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

TL;DR

The study tests magnetic-field geometry in the acceleration–collimation zone of the M87 jet by combining high-sensitivity, multifrequency VLBI polarimetry with forward axisymmetric MHD modeling that incorporates relativistic effects. It derives RM-corrected, intrinsic polarization maps across the ACZ, revealing a large-scale helical field with a substantial poloidal component and indicating dissipation of toroidal magnetic-field energy upstream of the observed region. Forward modeling shows that a near-unity toroidal-to-poloidal field ratio reproduces the data, which is inconsistent with pure ideal MHD toroidal dominance; this implies non-ideal MHD processes shaping the jet from roughly 10^4 Rg outward. The results constrain the black hole spin orientation and suggest a broader need to incorporate magnetic dissipation in jet formation theories across accreting black-hole systems.

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.

Figures (21)

• Figure 1: Observed and model images of the M87 jet. Top (a): Background color scale: Total intensity emission of the M87 jet (logarithmic scale) at 12.4 GHz in units of Jy per beam. Colored ticks: Intrinsic, RM-corrected EVPAs derived using the 12.4 -- 24.4 GHz frequency combination, with tick color indicating the fractional polarization ($m_L$) derived at 12.4 GHz. The image is rotated clockwise by 18 degrees to compare with the model images. The synthesized beam, corresponding to the lowest frequency (12.4 GHz) used in this combination, is indicated in the bottom right corner. All images within the combination were convolved to this resolution before deriving the intrinsic EVPAs. The white bracket in panel (a) and the connecting regions indicate the spatial range covered by the model images in panels (b) and (c). Bottom left (b): Model image: Total intensity emission in logarithmic scale using the fiducial parameter set, incorporating projection and relativistic aberration effects. Bottom right (c): Corresponding model polarization image: Linear polarization vectors, with color indicating the fractional polarization. It is noted that the black hole spin of $|a| = 0.998$ is used for this model (see Appendix \ref{['appendix:model']} for more details). These model results successfully reproduce key observed features: (i) nearly symmetric total intensity structure; (ii) higher fractional polarization in the northern than the southern jet part; and (iii) predominantly perpendicular EVPAs in the northern part, contrasted with mixed perpendicular/parallel EVPAs in the southern part.
• Figure 2: Geometric overview of the fiducial model and polarization vectors. The helical magnetic fields and jet flows of our fiducial jet model are shown in the observer's view, where the line of sight is perpendicular to the paper. The black and gray lines show two representative magnetic field lines measured in the fluid rest frame. The blue and light blue lines show two representative stream lines. The solid parts lie on the side near the observer while the dashed parts lie on the opposite side far from the observer. As indicated by arrows, the jet flows from left to right. Within each pair (upper/lower), the left and right panels display two different positions with the same coordinates $(X,Y)$, where the $X$-axis is aligned with the jet axis projected on the sky plane and the units of the $X$ and $Y$ axes are mas, but at different depths with respect to the observer as designated in the figure. Specifically, $(X/\mathrm{mas},Y/\mathrm{mas},z/R_g) = (20.9, 0.40,1.82\times10^4), (20.9, 0.40,1.93\times10^4), (20.9, -0.40,1.82\times10^4), (20.9, -0.40,1.93\times10^4)$, where $z$ is the physical height from the equatorial plane, for upper left, upper right, lower left, and lower right panels, respectively. Colored lines represent: purple, velocity field $\hat{\boldsymbol{v}}$; yellow, electric field of radiation $\hat{\boldsymbol{E}}_\mathrm{rad}$; black, magnetic field of the jet $\hat{\boldsymbol{B}}_\mathrm{jet}$, where we assumed $B'_p > 0$ and $B'_\phi > 0$, which is one possible choice consistent with the constraint $B'_p B'_\phi > 0$; pink, line of sight measured in the fluid rest frame $\boldsymbol{n}_\mathrm{obs}^\prime$; and green, rotational axis of the Lorentz transformation $\hat{\boldsymbol{\Omega}}\propto \boldsymbol{n}_\mathrm{obs}^\prime\times \hat{\boldsymbol{v}}$. The rotational angle is the same as