Constraints on the magnetic field in the intercluster bridge A399-A401

Galaxy cluster mergers are natural consequences of structure formation in the Universe. Such events involve the dissipation of a large amount of energy (similar to 10(63) erg) during the process. Part of this energy can be channelled in particle acceleration and magnetic field amplification, enhancing non-thermal emission of the intra- and intercluster environment. Recently, low-frequency observations led to the detection of a bridge of diffuse synchrotron emission connecting two merging galaxy clusters, Abell 399 and Abell 401. This result provides clear observational evidence of relativistic particles and magnetic fields in between clusters. In this work, we used LOw Frequency ARray (LOFAR) observations at 144 MHz to study the polarised emission in the A399-A401 bridge region for the first time. No polarised emission was detected from the bridge region. Assuming a model where polarisation is generated by multiple shocks, depolarisation can be due to Faraday dispersion in the foreground medium with respect to the shocks. We constrained its Faraday dispersion to be greater than 0.10 rad m(-2) at 95% confidence level, which corresponds to an average magnetic field in the bridge region of greater than 0.46 nG (or 0.41 nG if we include regions of the Faraday spectrum that are contaminated by Galactic emission). This result is largely consistent with the predictions from numerical simulations for megaparsec regions where the gas density is about 300 times higher than the mean gas density.