Jets and Rings in Images of Spinning Black Holes

We develop a "dual cone" model for millimeter wavelength emission near a spinning black hole. The model consists of optically thin, luminous cones of emission, centered on the spin axis, which are meant to represent jet walls. The resulting image is dominated by a thin ring. We first consider the effect of black hole's spin on the image, and show that the dominant effect is to displace the ring perpendicular to the projection of the spin axis on the sky by $2 a_* \sin i + \mathcal{O}(a_*^3)$. This effect is lower order in $a_*$ than changes in the shape and size of the photon ring itself, but is undetectable without a positional reference. We then show that the centerline of the jet can provide a suitable reference: its location is exactly independent of spin if the observer is outside the cone, and nearly independent of spin if the observer is inside the cone. If astrophysical uncertainties can be controlled for, then spin displacement is large enough to be detectable by future space VLBI missions. Finally, we consider ring substructure in the dual cone model and show that features in total intensity are not universal and depend on the cone opening angle.Comment: 9 pages, 5 figures, submitted to Ap