The purpose of the current Letter is to give some relations between gravitational lensing in the strong-deflection limit and the frequencies of the quasinormal modes of spherically symmetric, asymptotically flat black holes. On the one side, the relations obtained can give a physical interpretation of the strong-deflection limit parameters. On the other side, they also give an alternative method for the measurement of the frequencies of the quasinormal modes of spherically symmetric, asymptotically flat black holes. They could be applied to the localization of the sources of gravitational waves and could tell us what frequencies of the gravitational waves we could expect from a black hole acting simultaneously as a gravitational lens and a source of gravitational waves.

We study the optical appearance and the apparent radiation flux of a geometrically thin and optically thick accretion disk around the static Janis-Newman-Winicour naked singularity. We confine ourselves to the astrophysically most relevant case, when the solution possesses a photon sphere, assuming that the radiation emitted by the disk is described by the Novikov-Thorne model. The observable images closely resemble the visual appearance of the Schwarzschild black hole, as only quantitative differences are present. For the Janis-Newman-Winicour solution the accretion disk appears smaller, and its emission is characterized by a higher peak of the radiation flux. In addition, the most significant part of the radiation is concentrated in a closer neighborhood of the flux maximum. The results are obtained independently by two alternative methods, consisting of a semianalytical scheme using the spherical symmetry of the spacetime, and a fully numerical ray-tracing procedure valid for any stationary and axisymmetric spacetime.

We model massive compact objects in galactic nuclei as stationary, axially symmetric naked singularities in the Einstein-massless scalar field theory and study the resulting gravitational lensing. In the weak deflection limit we study analytically the position of the two weak field images, the corresponding signed and absolute magnifications as well as the centroid up to post-Newtonian order. We show that there are static post-Newtonian corrections to the signed magnification and their sum as well as to the critical curves, which are functions of the scalar charge. The shift of the critical curves as a function of the lens angular momentum is found, and it is shown that they decrease slightly for the weakly naked and vastly for the strongly naked singularities with the increase of the scalar charge. The pointlike caustics drift away from the optical axis and do not depend on the scalar charge. In the strong deflection limit approximation, we compute numerically the position of the relativistic images and their separability for weakly naked singularities. All of the lensing quantities are compared to particular cases as Schwarzschild and Kerr black holes as well as Janis-Newman-Winicour naked singularities.

We revisit the shadow of rotating traversable wormholes discussing the role of the wormhole throat in the shadow formation. For certain classes of wormholes the throat serves as a potential barrier...

In the present work we study numerically quasiequatorial lensing by the charged, stationary, axially symmetric Kerr-Sen dilaton-axion black hole in the strong deflection limit. In this approximation we compute the magnification and the positions of the relativistic images. The most outstanding effect is that the Kerr-Sen black hole caustics drift away from the optical axis and shift in the clockwise direction with respect to the Kerr caustics. The intersections of the critical curves on the equatorial plane as a function of the black hole angular momentum are found, and it is shown that they decrease with the increase of the parameter $Q^2/M$. All of the lensing quantities are compared to particular cases as Schwarzschild, Kerr, and Gibbons-Maeda black holes.