Braneworld Black Hole Gravitational Lensing

A class of braneworld black holes, which I called as Bronnikov–Melnikov–Dehen(BMD) black holes,are studied as gravitational lenses. I obtain the deflection angle in the strong deflection limit, and further calculate the angular positions and magnifications of relativistic images as well as the time delay between different relativistic images. I also compare the results with those obtained for Schwarzschild and two braneworld black holes, i.e., the tidal Reissner-Nordstr¨om(R-N) and the Casadio–Fabbri–Mazzacurati(CFM) black holes.     

Weak Gravitational Lensing from Regular Bardeen Black Holes

In this article we study weak gravitational lensing of regular Bardeen black hole which has scalar charge g and mass m. We investigate the angular position and magnification of non-relativistic images in two cases depending on the presence or absence of photon sphere. Defining dimensionless charge parameter \(q=\frac {g}{2m}\) we seek to disappear photon sphere in the case of \(|q|>{24\sqrt 5}/{125}\) for which the space time metric encounters strongly with naked singularities. We specify the basic parameters of lensing in terms of scalar charge by using the perturbative method and found that the parity of images is different in two cases: (a) The strongly naked singularities is present in the space time. (b) singularity of space time is weak or is eliminated (the black hole lens).     

Strong Field Limit of Black Hole Gravitational Lensing

We give the formulation of the gravitational lensing theory in the strong field limit for a Schwarzschild black hole as a counterpart to the weak field approach. It is possible to expand the full black hole lens equation to work a simple analytical theory that describes the physics in the strong field limit at a high accuracy degree. In this way, we derive compact and reliable mathematical formulae for the position of additional critical curves, relativistic images and their magnification, arising in this limit.     

Time Delay in Black Hole Gravitational Lensing as a Distance Estimator

We calculate the time delay between different relativistic images formed by black hole gravitational lensing in the strong field limit. For spherically symmetric black holes, it turns out that the time delay between the first two images is proportional to the minimum impact angle. Their ratio gives a very interesting and precise measure of the distance of the black hole. Moreover, using also the separation between the images and their luminosity ratio, it is possible to extract the mass of the black hole. The time delay for the black hole at the center of our Galaxy is just few minutes, but for supermassive black holes with M=10⁸ ÷10⁹ in the neighbourhood of the Local Group the time delay amounts to few days, thus being measurable with a good accuracy.     

Gravitational Lensing by Charged Black Holes

We formulate the lensing effects of a spherically symmetric electrically charged black hole using thin lens equations. The charged black hole leads to three images and could lead to three Einstein rings provided the parameters such as the mass, charge and the distances satisfy certain constraints. We have computed the exact positions of images and magnification properties for a super-massive black hole with electric charge.     

Gravitational Lensing of STU Black Holes

In this paper we study gravitational lensing by STU black holes. We considered extremal limit of two special cases of zero-charged and one-charged black holes, and obtain the deflection angle. We find that the black hole charge increases the deflection angle.     

A New Regular Black Hole

A spherically symmetric uncharged regular black hole is proposed in this paper. The black hole’s density in proportion to $r^{3n}e^{-r^{3n+3}}$ , and the curvature tensor in the region of r=0 keep finity. When n=0 in our model, this spacetime is no other than Dymnikova regular black hole. What’s more, there are better properties in this spacetime when n>0. We then discuss the temperature and Hawking radiation of the black hole’s horizon.     

Effects of Homogeneous Plasma on Strong Gravitational Lensing of Kerr Black Holes

Considering the Kerr black hole surrounded by a homogeneous unmagnetized plasma medium, we study the strong gravitational lensing on the equatorial plane of the Kerr black hole. It is found that the presence of the uniform plasma can increase the photon-sphere radius r_(ps), the coefficients a and b, the angular position of the relativistic images(θ_∞),the deflection angleα(θ)and the angular separation s. However, the relative magnitude r_m decreases in the presence of the uniform plasma medium. It is also shown that the impact of the uniform plasma on the effect of strong gravitational lensing becomes smaller as the spin of the Kerr black hole increases in the prograde orbit(a 0). In particular, for the extreme black hole(a = 0.5), the effect of strong gravitational lensing in the homogeneous plasma medium is the same as the case in vacuum for the prograde orbit.     

