The parsec-scale distributions of intensity, linear polarization and Faraday rotation in the core and jet of Mrk 501 at 8.4-1.6 GHz

Previous very long baseline interferometry (VLBI) observations of the nearby (z = 0.0337) active galactic nucleus (AGN) Mrk 501 have revealed a complex total-intensity structure with an approximately 90° misalignment between the jet orientations on parsec and kiloparsec scales. The jet displays a ‘spine’ of magnetic field orthogonal to the jet surrounded by a ‘sheath’ of magnetic field aligned with the jet. Mrk 501 is also one of a handful of AGN that are regularly detected at TeV energies, indicating the presence of high-energy phenomena in the core. However, multi-epoch analyses of the VLBI total-intensity structure have yielded only very modest apparent speeds for features in the VLBI jet. We investigate the total-intensity and linear-polarization structures of the parsec- to decaparsec-scale jet of Mrk 501 using VLBA observations at 8.4, 5, 2.2 and 1.6 GHz. The rotation-measure distribution displays the presence of a Faraday rotation gradient across an extended stretch of the jet, providing new evidence for a helical magnetic field associated with the jet of this AGN. The position of the radio core from the base of the jet follows the law rcore(ν) ∝ν−1.1±0.2, consistent with the compact inner jet region being in equipartition. Hence, we estimate a magnetic field strength of ∼40 mG at a distance of 1 pc.