Analytical approach to strong gravitational lensing from ultracompact objects

Understand

Strong gravitational lensing from black holes results in the formation of relativistic images, in particular, relativistic Einstein rings.

• For objects with event horizons, the radius of the unstable light ring (photon sphere) is the lowest radius at which a relativistic image might be formed.
• For horizonless ultracompact objects, additional relativistic images and rings can form inside this radius.
• In this paper, we provide an analytical approach to deal with strong gravitational lensing from such ultracompact objects, which is substantially different from the black hole cases, first reported by Bozza.