The Black Hole Explorer: Photon Ring Science, Detection and Shape Measurement

TL;DR

The paper proposes the Black Hole Explorer (BHEX), a space-based radio interferometry mission designed to image and analyze the photon rings of supermassive black holes M87* and Sgr A* to test general relativity and measure black hole parameters.

Contribution

It introduces a novel space-based interferometric experiment specifically aimed at imaging photon rings around black holes to confirm Kerr geometry and measure their properties.

Findings

Design of BHEX enables photon ring imaging at 100-300 GHz.

Expected to confirm Kerr black hole nature for M87* and Sgr A*.

Provides precise estimates of black hole masses and spins.

Abstract

General relativity predicts that black hole images ought to display a bright, thin (and as-of-yet-unresolved) ring. This "photon ring" is produced by photons that explore the strong gravity of the black hole, flowing along trajectories that experience extreme light bending within a few Schwarzschild radii of the horizon before escaping. The shape of the photon ring is largely insensitive to the precise details of the emission from the astronomical source surrounding the black hole and therefore provides a direct probe of the Kerr geometry and its parameters. 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 100 GHz through 300 GHz and from an orbital distance of ~30,000 km. This design will enable measurements of the photon rings around both M87* and Sgr A*, confirming the Kerr nature of these sources and delivering sharp estimates of their masses and spins.