We explore the shadow of a certain class of rotating traversable wormholes within classical general relativity. The images depend on the angular momentum of the wormhole and the inclination angle of the observer. We compare the results with the case of the Kerr black hole. For small angular momenta, the shadows for the two solutions are nearly identical; however, with the increasing of the angular momentum, they start to deviate considerably.

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 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...

Recently new scalarized black hole solutions were constructed in the extended scalar-tensor-Gauss-Bonnet gravity, where the scalar field is sourced by the curvature of the spacetime via the Gauss-Bonnet invariant. A natural extension of these results is to consider the case of nonzero black hole charge. In addition we have explored a large set of coupling functions between the Gauss-Bonnet invariant and the scalar field, that was not done until now even in the uncharged case, in order to understand better the behavior of the solutions and the deviations from pure general relativity. The results show that in the case of nonzero black hole charge two bifurcation points can exist—one at larger masses where the scalarized solutions bifurcated from the Reissner-Nordström one, and one at smaller masses where the scalar charge of the solutions decreases again to zero and the branch merges again with the GR one. All of the constructed scalarized branches do not reach an extremal limit. We have examined the entropy of the black holes with nontrivial scalar field and it turns out, that similar to the uncharged case, the fundamental branch which possesses scalar field with no nodes is thermodynamically favorable over the Reissner-Nordström one for the considered coupling functions, while the rest of the branches possessing scalar field with one or more zeros have lower entropy compared to the GR case and they are supposed to be unstable.

We study the axial perturbations of spontaneously scalarized black holes in Einstein-Gauss-Bonnet theories. We consider the nodeless solutions of the fundamental branch of the model studied by Doneva and Yazadjiev” [ Phys. Rev. Lett. 120, 131103 (2018)], which possesses a region of radially stable configurations, as shown by Blazquez-Salcedo et al. [ Phys. Rev. D 98, 084011 (2018)]. Here we show that almost all of the radially stable black holes are also stable under axial perturbations. When the axial potential is no longer strictly positive, we make use of the S-deformation method to show stability. As for the radial perturbations, hyperbolicity is lost below a certain critical horizon size for a fixed coupling constant. In the stable region, we determine the spectrum of the quasinormal modes by time evolution and by solving the associated time-independent eigenvalue problem.

We study the optical appearance of a thin accretion disk around compact objects within the Einstein–Gauss–Bonnet gravity. Considering static spherically symmetric black holes and naked singularities we search for characteristic signatures which can arise in the observable images due to the modification of general relativity. While the images of the Gauss–Bonnet black holes closely resemble the Schwarzschild black hole, naked singularities possess a distinctive feature. A series of bright rings are formed in the central part of the images with observable radiation $$10^3$$10 3 times larger than the rest of the flux making them observationally significant. We elucidate the physical mechanism, which causes the appearance of the central rings, showing that the image is determined by the light ring structure of the spacetime. In a certain region of the parametric space the Gauss–Bonnet naked singularities possess a stable and an unstable light ring. In addition the gravitational field becomes repulsive in a certain neighbourhood of the singularity. This combination of features leads to the formation of the central rings implying that the effect is not specific for the Einstein–Gauss–Bonnet gravity but would also appear for any other compact object with the same characteristics of the photon dynamics.