  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.
Here, our analysis indicates that the angular separations and magnifications of
relativistic images inside the unstable light ring (photon sphere) might be
several orders of magnitude higher compared to the ones outside it. This
indicates fundamental differences in the nature of strong gravitational lensing
from black holes and ultracompact objects.
