New constraints on the magnetization of the cosmic web using LOFAR
  Faraday rotation observations
  Measuring the properties of extragalactic magnetic fields through the effect
of Faraday rotation provides a means to understand the origin and evolution of
cosmic magnetism. 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 $\Delta$RM along the line of sight between
non-physical pairs. From our analysis, we find no significant difference
between the $\Delta$RM distributions of the physical and non-physical pairs,
limiting the excess Faraday rotation contribution to $< 1.9$ rad/m$^2$
($\sim$$95\%$ confidence). We use this limit with a simple model of an
inhomogeneous universe to place an upper limit of 4 nG on the cosmological
co-moving magnetic field strength on Mpc scales. We also compare the RM data
with a more realistic suite of cosmological MHD simulations, that explore
different magnetogenesis scenarios. Both magnetization of the large scale
structure by astrophysical processes such as galactic and AGN outflows, and
simple primordial scenarios with seed magnetic field strengths $< 0.5$ nG
cannot be rejected by the current data; while stronger primordial fields or
models with dynamo amplification in filaments are disfavoured.
