2024ApJ...975..103P

Faraday rotation is an important probe of the magnetic fields and magnetized plasma around active galactic nuclei jets. We present a Faraday rotation measure (RM) image of the M87 jet between 85.2 and 101.3 GHz with a resolution of ∼2″ with the Atacama Large Millimeter/submillimeter Array. We found that the RM of the M87 core is (4.5 ± 0.4) × 10⁴ rad m^–2 with a low linear polarization fraction of (0.88 ± 0.08)%. The spatial RM gradient in the M87 jet spans a wide range from ∼ -2 × 10⁴ rad m^–2 to ∼3 × 10⁴ rad m^–2 with a typical uncertainty of 0.3 × 10⁴ rad m^–2. A comparison with previous RM measurements of the core suggests that the Faraday rotation of the core may originate very close to the supermassive black hole. Both an internal origin and an external screen with a rapidly varying emitting source could be possible. As for the jet, the RM gradient indicates a helical configuration of the magnetic field that persists up to the kiloparsec scale. Combined with the kiloparsec-scale RM measurements at lower frequencies, we found that RM is frequency-dependent in the jet. One possible scenario to explain this dependence is that the kiloparsec-scale jet has a trumpet-like shape, and the jet coil unwinds near its end.