Resolving the n = 1 photon subring of M87* and Sgr A*: detection and percent-level Kerr shape test
Difficulty 53: the physics and the signature are well understood, but detection needs a space-VLBI mission that is not yet selected, and a percent-level shape test must marginalise over plasma physics at n = 1; this puts it just below B-modes (55) and the Earth-twin RV problem (55), which are similarly instrument-gated, and above the GW spin-origin problem (44). Application value 5.2: a strong-field test with modest technology spin-off. Leverage 35: it would provide an EM Kerr test and spin measurements complementary to ringdown (38), below neutron-star EOS (52).
The problem
Spot the thin extra rings of light that wrap around a black hole, and use their shape to test Einstein.
- 1Some light circles the black hole first
- 2Each lap makes a thinner, fainter ring
- 3Ring shape depends only on mass and spin
- 4Needs a telescope bigger than Earth
- 5Space dish could test Einstein's black hole
The physics. In a black-hole image, light rays that make n half-orbits around the hole produce the n-th "subring". For a Kerr black hole, the n-th subring approaches the critical curve (the edge of the shadow) with exponentially smaller flux: F_(n+1)/F_n is about e^(−γ) at large n, where γ is the Lyapunov exponent of the bound photon orbits (γ = π for Schwarzschild, a flux ratio of about 1/23). At n = 1 the ring still depends somewhat on the emission geometry, so it tracks the critical curve only approximately. The ring's diameter and shape are set by mass and spin, with a strongly constrained shape for Kerr.
Path to understanding
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- Radio interferometry and aperture synthesisAlgorithm · host
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- Resolving the n = 1 photon subring of M87* and Sgr A*: detection and percent-level Kerr shape testThe problem
State of the art
EHT images of M87* and Sgr A* resolve the bright n = 0 emission ring but not the n ≥ 1 subrings. Broderick et al. (ApJ 935, 61, 2022) claimed to isolate the n = 1 ring in EHT 2017 M87* data with a hybrid image model; the claim is contested because the 2017 baselines do not reach the ring's characteristic visibility signature, so there is no undisputed detection. Johnson et al. (arXiv:1907.04329, Sci. Adv. 2020) showed that each subring produces a universal damped-periodic visibility signature and that n = 1 needs baselines beyond the Earth at 230 GHz; Gralla, Lupsasca and Marrone (arXiv:2008.03879, PRD 2020) showed that the ring's shape gives a precise GR test. The Black Hole Explorer (BHEX; Johnson et al., arXiv:2406.12917; photon-ring case by Lupsasca et al., arXiv:2406.09498) is a space-VLBI concept observing in two bands near 100 and 300 GHz from an orbit of about 30,000 km; its 2025 SMEX target slipped, and as of April 2026 MIT Haystack said it will be proposed to NASA's Small Explorer call in mid-2026, with no selection announced by October 2026. Theory continues on non-Kerr alternatives: Suzuki et al. (arXiv:2511.20756, preprint) find that high-order rings of a Kerr naked singularity with a = 1.01 differ measurably from Kerr in visibility space.
Realistic applications
A photon-ring measurement would be a strong-field test of general relativity independent of gravitational waves and would measure black-hole spin; the space-VLBI technology (precision phase referencing, high-rate downlink) has spin-offs for deep-space communication and radio astronomy.
What counts as solved
Detection of the n=1 subring signature in visibility data of M87* or Sgr A* at ≥ 5σ, with a measured ring diameter consistent (or inconsistent) with the Kerr critical-curve prediction at ≲ 1–2% precision after marginalising over astrophysical plasma models.
How it gets verified
ObservationSettled by new measurements or observations.
Needs new interferometric visibility data detecting the n = 1 subring of M87* or Sgr A* at 5 sigma or more, with a ring diameter measured to about 1 to 2% precision after marginalising over plasma models, to test consistency with the Kerr critical curve.
Claims under verification
+ add a claimNo claimed results on the Verification board for this problem yet.
Where the work is
Mainly building and flying a space-VLBI element with stable phase referencing; analysis needs plasma-model simulations.
Bottlenecks
ranked by votes- #1EngineeringSpace VLBI at 100–300 GHz
A dish in medium Earth orbit must carry ultra-stable frequency standards, cryogenic receivers and high-rate data links, and maintain phase coherence with ground stations at these frequencies.
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Progress
- Jun 13, 2024BHEX photon-ring science casesource
Lupsasca et al. describe the space-VLBI design for photon-ring detection and Kerr tests.
- Aug 1, 2022Contested n = 1 claim in EHT 2017 datasource
Broderick et al. report isolating the M87* photon ring with a hybrid model; the claim is not accepted as a detection (month of publication; day set to the first).
- Aug 10, 2020Photon-ring shape as a GR testsource
Gralla, Lupsasca and Marrone derive the shape test for the critical curve.
- Jul 9, 2019Universal interferometric signature of photon ringssource
Johnson et al. show subrings dominate long-baseline visibilities with a damped periodic signature (arXiv submission month; day approximate).