A geometric crescent model for black hole images
  The Event Horizon Telescope (EHT), a global very long baseline interferometry
array operating at millimetre wavelengths, is spatially resolving the immediate
environments of black holes for the first time. The current observations of the
Galactic center black hole, Sagittarius A* (Sgr A*), and M87 have been
interpreted in terms of either geometric models (e.g., a symmetric Gaussian) or
detailed calculations of the appearance of black hole accretion flows. The
former are not physically motivated, while the latter are subject to large
systematic uncertainties. Motivated by the dominant relativistic effects of
Doppler beaming and gravitational lensing in many calculations, we propose a
geometric crescent model for black hole images. We show that this simple model
provides an excellent statistical description of the existing EHT data of Sgr
A* and M87, superior to other geometric models for Sgr A*. It also
qualitatively matches physically predicted models, bridging accretion theory
and observation. Based on our results, we make predictions for the
detectability of the black hole shadow, a signature of strong gravity, in
future observations.
