Speaker
Description
Large-scale intergalactic magnetic fields (IGMFs) may comprise both galactic and cosmogenic components, which can be probed via observations of delayed $\gamma$-ray emission from electromagnetic cascades initiated by the highest-energy photons emitted by distant sources. These components can, in principle, be distinguished through their redshift evolution; however, observational evidence for non-negligible magnetic fields has so far been largely limited to low redshifts.
This work extends constraints on the IGMF to redshifts $z \gtrsim 1$ using 17 years of all-sky observations of high-redshift active galactic nuclei with the Fermi/LAT $\gamma$-ray telescope. By combining Fermi/LAT data in the 0.1 GeV – 1 TeV energy range with Monte Carlo simulations of $\gamma$-ray-induced electromagnetic cascades, it is shown that the null hypothesis of zero magnetic field strength in the redshift interval $z \in [0.5,\,3]$ is disfavoured at the $\approx 8.6\sigma$ significance level. This corresponds to a lower bound of $B \gtrsim 1 \times 10^{-18}$ cG for magnetic field correlation lengths exceeding 1 Mpc.
The same dataset further constrains the volume-filling fraction of the IGMF to $f \gtrsim 90\%$ within the probed redshift range. The robustness of these results is verified against potential systematic effects, including source flux variability and uncertainties in the $\gamma$-ray attenuation model.
These results provide the first evidence to date for pervasive intergalactic magnetic fields at $z \gtrsim 1$, placing new constraints on their origin and evolution.
