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

TL;DR

This study uses high-sensitivity, multifrequency VLBI polarimetry to map the magnetic field structure in the M87 jet's acceleration-collimation zone, revealing a large-scale helical field with a significant poloidal component, challenging steady MHD predictions.

Contribution

It provides the first high-resolution, Faraday rotation-corrected polarization maps of the M87 jet's ACZ, showing evidence for a helical magnetic field with ongoing dissipation, contrary to expectations of toroidal dominance.

Findings

Detection of a large-scale, ordered helical magnetic field.

Evidence of ongoing magnetic dissipation limiting toroidal buildup.

Support for a black hole spin orientation away from the observer.

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 1.4-24.4GHz. We present high-fidelity, Faraday rotation-corrected maps of intrinsic linear polarization that continuously resolve the ACZ in the de-projected distance range of ~9e3 to ~3.6e5 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 ~20mas along the jet and remaining prominent out to ~100mas. 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 black hole'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.