Nonlogarithmic divergence of a deflection angle by a marginally unstable photon sphere of the Damour-Solodukhin wormhole in a strong deflection limit

Abstract: Static, spherically symmetric black holes and compact objects without an event horizon have unstable (stable) circular orbits of a light called photon (antiphoton) sphere. A Damour-Solodukhin wormhole has been suggested as a simple black hole mimicker and the difference of its metric tensors from a black hole is described by a dimensionless parameter $\lambda$. The wormhole with two flat regions has two photon spheres and an antiphoton sphere for $\lambda<\sqrt{2}/2$ and a photon sphere for $\lambda \geq \sqrt{2}/2$. When the parameter $\lambda$ is $\sqrt{2}/2$, the photon sphere is marginally unstable because of degeneration of the photon spheres and antiphoton sphere. We investigate gravitational lensing by the wormhole in weak and strong gravitational fields. We find that the deflection angle of a light ray reflected by the marginally unstable photon sphere diverges nonlogarithmically in a strong deflection limit for $\lambda=\sqrt{2}/2$, while the deflection angle reflected by the photon sphere diverges logarithmically for $\lambda \neq \sqrt{2}/2$. We extend a strong deflection limit analysis for the nonlogarithmic divergence case. We expect that our method can be applied for gravitational lenses by marginally unstable photon spheres of various compact objects.