the angle between $\boldsymbol{n}_\mathrm{obs}$ and $\boldsymbol{n}_\mathrm{obs}^\prime$, where quantities with prime denote those measured in the fluid rest frame. The solid and dashed lines are assigned to quantities measured in the laboratory and fluid rest frames, respectively. The velocity $\hat{\boldsymbol{v}}$ is determined by the jet structure of our model and determines the direction of $\boldsymbol{n}_\mathrm{obs}^\prime$ through the relativistic aberration effect, both of which fix the rotational axis $\hat{\boldsymbol{\Omega}}$ and angle of the Lorentz transformation of other vectors. The magnetic field is determined by our jet model and determines the plane of oscillating electric fields of radiation so that $\hat{\boldsymbol{E}}_\mathrm{rad}^\prime \propto \boldsymbol{n}_\mathrm{obs}^\prime \times \hat{\boldsymbol{B}}_\mathrm{jet}^\prime$ in the fluid rest frame. $\hat{\boldsymbol{E}}_\mathrm{rad}^\prime$ is Lorentz-transformed into $\hat{\boldsymbol{E}}_\mathrm{rad}$ by the rotation determined above. The observed radiation (total intensity and polarization) is obtained as a superposition of local electric field vectors $\hat{\boldsymbol{E}}_\mathrm{rad}$ along a ray.
• Figure 3: Alternative model polarization images of the M87 jet. Left (a): Linear polarization image from the model assuming the same physical parameters as those in Figure \ref{['fig:m87jet']}c but with an opposite black hole spin vector direction (toward the observer). This results in the image being flipped along the declination axis. Right (b): Linear polarization image from the model assuming the same physical parameters as presented in Figure \ref{['fig:m87jet']}c but with an assumed $a=0.5$ and $\Omega_{F} = \Omega_{\rm H}/2$ (i.e., assuming no dissipation between the horizon and the ACZ). Thus, this represents the expected image under ideal MHD conditions throughout the jet, leading to a toroidally dominated magnetic field structure at this scale. While these models can reproduce symmetric total intensity emission similar to that shown in Figure \ref{['fig:m87jet']}b, they do not replicate the observed linear polarization properties, particularly the asymmetry in the fractional polarization evident in Figure \ref{['fig:m87jet']}a.
• Figure 4: Schematic diagram summarizing our findings and inferred physics. The black hole's spin vector points away from the observer, consistent with interpretations of EHT observations EHT2019eEHT2025. Within the jet ACZ (the 'downstream region' depicted), the jet is threaded by a large-scale helical magnetic field (rest-frame pitch angle $45^\circ$). The discrepancy between this structure and the prediction under the ideal MHD condition implies significant dissipation of the toroidal magnetic field component, conceptually illustrated by the light green zone representing the inferred region where significant toroidal magnetic field dissipation is required to occur 'upstream' (closer to the black hole than the region dominated by the observed helical field). Note that while a single conceptual zone is depicted for simplicity, the actual underlying dissipation is likely an extended process, potentially distributed over considerable distances or occurring in multiple regions, and is not explicitly modeled here. The jet exhibits slow clockwise rotation (viewed from top) superimposed on its dominant poloidal motion.
• Figure 5: Intrinsic polarization maps for lower frequency combinations. Similar to Figure \ref{['fig:m87jet']}a, these panels show the total intensity emission (background color scale, logarithmic) and intrinsic, RM-corrected EVPAs (colored ticks; color indicates fractional polarization derived at the lowest observing frequency for each frequency combination) for the M87 jet, derived using lower frequency combinations. Top: Results from the 8.2--12.4 GHz combination, which probe the jet structure at larger distances. The image has been rotated clockwise by $21^\circ$ relative to the observed images to align the jet axis with the $x$-axis. The cyan box indicates the region shown in Figure \ref{['fig:m87jet']}a. Bottom: Results from the 2.2--4.9 GHz combination. The image has been rotated clockwise by $23^\circ$ relative to the observed images. Note the different spatial scale used in each panel compared to Figure \ref{['fig:m87jet']}a. The synthesized beam for each combination (corresponding to the lowest frequency in that set) is shown in the bottom right corner of each panel.