Gravitational Lensing of Fermion Stars

We discuss the fermion stars, the self-gravitating systems of Fermi gases, as possible gravitational lenses. It is supposed that the fermions interact with themselves and other particles only by gravity, so they are the candidates of dark matter. We calculate Einstein deflection angles, study the image configurations, and calculate the magnification factors for a number of fermion stars that range from strong relativistic configurations to nonrelativistic ones. We find that typically there are three images, one Einstein ring and one radial critical curve for both cases. Two of the images are within the Einstein ring, and the other is outside, which may be very far. All these lensing characteristics can help to identify fermion stars as potential lensing objects, thus might give direct evidence that dark fermion stars exist in the universe.     

Gravitational Perturbations in Einstein Aether Black Hole Spacetime

We study the gravitational perturbations in Einstein aether black hole spacetime and find that the quasinormal modes(QNMs) of the first kind of aether black hole are similar to that of a Lorentz violation(LV) model,the quantum electrodynamics(QED) extension limit of standard model extension. These similarities between completely different backgrounds may imply that LV in the gravity sector and LV in the matter sector have some connections: damping QNMs more rapidly and prolonging its oscillation period. Compared to the Schwarzschild case, the first kind of black holes have larger damping rates and the second ones have lower damping rates, and they all have smaller real oscillation frequency. These differences could be detected by the new generation of gravitational antennas.     

Regular and Black Hole Skyrmions with Axisymmetry

It has been known that a B=2 skyrmion is axially symmetric. We consider the Skyrme model coupled to gravity and obtain static axially symmetric regular and black hole solutions numerically. Computing the energy density of the skyrmion, we discuss the effect of gravity to the energy density and baryon density of the skyrmion.     

Quasinormal Modes of Hayward Regular Black Hole

In this paper, using asymptotic iteration method and eikonal limit, the massive scalar quasinormal modes (QNM) is studied in regular Hayward spacetime, which is much similar to Schwarzschild black hole when r→∞ but there is no singularity at the center. We analyze the QNM frequencies ω by varying the parameter β (it is related to mass of black hole and cosmological constant), spherical harmonic index L and the mass of scalar field m. The results show that the effect of β could lead to the real part of ω increase but the imaginary part decrease, which imply that the existence of cosmological constant would impact on the process of a black hole relaxing after it has been perturbed.     

Holographic Superconductor of Regular Phantom Black Hole

Holographic superconductors containing a non-minimal derivative coupling for the scalar field in a regular phantom plane symmetric black hole have been considered. We show that the parameter of the regular black hole b as well as the non-minimal derivative coupling parameter η affect the formation of the condensate as well as the conductivity in the superconductor. Moreover, b has a critical value in which the critical temperature T_c increases without a bound.     

Statistical Mechanics of a New Regular Black Hole

In this work we consider a new proposed regular black hole and study statistics of this black hole. We calculate partition function and related quantities which determine statistics of this black hole. We confirm that the microscopic entropy coincides with BH entropy.     

Letter: Gravitational Thermodynamics of a Vaidya Black Hole

The path integral approach is applied to the statistical thermodynamics of a radiating Vaidya black hole. The entropy still satisfies the Bekenstein-Hawking formula, except for a negligible term. The entropy production, as a measurement of the irreversibility, is also obtained.     

Lensing by Gravitational Waves from Supernovae

Gravitational waves can act as gravitationallenses and create multiple images of a light source.This situation is much more interesting thansingle-image lensing because of the associatedhigh-amplification events that may lead to the indirect detectionof gravitational waves. It is proposed to observe theeffect due to gravitational waves generated in supernovaexplosions.