Fast-spinning black holes inferred from symmetrically limb-brightened radio jets

TL;DR

This study models relativistic jets from black holes to explain symmetrically limb-brightened radio images, linking jet structure to black hole spin and magnetic field configurations, and providing insights for interpreting observations like those from the Event Horizon Telescope.

Contribution

It demonstrates that symmetric limb-brightened jets are produced by magnetic fields penetrating fast-spinning black holes, offering a theoretical basis for interpreting observed jet images.

Findings

Symmetric limb-brightening occurs with fast-spinning black holes.

Asymmetric images are linked to slowly spinning black holes or disk fields.

Counter-jet edges can appear luminous even at small viewing angles.

Abstract

This paper theoretically investigates the relations between the structure of relativistic jets and produced synchrotron images, by using a steady axisymmetric force-free jet model. We especially focus on the limb-brightened jets that are largely symmetric to the jet axes and observed in some active galactic nuclei such as M87, Mrk~501, Cyg~A, and 3C84. We find that symmetrically limb-brightened images can be produced when magnetic field lines of the jet penetrate a fast-spinning black hole as motivated by the Blandford-Znajek mechanism. On the other hand, jets with magnetic field lines that pass through a slowly spinning black hole or the Keplerian accretion disk produce highly asymmetric radio images. In addition, the edge of a counter jet tends to be luminous in the accretion-disk model even for rather small viewing angles, which may be problematic for some observed jets. We also suggest that the site of particle accelerations in relativistic jets can be constrained by fitting the radio images to observations. This kind of study focusing on the jet images far away from the central engine is complimentary to those concentrating directly on the innermost region with upcoming data of the Event Horizon Telescope.