IGMF limits with LOFAR

Here, we use data from the LOFAR Two-Metre Sky Survey (LoTSS) to calculate the Faraday rotation measure (RM) of close pairs of extragalactic radio sources. By considering the RM difference ({Delta}RM) between physical pairs (e.g. double-lobed radio galaxies) and non-physical pairs (i.e. close projected sources on the sky), we statistically isolate the contribution of extragalactic magnetic fields to RM along the line of sight between non-physical pairs.

From our analysis, we find no significant difference between the RM distributions of the physical and non-physical pairs, limiting the excess Faraday rotation contribution to <1.9rad/m^2^ (~95 per cent confidence). We use this limit with a simple model of an inhomogeneous universe to place an upper limit of 4nG on the cosmological co-moving magnetic field strength on Mpc scales. We also compare the RM data with a more realistic suite of cosmological magnetohydrodynamical simulations that explore different magnetogenesis scenarios.