2010ApJ...720...41A

We perform multi-frequency polarimetry toward 0836+710 using data from the Very Long Baseline Array. These observations allow us to measure both the distributions of the polarization position angle and the Faraday rotation measure (RM). We find a systematic gradient in the RM distribution as has been reported in several observations of relativistic jets emanating from active galactic nuclei. The RM corresponds to the line-of-sight component of the magnetic field. Thus, a systematic gradient of the RM along the transverse direction of the jet implies the existence of helical magnetic components associated with the jet itself. We derive the pitch angle of the helical magnetic field independently from the distribution of the projected magnetic field and from the RM data. Their discrepancies can be understood in a spine/sheath structure of the jet.