What is the photon ring in black hole imaging?

• A. It is the same as the event horizon.
• B. It is the dark region around the shadow.
• C. It is the bright, highly lensed light from photons orbiting near the photon sphere, surrounding the shadow.
• D. It is formed by the accretion disk's outer edge.

Answer: (C)

The photon ring is the bright, highly bent light produced by photons that skim extremely close to the black hole, near the photon sphere, and wind around the hole before escaping toward us. This intense gravitational lensing wraps light around the black hole and creates a luminous ring that surrounds the dark, shadowed region. It sits outside the event horizon—the boundary beyond which light cannot escape—so the ring is a visual consequence of light bending just outside that boundary, not the horizon itself. The ring’s brightness and exact shape depend on the black hole’s spin and the observer’s angle, and it can be distorted for rotating holes, unlike the dark shadow, which is the region lacking escaping photons. This description aligns with the bright ring formed by photons on highly curved paths around the photon sphere.