred by different authors vary from B ∼ 10 −16 −10 −15 G (Neronov & Vovk 2010;Dolag et al 2011;Tavecchio et al 2010) to B ∼ 10 −18 G (Dermer et al 2011). The reasons for this broad range of IGMF estimates have been critically discussed by Taylor et al (2011).…”