Abstract
Abstract A distinct visual signature occurs in black holes that are surrounded by optically thin and geometrically thick emission regions. This signature is a sharp-edged dip in brightness that is coincident with the black hole’s shadow, which is the projection of the black hole’s unstable-photon region on the observer’s sky. We highlight two key mechanisms that are responsible for producing the sharp-edged dip: (i) the reduction of intensity observed in rays that intersect the unstable-photon region, and thus the perfectly absorbing event horizon, versus rays that do not (blocking); and (ii) the increase of intensity observed in rays that travel along extended, horizon-circling paths near the boundary of the unstable-photon region (path-lengthening). We demonstrate that the black hole shadow is a distinct phenomenon from the photon ring, and that models exist in which the former may be observed but not the latter. Additionally, we show that the black hole shadow and its associated visual signature differ from the more model-dependent brightness depressions associated with thin-disk models because the blocking and path-lengthening effects are quite general for geometrically thick and optically thin emission regions. Consequentially, the black hole shadow is a robust and fairly model-independent observable for accreting black holes that are in the deep sub-Eddington regime, such as low-luminosity active galactic nuclei.
Highlights
As long as the emission region is optically thin and geometrically thick, a strong visual signature that is coincident with the shadow should be observable
A concern exists that the Event Horizon Telescope (EHT) has produced images of the detailed astrophysical circumstances of the accretion disk/jet system, rather than of the black-hole shadow as an observable that is directly informative about the black hole’s spacetime geometry. It is claimed in Gralla (2020) that the black-hole shadow, as an observable feature, is “not generic”, but that the photon ring - a term which has been defined in several ways, but which we here take to mean: the complex system of lensed images of the emission region which nearly coincides with the boundary of the shadow - is generic
If the black hole is surrounded by an optically thin, geometrically thick emission region, a step-like dip in brightness will be observed that is coincident with the shadow
Summary
In the recreational practice of coin rubbing, a sheet of paper is placed over a coin, and a pencil or crayon is rubbed on the paper. The presence of a geometrically thick, optically thin emission region directly surrounding a black hole’s event horizon gave rise to the concept of the black-hole shadow (Falcke et al 2000a) that is observed when imaging such accreting black holes, and whose features depend only on spacetime/observer geometry (Johannsen 2016; Johannsen & Psaltis 2010a; Bronzwaer et al 2020). The concept was derived from the work by Bardeen (1973), who calculated the optical appearance of a black hole in front of a plane-parallel, uniformly radiating disk While this scenario, e.g., a black hole in front of a star, is impossible to observe with current technology, the key visual element of this appearance is the so-called photon sphere - the boundary of the region around the black hole in which closed, circular photon orbits exist. For these reasons, observing the black-hole shadow was a key driver for the Event Horizon Telescope (EHT, Event Horizon Telescope Collaboration et al 2019a) and one of its clearest predictions (Falcke et al 2000a; Broderick & Loeb 2006; Kamruddin & Dexter 2013; Moscibrodzka et al 2016)
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