Polarised structures in the radio lobes of B2 0258+35 Evidence of magnetic draping?

Context. The contribution of active galactic nuclei to the magnetisation of the Universe can be constrained by knowing their duty cycles, jet and magnetic field morphologies, and the physical processes dominating their interaction with the surrounding environment. Aims. The magnetic field morphology and strength of radio lobes of AGN has an influence on the mechanisms for the propagation of cosmic rays into intergalactic space. Using the source B2 0258+35 we want to investigate the interaction of its radio lobes with the surrounding environment and examine the underlying physical effects. Methods. Published H I and radio continuum data at lambda 21 cm were combined with newly reduced archival Westerbork Radio Synthesis Telescope polarisation data at the same wavelength to investigate the polarised emission in the radio lobes of B2 0258+35. We assumed energy equipartition between the cosmic rays and the magnetic field to calculate their pressure and investigate the physical processes leading to the detected emission. Results. We detected a unique S-shaped diffuse polarised structure. The lobes have a pressure of p = 1.95 +/- 0.4 x 10(-14) dyn cm(-2). The calculated total magnetic field strengths are low (B-eq = 1.21 +/- 0.12 mu G). We observe depolarisation in the northern lobe, which might originate from the H I-disc in the foreground. In addition we see an anti-correlation between the pressure and the fractional polarisation along the S-shaped structure. Therefore we consider magnetic draping and magnetic field compression as possible effects that might have created the observed S-shape. Conclusions. Our results suggest that magnetic draping can be effectively used to explain the observed polarised structures. This is likely due to the combination of a relatively low magnetic field strength, enabling super-Alfvenic motion of the rising lobes (with M-A = 2.47-3.50), and the coherency of the surrounding magnetic field. Moreover, the draped layer tends to suppress any mixing of the material between the radio lobes and the surrounding environment, but can enhance the mixing and re-acceleration efficiencies inside the lobes, providing an explanation for the average flat spectral index observed in the lobes.