Multifrequency simultaneous VLBA view of the radio source 3C 111

Context. Relativistic jets originating at the center of active galactic nuclei are embedded in extreme environments with strong magnetic fields and high particle densities, which makes them a fundamental tool for studying the physics of magnetized plasmas. Aims. We aim to investigate the magnetic field structure and the parsec and sub-parsec properties of the relativistic jet in the radio galaxy 3C 111. Rotation measure (RM) studies of nearby radio-galaxies, such as this one, provide a valuable tool with which to investigate the transversal magnetic field properties. Methods. We used multifrequency simultaneous Very Long Baseline Array (VLBA) data from 5 ? GHz up to 87.6 ? GHz. We performed an analysis of both total intensity and polarization maps to study the jet magnetic field and infer the spectral properties of the synchrotron emission. We modeled the brightness distribution of the source with multiple 2D Gaussian components to characterize individual emission features. Results. After determining the core shift (rcv1.27±0.17), we computed the spectral index maps for all the adjacent frequency pairs and found different distributions for the core region (αmax1.5 and αmin0.2) and the jet (α1.5 on average) with an unusual optically thick or flat feature in it at 12 pc from the core. Using modelfit, we found a total of 56 components at different frequencies. By putting constraints on the size and position, we identified 22 components at different frequencies for which we computed the equipartition magnetic field. We computed the rotation measure at two different triplets of frequencies. At 15.2 21.9 43.8 GHz, we discovered high values of RM in the same region where the optically thick or flat feature was found. This can be associated with a high electron density value at 12 pc from the core that we interpret as originating in a cloud of the clumpy torus. At 58.415.2 ? GHz, we found a distribution of the electric vector position angles and significant RM transverse gradient that provide strong evidence of a helical configuration of the magnetic field, as found in simulations.