Faraday rotation at low frequencies: magnetoionic material of the large FRII radio galaxy PKS J0636−2036

We present a low-frequency, broad-band polarization study of the FRII radio galaxy PKS J0636−2036 (z = 0.0551), using the Murchison Widefield Array (MWA) from 70 to 230 MHz. The northern and southern hotspots (separated by ∼14.5 arcmin on the sky) are resolved by the MWA (3.3 arcmin resolution) and both are detected in linear polarization across the full frequency range. A combination of Faraday rotation measure (RM) synthesis and broad-band polarization model fitting is used to constrain the Faraday depolarization properties of the source. For the integrated southern hotspot emission, two-RM-component models are strongly favoured over a single RM component, and the best-fitting model requires Faraday dispersions of approximately 0.7 and 1.2 rad m−2 (with a mean RM of ∼50 rad m−2). High-resolution imaging at 5 arcsec with the Australia Telescope Compact Array shows significant sub-structure in the southern hotspot and highlights some of the limitations in the polarization modelling of the MWA data. Based on the observed depolarization, combined with extrapolations of gas density scaling relations for group environments, we estimate magnetic field strengths in the intergalactic medium between ∼0.04 and 0.5 μG. We also comment on future prospects of detecting more polarized sources at low frequencies.