Images and photon ring signatures of thick disks around black holes

Abstract

High-frequency very-long-baseline interferometry (VLBI) observations can now resolve the horizon-scale emission from sources in the immediate vicinity of nearby supermassive black holes. Future space-VLBI observations will access highly lensed features of black hole images -- photon rings -- that will provide particularly sharp probes of strong-field gravity. Focusing on the particular case of the supermassive black hole M87*, our goal is to explore a wide variety of accretion flows onto a Kerr black hole and to understand their corresponding images and visibilities. We are particularly interested in the visibility on baselines to space, which encodes the photon ring shape and whose measurement could provide a stringent test of the Kerr hypothesis. We develop a fully analytical model of stationary, axisymmetric accretion flows with a variable disk thickness and a matter four-velocity that can smoothly interpolate between purely azimuthal rotation and purely radial infall. We then determine the observational appearance of such flows, taking care to include the effects of thermal synchrotron emission and absorption. Our images generically display a "wedding cake" structure composed of discrete, narrow photon rings (n=1,2,...) stacked on top of broader primary emission that surrounds a central brightness depression of model-dependent size. We find that the "black hole shadow" is a model-dependent phenomenon -- even for diffuse, optically thin sources -- and should not be regarded as a generic prediction of general relativity. At 230 GHz, the n=1 ring is always visible, but the n=2 ring is sometimes suppressed due to absorption. At 345 GHz, the medium is optically thinner and the n=2 ring displays clear signatures in both the image and visibility domains, identifying this frequency as more promising for future space-VLBI measurements of the photon ring shape.

21 pages, 9 figure, accepted by A&A, minor changes

References in corpus (12)

• First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole
• First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
• First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole
• First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
• First M87 Event Horizon Telescope Results. II. Array and Instrumentation
• Black Hole Shadows, Photon Rings, and Lensing Rings
• The Shadow of a Spherically Accreting Black Hole
• Lensing by Kerr Black Holes
• Observing the Inner Shadow of a Black Hole: A Direct View of the Event Horizon
• Monitoring the Morphology of M87* in 2009-2017 with the Event Horizon Telescope
• The Effects of Accretion Flow Dynamics on the Black Hole Shadow of Sagittarius A${}^{\ast }$
• Measuring the shape of a black hole photon ring

Cited by in corpus (4)

• Investigating shadow images and rings of the charged Horndeski black hole illuminated by various thin accretions
• Observational properties of relativistic fluid spheres with thin accretion disks
• Observational imprints of gravastars from accretion disks and hot-spots
• Orbital configurations of spaceborne interferometers for studying photon rings of supermassive black holes