Rotating black hole shadow in Rastall theory

arXiv (Cornell University), 2017

Due to the gravitational lensing effect, a black hole casts a shadow larger than its horizon over a bright background, and the shape and size can be calculated. The Event Horizon Telescope collaboration has produced the first direct image (shadow) of the black hole and it is in accordance with the shadow of a Kerr black hole of general relativity. However, deviations from the Kerr black hole arising from modified theories of gravity are not ruled out and they are important as they offer an arena to test these theories through astrophysical observation. This stimulates us to investigate rotating black holes surrounded by anisotropic fluid in Rastall theory namely a rotating Rastall black hole, which is characterized by mass $M$, spin $a$, field structure parameter $N_s$ and the Rastall parameter $\psi$. It encompasses, as special cases, Kerr ($N_s \to 0$) and Kerr-Newman ($s=0$ and $N_s = -Q^2 $) black holes. The rotating Rastall black hole is characterized by an additional cosmological-like horizon apart from Cauchy and event horizons. We derive an analytical formula for the shadow of a rotating Rastall black hole and go on to visualize the shadow of black holes for various values of the parameters for an observer at a given coordinate ($r_O, \theta_O$) in the domain $[r_+,r_q]$.

Physical Review D, 2016

We study the shadows cast by the different types of rotating regular black holes viz. Ayón-Beato-García (ABG), Hayward, and Bardeen. These black holes have in addition to the total mass (M) and rotation parameter (a), different parameters as electric charge (Q), deviation parameter (g), and magnetic charge (g *), respectively. Interestingly, the size of the shadow is affected by these parameters in addition to the rotation parameter. We found that the radius of the shadow in each case decreases monotonically and the distortion parameter increases when the value of these parameters increase. A comparison with the standard Kerr case is also investigated. We have also studied the influence of the plasma environment around regular black holes to discuss its shadow. The presence of the plasma affects the apparent size of the regular black hole's shadow to be increased due to two effects (i) gravitational redshift of the photons and (ii) radial dependence of plasma density.

The European Physical Journal Plus

This work is devoted to the study of the optical properties of the charged-rotating-NUT-Kiselev (CRNK) black hole in the Rastall theory of gravity. By investigating the motion of photons in the CRNK black hole spacetime in the Rastall gravity we show that the deflection angle of photons due to the gravitational lensing is mostly influenced by the NUT charge, parameter of the equation of state for the quintessence and the quintessential intensity. We observe that the effects of the rest of the spacetime parameters are negligible on the deflection angle. We present the shape of the shadow cast by the CRNK black hole in the Rastall gravity for various values of the spacetime parameters. We demonstrate that spin parameter a of the black hole changes the position and size of the shadow of the black hole very little. While changing the values of the rest of the spacetime parameters can alter the shape and the size of the observed shadow of the black hole significantly. We observe that the radius of the black hole shadow and the deflection angle of photon are bigger for the CRNK black hole in the Rastall gravity, as compared with the case of the Kerr black hole. Lastly, we show how the spacetime parameters can change the observables Rs and δs, being the average radius of the shadow and the distortion parameter that measures the deviation of the shape of the shadow from a perfect circle with radius Rs, respectively. Our results show that in most scenarios, for bigger values of Rs we get smaller values of the parameter δs.

Physical Review D, 2014

A black hole casts a shadow as an optical appearance because of its strong gravitational field. We study the shadow cast by the five-dimensional Myers-Perry black hole with equal rotation parameters. We demonstrate that the null geodesic equations can be integrated that allows us to investigate the shadow cast by a black hole. The shadow of a black hole is found to be a dark zone covered by deformed circle. Interestingly, the shapes of the black hole shadow are more distorted and size decreases for larger black hole spins. Interestingly, it turns out that, for fixed values of rotation parameter, the shadow is slightly smaller and less deformed than for its four-dimensional Kerr black counterpart. Further, the shadow of the five-dimensional Kerr black hole is concentric deformed circles. The effect of rotation parameter on the shape and size of a naked singularity shadow is also analyzed.

