What is a photon ring?

• A. The event horizon.
• B. The dark region around the shadow.
• C. The bright, highly lensed light from photons orbiting near the photon sphere, surrounding the shadow.
• D. The accretion disk inner edge.

Answer: (C)

Light near a black hole is bent so strongly that some photons can execute near-orbits around the hole. The photon ring is the bright, narrow band of light produced by photons that skim close to the photon sphere and loop around the hole many times before escaping toward us. This extreme gravitational lensing concentrates light into a ring that surrounds the dark shadow, making it the most distinctive feature in images of black holes. The event horizon is the boundary from which nothing escapes, not a bright feature you’d see as a ring. The dark region around the ring is the shadow itself, not the light ring. The accretion disk inner edge is where hot material in the disk ends up emitting strongly, but it’s a different structure than the photons that form the ring.