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We present results from multifrequency polarimetry of NRAO 140 using the VeryLong Baseline Array. These observations allow us to reveal the distributions ofboth the polarization position angle and the Faraday rotation measure (RM).These distributions are powerful tools to discern the projected andline-of-sight components of the magnetic field, respectively. We find asystematic gradient in the RM distribution, with its sign being opposite ateither side of the jet with respect to the jet axis. The sign of the RM changesonly with the direction of the magnetic field component along the line ofsight, so this can be explained by the existence of helical magnetic componentsassociated with the jet itself. We derive two constraints for the pitch angleof the helical magnetic field from the distributions of the RM and theprojected magnetic field; the RM distribution indicates that the helical fieldsare tightly wound, while that of the projected magnetic field suggests they areloosely wound around the jet axis. This inconsistency may be explained if theFaraday rotator is not cospatial with theemitting region. Our results may pointtoward a physical picture in which an ultra-relativistic jet (spine) with aloosely wound helical magnetic field is surrounded by a sub-relativistic windlayer (sheath) with a tightly wound helical magnetic field.

Multifrequency Polarimetry of the Nrao 140 Jet: Possible Detection of a Helical Magnetic Field and Constraints on its Pitch Angle