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

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

• The paper establishes a lower limit on extragalactic magnetic field strength of B ≥ 3×10⁻¹⁶ gauss based on the absence of GeV cascade emissions.
• The researchers analyzed gamma-ray spectra from several TeV blazars using Fermi and HESS observations combined with numerical modeling.
• These findings narrow the parameter space for cosmological magnetogenesis models by reinforcing the hypothesis of a primordial origin for magnetic fields.

Evidence for Strong Extragalactic Magnetic Fields from Fermi Observations of TeV Blazars

The paper authored by Andrii Neronov and Ievgen Vovk presents significant findings on the nature and strength of extragalactic magnetic fields (EGMFs) derived from the Fermi and HESS gamma-ray telescope data. These results bear critical implications for understanding the primordial conditions of the universe and the mechanisms underlying cosmic magnetism generation.

Summary of Findings

The research identifies a lower bound on the EGMF strength, establishing it at $B \ge 3 \times 10^{-16}$ gauss. This conclusion arises from the absence of GeV gamma-ray emissions, which would typically constitute a result of electromagnetic cascades initiated by TeV gamma rays in the intergalactic medium. The lack of such emissions suggests stronger EGMFs than previously assumed, potentially constraining both astrophysical and cosmological models of cosmic magnetic field origins.

Methodological Approach

The researchers analyze the gamma-ray spectra of multiple blazars—1ES 1101-232, 1ES 0229+200, 1ES 0347-121, and H 2356-309—using Fermi’s observational data to set upper limits on gamma-ray fluxes. These sources, notable for their high redshifts and hard TeV spectra, offer insights into possible cascade emissions resultant from interactions with the extragalactic background light (EBL). Utilizing numerical models, the authors assess the resulting interactions and gamma-ray absorption, deducing constraints on EGMF strengths from the spectroscopic data.

Results and Implications

The findings suggest a non-negligible EGMF, notably contributing to the suppression of cascade emissions below certain energy thresholds. The authors provide estimates for magnetic field strengths across different correlation lengths, influencing our understanding of the magnetic field structure on cosmological scales. The observed constraints imply that the existence of weak EGMFs aligns with cosmological models, which assert their formation in the early universe—across epochs such as inflation or the electroweak phase transition.

The presence of EGMFs in intergalactic voids supports theories suggesting a cosmological origin over an astrophysical one, since localized astrophysical processes are less likely to result in pervasive, universe-spanning magnetic fields.

Theoretical and Practical Implications

The paper's conclusions have significant theoretical implications, particularly for the testing of cosmological magnetogenesis theories. A stronger EGMF constrains these models, effectively shrinking the permissible parameter space for scenarios positing the generation of primordial magnetic fields during different universe epochs. Practically, this insight impacts the interpretation of cosmic background radiation and the propagation of high-energy astrophysical phenomena.

Future Directions

Further research is warranted to refine these bounds and gain deeper insight into the spatial distribution and dynamics of EGMFs. Future observational advances, particularly in high-resolution gamma-ray astronomy and cosmic microwave background polarization studies, could further elucidate the fundamental properties of these fields. An understanding of EGMF variations could potentially resolve longstanding questions about cosmic structure formation and evolution.

In conclusion, Neronov and Vovk’s work on EGMFs provides a crucial perspective on the underlying forces and histories that shaped our universe, offering essential data that challenges and refines existing astrophysical and cosmological models alike.