  Accreting supermassive black holes can now be observed at the event-horizon
scale at mm wavelengths. Current predictions for the image rely on hypotheses
(fluid modeling, thermal electrons) which might not always hold in the vicinity
of the black hole, so that a full kinetic treatment is in order. In this
letter, we describe the first 3D global general-relativistic particle-in-cell
simulation of a black-hole magnetosphere. The system displays a persistent
equatorial current sheet. Synthetic images are computed by ray-tracing
synchrotron emission from nonthermal particles accelerated in this current
sheet by magnetic reconnection. We identify several time-dependent features of
the image at moderate viewing angles: a variable radius of the ring, and hot
spots moving along it. In this regime, our model predicts that most of the flux
of the image lies inside the critical curve. These results could help
understand future observations of black-hole magnetospheres at improved
temporal and spatial resolution.
