# Universal Interferometric Signatures of a Black Hole's Photon Ring

## TL;DR

This paper demonstrates that the photon ring substructures around black holes produce universal interferometric signatures, enabling precise measurements of black hole properties and tests of general relativity with sparse arrays.

## Contribution

It reveals that the self-similar subrings of the photon ring generate universal interferometric signatures, providing new methods for black hole parameter estimation and relativistic tests.

## Key findings

- Photon ring subrings produce strong interferometric signatures
- Signatures enable measurements of black hole mass and spin
- Method allows tests of general relativity with sparse arrays

## Abstract

The Event Horizon Telescope image of the supermassive black hole in the galaxy M87 is dominated by a bright, unresolved ring. General relativity predicts that embedded within this image lies a thin "photon ring," which is composed of an infinite sequence of self-similar subrings that are indexed by the number of photon orbits around the black hole. The subrings approach the edge of the black hole "shadow," becoming exponentially narrower but weaker with increasing orbit number, with seemingly negligible contributions from high order subrings. Here, we show that these subrings produce strong and universal signatures on long interferometric baselines. These signatures offer the possibility of precise measurements of black hole mass and spin, as well as tests of general relativity, using only a sparse interferometric array.