Browsing by Author "Azreg Ainou, Mustapha"

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Comment on 'Gravitational Analysis of Rotating Charged Black-Hole-Like Solution in Einstein-Gauss-Bonnet Gravity'
(2022) Azreg Ainou, Mustapha
The author reviews the derivations of the rotating metric made in Ann. Phys. (Berlin), 2022, 2200074, and shows that the analysis yielding the solution is flawed throughout the paper.
Equatorial and Polar Quasinormal Modes and Quasiperiodic Oscillations of Quantum Deformed Kerr Black Hole
(2022) Jusufi, Kimet; Azreg Ainou, Mustapha; Jamil, Mubasher; Wu, Qiang; 0000-0002-3244-7195
In this paper, we focus on the relation between quasinormal modes (QNMs) and a rotating black hole shadow. As a specific example, we consider the quantum deformed Kerr black hole obtained via Newman-Janis-Azreg-Ainou algorithm. In particular, using the geometric-optics correspondence between the parameters of a QNMs and the conserved quantities along geodesics, we show that, in the eikonal limit, the real part of QNMs is related to the Keplerian frequency for equatorial orbits. To this end, we explore the typical shadow radius for the viewing angles, theta(0) = pi/2, and obtained an interesting relation in the case of viewing angle theta(0) = 0 (or equivalently theta(0) = pi). Furthermore we have computed the corresponding equatorial and polar modes and the thermodynamical stability of the quantum deformed Kerr black hole. We also investigate other astrophysical applications such as the quasiperiodic oscillations and the motion of S2 star to constrain the quantum deforming parameter
On the Parameters of the Spherically Symmetric Parameterized Rezzolla-Zhidenko Spacetime through Solar System Tests, the Orbit of the S2 Star about Sgr A*, and Quasiperiodic Oscillations
(2023) Shaymatov, Sanjar; Ahmedov, Bobomurat; De Laurentis, Mariafelicia; Jamil, Mubasher; Wu, Qiang; Wang, Anzhong; Azreg Ainou, Mustapha
In this paper, we find the higher-order expansion parameters alpha and lambda of spherically symmetric parameterized Rezzolla-Zhidenko (PRZ) spacetime by using its functions of the radial coordinate. We subject the parameters of this spacetime to classical tests, including weak gravitational field effects in the solar system, observations of the S2 star that is located in the star cluster close to the Sgr A*, and of the frequencies of selected microquasars. Based on this spherically symmetric spacetime, we perform the analytic calculations for solar system effects such as perihelion shift, light deflection, and gravitational time delay to determine limits on the parameters by using observational data. We restrict our attention to the limits on the two higher-order expansion parameters alpha and lambda that survive at the horizon or near the horizon of spherically symmetric metrics. The properties of the expansion of these two small parameters in PRZ parameterization are discussed. We further apply Markov Chain Monte Carlo simulations to analyze and obtain the limits on the expansion parameters by using observations of the phenomena of the S2 star. Finally, we consider the epicyclic motions and derive analytic expressions of the epicyclic frequencies. Applying these expressions to the quasiperiodic oscillations of selected microquasars allows us to set further limits on the parameters of the PRZ spacetime. Our results demonstrate that the higher-order expansion parameters can be given in the range alpha, lambda = (-0.09, 0.09) and of order similar to 10(-2) as a consequence of three different tests and observations.
A Stationary Axisymmetric Vacuum Solution for Pure R2 Gravity
(2023) Azreg Ainou, Mustapha; Nguyen, Hoang Ky; 0000-0002-3244-7195
The closed-form expression for pure R-2 vacuum solution obtained in Phys. Rev. D 107, 104 008 (2023) lends itself to a generalization to axisymmetric setup via the modified Newman-Janis algorithm. We adopt the procedure put forth in Phys. Rev. D 90, 064 041 (2014) bypassing the complexification of the radial coordinate. The procedure presumes the existence of Boyer-Lindquist coordinates. Using the Event Horizon Telescope Collaboration results, we model the central black hole M87* by the thus obtained exact rotating metric, depending on the mass, rotation parameter and a third dimensionless parameter. The latter is constrained upon investigating the shadow angular size assuming mass and rotation parameters are those of M87*. Stability is investigated.