Citation:
Abstract:
The photon ring is a narrow ring-shaped feature (predicted by general relativity, but not yet observed) that appears in black hole images. It is caused by the extreme bending of light within a few Schwarzschild radii of the event horizon and provides a direct probe of the unstable bound photon orbits of the Kerr black hole geometry. The precise shape of the observable photon ring is remarkably insensitive to the details of the astronomical source and can therefore be used as a precise probe of strong-field gravity. The Black Hole Explorer (BHEX) is a proposed space-based experiment targeting the supermassive black holes M87* and Sgr A* with radio-interferometric observations at frequencies of 86 through 345 GHz and from an orbital distance of ~40,000km. We forecast that its design will enable a measurement of the photon rings around M87* and Sgr A* and confirm the Kerr nature of these two sources.
