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