Fast-spinning Black Holes Inferred from Symmetrically Limb-brightened Radio Jets

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 (BH), as motivated by the Blandford–Znajek mechanism. On the other hand, jets with magnetic field lines that pass through a slowly spinning BH or the Keplerian accretion disk produce highly asymmetric radio images. In addition, the edge of a counterjet 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 complementary to those concentrating directly on the innermost region with upcoming data from the Event Horizon Telescope.