Multifrequency polarimetry of the NRAO 140 jet: Possible detection of a helical magnetic field and constraints on its pitch angle

We present results from multifrequency polarimetry of NRAO 140 using the Very Long Baseline Array. These observations allow us to reveal the distributions of both the polarization position angle and the Faraday rotation measure (RM). These distributions are powerful tools to discern the projected and line-of-sight components of the magnetic field, respectively. We find a systematic gradient in the RM distribution, with its sign being opposite at either side of the jet with respect to the jet axis. The sign of the RM changes only with the direction of the magnetic field component along the line of sight, so this can be explained by the existence of helical magnetic components associated with the jet itself. We derive two constraints for the pitch angle of the helical magnetic field from the distributions of the RM and the projected magnetic field; the RM distribution indicates that the helical fields are tightly wound, while the distribution of the projected magnetic field suggests they are loosely wound around the jet axis. This inconsistency may be explained if the Faraday rotator is not cospatial with the emitting region. Our results may point toward a physical picture in which an ultrarelativistic jet ("spine'') with a loosely wound helical magnetic field is surrounded by a subrelativistic wind layer ("sheath'') with a tightly wound helical magnetic field.