Null geodesics and observables around the Kerr–Sen black hole

The time-like and null-like geodesics around compact objects are some of the best tools to classify and understand the structure of a space-time. In this paper, we study the null geodesics around charged static dilaton black holes in Einstein-Maxwell-dilaton gravity. The physical parameters for non-radial geodesics including the effective potential, effective force, radius of the photon sphere and impact parameter are obtained and effects of the charge parameter and dilaton coupling constant on these quantities are studied. Possible photon motions for different values of the impact parameter are analyzed and unstable circular orbits and unbounded orbits are plotted. These results are compared to that of the Schwarzschild, Reissner-Nordstrom and GMGHS black holes. Also, we study the shadow cast by a dilaton black hole and investigate how the dilaton coupling affects the size of the black hole shadow. Finally, as an application of null geodesics, we calculate the deflection of light and investigate the effects of the model parameters on the bending angle.

On null geodesics and shadow of hairy black holes in Einstein-Maxwell-dilaton gravity

In order to classify and understand structure of the spacetime, investigation of the geodesic motions of massive and massless particles is a key tool. So the geodesic equation is a central equation of gravitating systems and the subject of geodesics in the black hole dictionary attracted much attention. In this paper, we give a full description of geodesic motions in three-dimensional spacetime. We investigate the geodesics near charged BTZ black holes and then generalize our prescriptions to the case of massive gravity. We show that electric charge is a critical parameter for categorizing the geodesic motions of both lightlike and timelike particles. In addition, we classify the type of geodesics based on the particle properties and geometry of spacetime.

/pdf/simulation-of-geodesic-trajectory-of-charged-btz-black-holes-19w28fdf9m.pdf

Simulation of geodesic trajectory of charged BTZ black holes in massive gravity

The dynamic phase transition of charged dilaton black holes is investigated in this paper. The Gibbs free energy landscape is introduced, and the corresponding \begin{document}$G_{\rm L}$\end{document} is calculated for the dilaton black hole. We numerically solve the Fokker-Planck equation constrained by only the reflecting boundary condition. The effects of dilaton gravity on the probabilistic evolution of dilaton black holes are explored. Firstly, the horizon radius difference between a large dilaton black hole and a small dilaton black hole increases with the parameter \begin{document}$\alpha$\end{document} . Secondly, with increasing \begin{document}$\alpha$\end{document} , the system needs much more time to achieve a stationary distribution. Finally, the values attained for \begin{document}$\rho(r_{\rm l},t)$\end{document} and \begin{document}$\rho(r_{\rm s},t)$\end{document} vary with \begin{document}$\alpha$\end{document} . Additionally, by resolving the Fokker-Planck equation constrained by both the reflecting boundary condition and absorbing boundary condition, we investigate the first passage process of dilaton black holes. The initial peak decays more slowly with increasing \begin{document}$\alpha$\end{document} , which can also be observed via the slowing decay of \begin{document}$\Sigma(t)$\end{document} (the sum of the probability of the black hole system not having completed a first passage by time t). Moreover, the time corresponding to the single peak of the first passage time distribution is found to increase with the parameter \begin{document}$\alpha$\end{document} . Considering these observations, the dilaton field is found to slow down the dynamic phase transition process between a large black hole and a small black hole.

pdf/dynamic-phase-transition-of-charged-dilaton-black-holes-5gkaac7mfq.pdf

Dynamic phase transition of charged dilaton black holes

It seems surprising that the emissivity properties of the accretion disk (\textit{a la} Page and Thorne) surrounding the Gibbons-Maeda-Garfinkle-Horowitz-Strominger (GMGHS) black holes of heterotic string theory have not yet been studied. To fill this gap in the literature, we study the emissivity properties of the thin accretion disks around these black holes both in the Einstein and in the string frame using the Page-Thorne model. For illustration, we choose as a toy model a stellar-sized spherically symmetric black hole and find that, while the emissivity properties do not significantly differ from those of Reissner-Nordstrom and Schwarzschild black holes, they remarkably differ at GMGHS extreme limits corresponding to naked singularity and wormhole at higher frequencies. These differences provide a novel way to speculatively conclude about different types of objects from the observational point of view.

/pdf/accretion-disks-around-the-gibbons-maeda-garfinkle-horowitz-2tdqot8fvr.pdf

Accretion disks around the Gibbons-Maeda-Garfinkle-Horowitz-Strominger charged black holes.

In this paper, we consider the timelike and null geodesics around the static (GMGHS, magnetically charged GMGHS, electrically charged GMGHS) and the rotating (Kerr-Sen dilaton-axion) dilaton black holes. The geodesic equations are solved in terms of Weierstrass elliptic functions. To classify the trajectories around the black holes, we use the analytical solution and effective potential techniques and then characterize the different types of the resulting orbits in terms of the conserved energy and angular momentum. Also, using the obtained results we study astrophysical applications.

Geodesic equations in the static and rotating dilaton black holes: Analytical solutions and applications

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Geodesic equations in the static and rotating dilaton black holes: Analytical solutions and applications