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

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.

💡 Research Summary

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The paper presents a novel indirect method for constraining the strength of intergalactic magnetic fields (IGMFs) by exploiting the non‑detection of GeV‑scale gamma‑ray emission from electromagnetic cascades initiated by TeV photons emitted by distant blazars. High‑energy TeV photons traveling through the intergalactic medium (IGM) encounter the extragalactic background light (EBL) and undergo photon‑photon pair production, creating electron‑positron pairs. These relativistic pairs subsequently inverse‑Compton scatter cosmic microwave background (CMB) photons, up‑scattering them to GeV energies. In the absence of magnetic fields, the cascade photons would be beamed along the line of sight, producing a detectable GeV halo around the original TeV source. However, if an IGMF is present, the Lorentz force deflects the charged pairs, spreading the cascade emission over a larger solid angle and reducing the flux that reaches Earth‑bound detectors.

The authors construct a comprehensive Monte‑Carlo simulation that incorporates (i) the intrinsic TeV spectra of several well‑studied blazars (e.g., 1ES 0229+200, PKS 2155‑304), (ii) realistic models of the EBL, (iii) the inverse‑Compton cooling of the pairs, and (iv) a parametrized magnetic field characterized by its strength (B) and correlation length (\lambda_B). By varying (B) and (\lambda_B), they compute the expected GeV flux for each source and compare it with the upper limits derived from long‑term observations by the Fermi Large Area Telescope (LAT).

The key observational result is that none of the examined blazars exhibit the predicted GeV excess; the LAT data provide stringent upper limits that are well below the cascade flux expected for magnetic fields weaker than roughly (3\times10^{-16}) gauss. Consequently, the authors infer a lower bound (B \ge 3\times10^{-16}) G for a correlation length of order a megaparsec. Moreover, when (\lambda_B \ll 1) Mpc, the bound scales as (B \propto \lambda_B^{-1/2}), reflecting the enhanced deflection efficiency of small‑scale turbulent fields. For instance, if (\lambda_B) is as small as 10 kpc, the required field strength rises to about (10^{-15}) G.

These findings have profound implications for theories of cosmic magnetogenesis. Primordial scenarios that generate ultra‑weak seed fields during inflation or phase transitions must now accommodate a minimum amplification factor sufficient to reach the observed lower bound by the present epoch. Conversely, astrophysical mechanisms that rely solely on outflows from galaxies or galaxy clusters appear insufficient to seed the IGM on megaparsec scales at the required level. The result therefore favors models in which a non‑negligible primordial magnetic component exists, subsequently processed by large‑scale structure formation and turbulent dynamo action.

In summary, the paper demonstrates that the absence of GeV cascade emission in Fermi‑LAT data imposes a robust lower limit on the intergalactic magnetic field strength, (B \gtrsim 3\times10^{-16}) G (with a (\lambda_B^{-1/2}) dependence for smaller correlation lengths). This constraint narrows the viable parameter space for the origin of cosmic magnetic fields and sets a benchmark for future high‑energy gamma‑ray observatories such as the Cherenkov Telescope Array (CTA) and LHAASO, which will be capable of probing even weaker fields and finer magnetic structures.