Universal interferometric signatures of a black hole’s photon ring

[BillTre]

TL;DR Summary

Photon rings around black holes are proposed to provide diagnostic signals concerning some black hole's properties.

This sounds interesting to me.
I would appreciate the opinions of more knowledgeable and physics oriented forumists.

The paper is in Science (I don't think its open access).
Here is the 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.

 

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In particular, we highlight the possibility of detecting the leading n = 1 subring using a station in low Earth orbit, the n = 2 subring using a station on the Moon, and the n = 3 subring using a station in the Sun-Earth L₂ orbit.

The EHT collaboration is looking into telescopes in Earth orbit to improve the angular resolution. Telescopes farther out are quite a bit into the future. With a single telescope having a long baseline they probably need many observations and long observation times, too.