Constraints on the Intergalactic Magnetic Field Using Fermi-LAT and H.E.S.S. Blazar Observations

Abstract

Magnetic fields in galaxies and galaxy clusters are believed to be the result of the amplification of intergalactic seed fields during the formation of large-scale structures in the universe. However, the origin, strength, and morphology of this intergalactic magnetic field (IGMF) remain unknown. Lower limits on (or indirect detection of) the IGMF can be obtained from observations of high-energy gamma rays from distant blazars. Gamma rays interact with the extragalactic background light to produce electron−positron pairs, which can subsequently initiate electromagnetic cascades. The gamma-ray signature of the cascade depends on the IGMF since it deflects the pairs. Here we report on a new search for this cascade emission using a combined data set from the Fermi Large Area Telescope and the High Energy Stereoscopic System. Using state-of-the-art Monte Carlo predictions for the cascade signal, our results place a lower limit on the IGMF of B > 7.1 × 10−16 G for a coherence length of 1 Mpc even when blazar duty cycles as short as 10 yr are assumed. This improves on previous lower limits by a factor of 2. For longer duty cycles of 104 (107) yr, IGMF strengths below 1.8 × 10−14 G (3.9 × 10−14 G) are excluded, which rules out specific models for IGMF generation in the early universe.

Detecting ALP wiggles at TeV energies

“…As already mentioned, we will focus on the effect of the intergalactic magnetic field. From the non-detection of electromagnetic cascades from blazars, it was concluded that the extragalactic space must be filled with an turbulent magnetic field with a strength of B 10 −14 G with a large filling factor [62][63][64], while an upper limit of B 10 −9 G is derived from Faraday rotation measurements [65]. The nature and the production of the extragalactic magnetic field remain however unknown: A large range of magnetic field strengths, spectral index at small k (i.e.…”

Constraining the Astrophysical Origin of Intergalactic Magnetic Fields

“…Recently, the observed Fermi LAT and H.E.S.S. spectra of 1ES 0229+200 were made publicly available (Aharonian et al 2023), 6 which we make use of in the present work.…”

“…The absorption of gamma-rays by the EBL can account for the entire spectral softening. Further details on the analysis can be found in Aharonian et al (2023).…”

“…Currently, the most robust lower limit from blazar observations is B > 1.8 × 10 −14 G (B > 3.9 × 10 −14 G) for an activity time of 10 4 yr (10 7 yr), set by a combined analysis of Fermi LAT and H.E.S.S. blazar observations (Aharonian et al 2023).…”

“…S. observations of 1ES 0229+200 by Aharonian et al (2023) to constrain the astrophysical origin of the IGMF. By comparing observations to Monte Carlo simulations performed using the photon propagation program ELMAG (Kachelriess et al 2012;Blytt et al 2020), we find a lower limit B 0 > 5.1 × 10 −15 G (B 0 > 1.0 × 10 −14 G) assuming a blazar activity time of Δt = 10 4 yr (Δt = 10 7 yr), comparable to the results of Aharonian et al (2023). The main emphasis in this work is, however, on the astrophysical origin of the IGMF.…”