Self-Lensing Flares from Black Hole Binaries: Observing Black Hole Shadows via Light Curve Tomography.

Supermassive black hole (BH) binaries are thought to produce self-lensing flares (SLFs) when the two BHs are aligned with the line of sight. If the binary orbit is observed nearly edge-on, we find a distinct feature in the light curve imprinted by the relativistic shadow around the background ("source") BH. We study this feature by ray tracing in a binary model and predict that 1% of the current binary candidates could show this feature. Our BH tomography method proposed here could make it possible to extract BH shadows that are spatially unresolvable by high-resolution very long base line interferometry (VLBI).

Fuzzball Shadows: Emergent Horizons from Microstructure.

We study the physical properties of four-dimensional, string-theoretical, horizonless "fuzzball" geometries by imaging their shadows. Their microstructure traps light rays straying near the would-be horizon on long-lived, highly redshifted chaotic orbits. In fuzzballs sufficiently near the scaling limit this creates a shadow much like that of a black hole, while avoiding the paradoxes associated with an event horizon. Observations of the shadow size and residual glow can potentially discriminate between fuzzballs away from the scaling limit and alternative models of black compact objects.

Gravitational Test beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole.

The 2017 Event Horizon Telescope (EHT) observations of the central source in M87 have led to the first measurement of the size of a black-hole shadow. This observation offers a new and clean gravitational test of the black-hole metric in the strong-field regime. We show analytically that spacetimes that deviate from the Kerr metric but satisfy weak-field tests can lead to large deviations in the predicted black-hole shadows that are inconsistent with even the current EHT measurements. We use numerical calculations of regular, parametric, non-Kerr metrics to identify the common characteristic among these different parametrizations that control the predicted shadow size. We show that the shadow-size measurements place significant constraints on deviation parameters that control the second post-Newtonian and higher orders of each metric and are, therefore, inaccessible to weak-field tests. The new constraints are complementary to those imposed by observations of gravitational waves from stellar-mass sources.