Algaba Marcos, Juan Carlos

A Rotation Measure Gradient on the M87 VLA Jet.

Author list: Juan Carlos Algaba, Keiichi Asada, Masanori Nakamura

Rotation measures (RMs) have proven to be an excellent tool to study magnetic field structures in AGNs. Here we study RM properties on kiloparsec scales of the M87 jet via stacked multi wavelength polarized VLA observations. Our results show for the first time an indication of the RM gradient transverse to the jet in knot A, and possibly knot C and HST-1. Motivated by the shape of the RM in knots A and B, we discuss that part of it may be a filamentary structure of higher RM due to an external Faraday screen, although we consider this unlikely because i) there is no total intensity counterpart, ii) no feature from external Faraday screen matches this structure and iii) we wouldn't detect the expected offset of the RM in the jet axis from the poloidal component of the magnetic field. The data presented here can be easily explained by a helical magnetic field. By combining this result together with polarization direction plus the U-shape and degree (~60%) of the fractional polarization across the jet, we can fairly conclude the presence of systematically wrapped, possibly helical, magnetic fields tightly wounded in knots A and C, in agreement with an MHD quad shock model. This also indicates a role of magnetic fields in the jet at scales far away (~10⁷ Rs) from the central engine.