Discovery of femto Gauss intergalactic magnetic fields towards Mkn 501

Abstract: The existence of intergalactic magnetic fields (IGMF) has so far not been established through observations. The IGMF is expected to be generated either via processes mainly connected to astrophysical processes or it could be a relic of phase transitions in the early universe. Upper bounds to the average field present are set via observations of Faraday rotation measure. Lower bounds have been derived from the non-detection of secondary gamma-rays possibly produced in electromagnetic cascades. We investigate the presence of IGMFs by studying the GeV gamma-ray emission from the nearby blazar Mkn~501 ($z=0.034$), searching for evidence of the extended halo expected to be observed around the point source. We analyse 14 years of data from Fermi-LAT and Swift-XRT/BAT to construct a time-average synchrotron-self Compton model for the TeV spectrum of Mkn~501. This injection spectrum is used to simulate the resulting cascade emission with the ELMAG code for different magnetic field and coherence length configurations. These templates are fit to the Fermi-LAT data to find a best-fitting model for the cascade emission. We find significant ($\ge 5 \sigma$ trial-corrected) evidence for extended secondary emission around Mkn~501, which is consistent with an IGMF with $B_\mathrm{rms}=1.5_{-0.6}^{+1.6}\times 10^{-15}~\mathrm{G}$ and a coherence length of $\ell_C=(10\pm 3)~\mathrm{kpc}$. The source needs to actively inject TeV gamma-rays for at least 45000 years to match the level of secondary emission. Our results indicate that the secondary gamma-rays are significantly present in the $\it{Fermi}$-LAT data. The effect of plasma-heating by pairs in the cascade appears to be negligible for Mkn 501. This is consistent with the observation that Mkn~501 is one of the objects with the lowest injection power among the blazars studied in the context of cascade emission.