Helical Magnetic Field in the Acceleration–Collimation Zone of the M87 Jet

Abstract Relativistic jets from supermassive black holes are expected to be magnetically launched and guided, with magnetic energy systematically converted to bulk kinetic energy throughout an extended acceleration–collimation zone (ACZ). A key prediction of magnetohydrodynamic (MHD) models is a transition from poloidally dominated fields near the engine to toroidally dominated fields downstream, yet direct tests within the ACZ are hampered by weak polarization and strong Faraday rotation. We report quasi-simultaneous, high-sensitivity, multifrequency very long baseline interferometric polarimetry of M87 spanning 1.4–24.4 GHz. We present high-fidelity, Faraday-rotation-corrected maps of intrinsic linear polarization that continuously resolve the ACZ in the deprojected distance range of ∼9.0 × 10 3 to ∼3.6 × 10 5 gravitational radii from the black hole. The maps reveal pronounced north–south asymmetries in fractional linear polarization and electric vector position angle (EVPA), peaking in the inner ACZ at a projected distance of ∼20 mas along the jet and remaining prominent out to ∼100 mas. These signatures are best reproduced by models with a large-scale ordered helical field that retains a substantial poloidal component—contrary to the rapid toroidal dominance expected under steady ideal MHD. This tension implies ongoing magnetic dissipation that limits toroidal buildup over the ACZ. The handedness of the helix provides an independent constraint on the BH’s spin direction, supporting a spin vector oriented away from the observer, consistent with the orientation inferred from horizon-scale imaging. Farther downstream, the asymmetries diminish, and the EVPA and fractional polarization distributions become more symmetric; we tentatively interpret this as evolution toward a more poloidally dominated configuration, while noting current sensitivity and dynamic range limits.