Physical Review D, 2012

We investigate the shadow cast by a rotating braneworld black hole, in the Randall-Sundrum scenario. In addition to the angular momentum, the tidal charge term deforms the shape of the shadow. For a given value of the rotation parameter, the presence of a negative tidal charge enlarges the shadow and reduces its deformation with respect to Kerr spacetime, while for a positive charge, the opposite effect is obtained. We also analyze the case in which the combination of the rotation parameter and the tidal charge results in a naked singularity. We discuss the observational prospects corresponding to the supermassive black hole at the Galactic center.

Physical Review D, 2019

Physical Review D, 2020

In this work, starting from a spherically symmetric scale-dependent black hole, a rotating solution is obtained by following the Newman-Janis algorithm without complexification. Besides studying the horizon, the static conditions and causality issues of the rotating solution, we get and discuss the shape of its shadow.

JCAP 05 (2020) 040, 2020

The recent detection of gravitational waves from black hole coalescences and the first image of the black hole shadow enhance the possibilities of testing gravitational theories in the strong-field regime. In this paper, we study the physical properties and the shadow image of a class of Kerr-like rotating black holes, whose Z2 symmetry is generically broken. Such black hole solutions could arise in effective low-energy theories of a fundamental quantum theory of gravity, such as string theory. Within a theory-agnostic framework, we require that the Kerr-like solutions are asymptotically flat, and assume that a Carter-like constant is preserved, enabling the geodesic equations to be fully separable. Subject to these two requirements, we find that the Z2 asymmetry of the spacetime is characterized by two arbitrary functions of polar angle. The shadow image turns out to be Z2 symmetric on the celestial coordinates. Furthermore, the shadow is completely blind to one of the arbitrary functions. The other function, although would affect the apparent size of the shadow, it hardly distorts the shadow contour and has merely no degeneracy with the spin parameter. Therefore, the parameters in this function can be constrained with black hole shadows, only when the mass and the distance of the black hole from the earth are measured with great precision.

Dark Matter in Astro- and Particle Physics, 2006

Recently Holz & Wheeler [1] considered a very attracting possibility to detect retro-MACHOs, i.e. retro-images of the Sun by a Schwarzschild black hole. In this paper we discuss glories (mirages) formed near rapidly rotating Kerr black hole horizons and propose a procedure to measure masses and rotation parameters analyzing these forms of mirages. In some sense that is a manifestation of gravitational lens effect in the strong gravitational field near black hole horizon and a generalization of the retro-gravitational lens phenomenon. We analyze the case of a Kerr black hole rotating at arbitrary speed for some selected positions of a distant observer with respect to the equatorial plane of a Kerr black hole. We discuss glories (mirages) formed near rapidly rotating Kerr black hole horizons and propose a procedure to measure masses and rotation parameters analyzing these forms of mirages. Some time ago Falcke, Melia & Agol [2] suggested to search shadows at the Galactic Center. In this paper we present the boundaries for shadows calculated numerically. We also propose to use future radio interferometer RADIOASTRON facilities to measure shapes of mirages (glories) and to evaluate the black hole spin as a function of the position angle of a distant observer.

arXiv: General Relativity and Quantum Cosmology, 2016

We study the shadow of a charged rotating black hole in $f(R)$ gravity. This black hole is characterized by mass, $M$, spin, $a$, electric charge, $Q$ and $R_{0}$ which is proportional to cosmological constant. We analyze the image of the black hole's shadow in four types 1) at $r\rightarrow\infty$, 2) at $r\rightarrow r_{o}$, in vacuum, 3) at $r\rightarrow\infty$, 4) at $r\rightarrow r_{o}$, for an observer at the presence of plasma. Moreover, we investigate the effect of spin, charge and modfication of gravity on the shape of shadow. In addition, we use two observables, the radius $R_{s}$ and the distortion parameter $\delta_{s}$, characterizing the apparent shape. We show that for all cases, the shadow becomes smaller with increasing electric charge. Also, by increasing the rotation parameters, circular symmetry of the image of black hole's shadow will change. Furthermore, in the presence of plasma, plasma parameter also effects on size of the shadow.