# A Mechanism for Triple-Ridge Emission Structure of AGN Jets

## TL;DR

This paper presents a steady axisymmetric force-free model of AGN jets that explains the observed triple-ridge structure in M87, linking electromagnetic fields, relativistic beaming, and electron distribution to jet imaging.

## Contribution

It introduces a novel force-free jet model that reproduces the triple-ridge emission structure observed in AGN jets, connecting electromagnetic structure with observable features.

## Key findings

- The model produces triple-ridge images consistent with observations.
- Relativistic beaming causes the central ridge component.
- Magnetic field structure influences limb-brightening and electron density.

## Abstract

Recent radio VLBI observations of the relativistic jet in M87 radio galaxy have shown a triple-ridge structure that consists of the conventional limb-brightened feature and a central narrow ridge. Motivated by these observations, we examine a steady axisymmetric force-free model of a jet driven by the central black hole (BH) with its electromagnetic structure being consistent with general relativistic magnetohydrodynamic simulations, and find that it can produce triple-ridge images even if we assume a simple Gaussian distribution of emitting electrons at the base of the jet. We show that the fluid velocity field associated with the electromagnetic field produces the central ridge component due to the relativistic beaming effect, while the limb-brightened feature arises due to strong magnetic field around the jet edge which also induces the electrons to be dense there. We also show that the computed image strongly depends on the electromagnetic field structure, viewing angle, and parameters related to the electrons' spatial distribution at the jet base. This study will help constraining the non-thermal electron injection mechanism of BH jets and be complementary to theoretical analyses of the upcoming data of Event Horizon Telescope.