Photon Shells, Black Hole Shadows, and Accretion Toroids

Abstract We analyze the properties of the Kerr black hole (BH) photon shell, focusing on the influence of aggregates of corotating and counterrotating toroids (ringed accretion disks, hereafter RAD), orbiting in the BH photon shell (photon shell “obscuration”). The particular case of a corotating accretion disk orbiting in the BH ergoregion is also investigated. We study influence on the BH shadow boundary, fixing the conditions under which it is possible to observe the ergoregion and RAD “imprint” on the shadows boundary. In general, remarkable differences appear between the counterrotating photon components of the boundary with respect to the corotating ones. Various regions of the shadow boundary generated from orbital regions linked to the aggregates of orbiting structures have been analyzed. In particular, we investigate the bound unstable spherical photon orbits in the ergoregion, or at the inversion point, where (toroidal) <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>ϕ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>=</mml:mo> <mml:mn>0</mml:mn> </mml:math> , or with the impact parameter ℓ = 0 to characterize the effects of the frame dragging on the boundary. Five main classes of BHs have been identified by distinguishable features of their photon shells (and shadow boundaries) in the context of the orbiting RAD.