Universal Interferometric Signatures of a Black Hole's Photon Ring (1907.04329v2)

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.

• The paper introduces a theoretical model of the black hole photon ring, predicting multiple self-similar subrings with damped oscillatory interferometric signatures.
• Advanced VLBI techniques are proposed to detect these narrow subrings, offering precise measurements of black hole mass, spin, and spacetime geometry.
• The study outlines future observational strategies including high-frequency and space-based arrays to validate general relativity in extreme gravitational fields.

Overview of "Universal Interferometric Signatures of a Black Hole's Photon Ring"

The research paper authored by Michael D. Johnson et al. addresses the theoretically predicted photon ring of a black hole and its potential observable signatures using very long baseline interferometry (VLBI). This subject is garnering more attention following the Event Horizon Telescope (EHT) observations, which confirmed the presence of a bright ring around the supermassive black hole (SMBH) in galaxy M87. Notably, within this observed structure, general relativity predicts a "photon ring" comprising an infinite sequence of increasingly faint and narrow subrings. These subrings emanate from photons that complete multiple orbits around the black hole before reaching the observer's telescope.

Photon Ring Structure and Subrings

The illuminating core of this paper is the theoretical elucidation of the photon ring and its constitutive subrings. The photon ring is a consequential feature in the image of a black hole as it represents a boundary close to the so-called black hole shadow. It is characterized by self-similar subrings indexed via half-orbit numbers of photons. Each subring approaches the boundary of the shadow and attenuates proportional to the Lyapunov exponents, essential metrics of orbital instability in Kerr black hole spacetime. The photon ring's observable features are thus encased in their exponential narrowness and diminishing brightness.

Interferometric Signatures and Observational Implications

A significant portion of the paper delineates how these subrings can impart distinct interferometric signatures on long baselines. The authors argue that interferometric arrays, even with sparse sensor configurations, could resolve these signatures, enabling not just mass and spin measurements, but also precision tests of spacetime geometry through general relativity. The paper elucidates this using mathematical extensions from geometrical models of the black hole's photon ring extrapolated from prior EHT experiments.

The team's exploration into the interferometric signatures includes derivation of expected visibilities for thin, thick, uniform, and non-uniform rings. The capture and analysis of such visibility signatures through reconciling damped oscillatory patterns in baseline data provide means to infer details of the space-time geometry. Notably, subsequent subrings serve as a series of radial damped oscillations, with each subring conveying detailed information about the black hole's characteristics.

Future Observational Prospects

The potential realization of these theoretical predictions requires enhancements in current observational arrays. However, according to the authors, these endeavors are feasible. The EHT or similar frameworks could achieve the necessary baseline organization with either concentrating on higher observing frequencies or by extending the array with space-based interferometers. Such modifications would allow the measurement of specific subring characteristics, notably the primordial $n=1$ subring reaching into low Earth orbit capacities and the $n=2,3$ subrings with more extensive setups, including lunar and Lagrange-point-based observatories.

Conclusion

In summary, this paper contributes a detailed theoretical and practical roadmap for unveiling a black hole’s photon ring and leveraging these structures to unlock more profound insights into the dynamics of photon orbits around black holes. The tonal restraint in focusing purely on the feasibility and mechanics of such observations encourages future collaborations and developments that converge theoretical predictions with practical astronomical measurements. This could spearhead subsequent astrophysical research and potentially validate, or refine, the models within general relativity by providing strict tests of its predictions near the extreme gravitational domains surrounding black holes.