Petya Nedkova

Shadow of a rotating traversable wormhole

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.

Thermodynamics of 5D black holes on asymptotically locally flat gravitational instantons

We investigate the thermodynamical properties of solutions to the 5D Einstein and Einstein-Maxwell-dilaton equations describing static black holes on ALF gravitational instantons. Some general expressions regarding the nut charge and potential are derived, as well as Smarr-like relations. The thermodynamics of two particular solutions representing a static black hole on Taub-Nut and Taub-Bolt instantons are examined in detail. Two different approaches for computation of the mass and the tension the Komar and counterterm method, are applied and compared. A new charged black hole on Taub-Bolt instanton is constructed and investigated.

Magnetized black hole on the Taub-NUT instanton

We present an exact solution to the 5D Einstein-Maxwell-dilaton equations describing a static black hole on Taub-Nut instanton. By construction the solution does not possess a charge, but is magnetized along the compact dimension. As a limit we obtain a new regular solution representing a magnetized Kaluza- Klein monopole. We investigate the relevant physical properties and derive the Smarr-like relations.

Rotating black ring on Kaluza-Klein bubbles

We construct a new exact solution to the 5D Einstein equations describing a rotating black ring with a single angular momentum surrounded by two Kaluza-Klein bubbles. The solution is generated by two-soliton Baecklund transformation. Its physical properties are computed and analyzed. The corresponding static solution, the rotating black string, and the boosted black string are reproduced as limits.

New magnetized squashed black holes—thermodynamics and Hawking radiation

We construct a new exact solution in the 5D Einstein–Maxwell-dilaton gravity describing a magnetized squashed black hole. We calculate its physical characteristics, and derive the Smarr-like relations. The Hawking radiation of the solution is also investigated by calculating the graybody factor for the emission of massless scalar particles in the low-energy regime. In distinction to the case of asymptotically flat magnetized black holes, the thermodynamical characteristics and the Hawking radiation of the solution depend on the external magnetic field, and consequently can be tuned by its variation.

Properties of the distorted Kerr black hole

We investigate the properties of the ergoregion and the location of the curvature singularities for the Kerr black hole distorted by the gravitational field of external sources. The particular cases of quadrupole and octupole distortion are studied in detail. We also investigate the scalar curvature invariants of the horizon and compare their behaviour with the case of the isolated Kerr black hole. In a certain region of the parameter space the ergoregion consists of a compact region encompassing the horizon and a disconnected part extending to infinity. The curvature singularities in the domain of outer communication, when they exist, are always located on the boundary of the ergoregion. We present arguments that they do not lie on the compact ergosurface. For quadrupole distortion the compact ergoregion size is negatively correlated with the horizon angular momentum when the external sources are varied. For octupole distortion infinitely many ergoregion configurations can exist for a certain horizon angular momentum. For some special cases we can have

Charged rotating dilaton black holes with Kaluza-Klein asymptotics

J^2/M^4 > 1

On the shadow of rotating traversable wormholes

and yet avoid the naked singularity.

Uniqueness theorem for static phantom wormholes in Einstein–Maxwell-dilaton theory

We construct a class of stationary and axisymmetric solutions to the 5D Einstein-Maxwell-dilaton gravity, which describe configurations of charged rotating black objects with Kaluza-Klein asymptotics. The solutions are constructed by uplifting a vacuum seed solution to six dimensions, performing a boost, and a subsequent circle reduction. We investigate the physical properties of the charged solutions, and obtain their general relations to the properties of the vacuum seed. We also derive the gyromagnetic ratio and the Smarr-like relations. As particular cases we study three solutions, which describe a charged rotating black string, a charged rotating black ring on Kaluza-Klein bubbles, and a superposition of two black holes and a Kaluza-Klein bubble.

CHARGED BLACK HOLES ON THE TAUB-BOLT INSTANTON AND THEIR THERMODYNAMICS

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 for light rays with particular impact parameters, thus modifying the shadow shape. We consider a couple of wormhole solutions and examine the structure of their shadow images, and the intrinsic mechanisms for their formation. Some of the shadows possess cuspy edges, which arise due to the interplay of two distinct families of unstable spherical orbits. These solutions provide examples, in which the explicit mechanism for cusp formation can be uncovered.