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

Abstract

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. 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. Published HI and radio continuum data at 21cm were combined with newly reduced archival WSRT polarisation data at the same wavelength to investigate the polarised emission in the radio lobes of B2 0258+35 where we detected a unique S-shaped polarised structure. We calculated the pressure to \(p=1.95 \pm 0.4 \cdot 10^{-14}\) dyn cm\(^{-2}\) using an energy equipartition approach and investigate the physical processes leading to the detected emission. We observe depolarisation in the northern lobe, which might originate from the HI-disc in the foreground. In addition we see an anti-correlation between the pressure and the fractional polarisation along the S-shaped structure. 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 (\(B_{eq}=1.21 \pm 0.12 \mu\)G), enabling super-Alfv\'enic motion of the rising lobes (\(M_A=2.47-3.50\)), 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.