Mühendislik Fakültesi / Faculty of Engineering
Permanent URI for this collectionhttps://hdl.handle.net/11727/1401
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Item 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, MustaphaIn 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.Item Constraining Wormhole Geometries Using The Orbit Of S2 Star And The Event Horizon Telescope(2022) Jusufi, Kimet; Kumar, Saurabh; Azreg-Ainou, Mustapha; Jamil, Mubasher; Wu, Qiang; Bambi, CosimoIn this paper we study the possibility of having a wormhole (WH) as a candidate for the Sgr A(star) central object and test this idea by constraining their geometry using the motion of S2 star and the reconstructed shadow images. In particular, we consider three WH models, including WHs in Einstein theory, brane-world gravity, and Einstein-Dirac-Maxwell theory. To this end, we have constrained the WH throat using the motion of S2 star and shown that the flare out condition is satisfied. We also consider the accretion of infalling gas model and study the accretion rate and the intensity of the electromagnetic radiation as well as the shadow images.Item Equatorial and Polar Quasinormal Modes and Quasiperiodic Oscillations of Quantum Deformed Kerr Black Hole(2022) Jusufi, Kimet; Azreg Ainou, Mustapha; Jamil, Mubasher; Wu, Qiang; 0000-0002-3244-7195In 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 parameterItem Constraining the generalized uncertainty principle through black hole shadow, S2 star orbit, and quasiperiodic oscillations(2022) Jusufi, Kimet; Azreg-Ainou, Mustapha; Jamil, Mubasher; Zhu, TaoIn this paper, we study the effect of the Generalized Uncertainty Principle (GUP) on the shadow of GUP-modified Kerr black hole and the correspondence between the shadow radius and the real part of the quasinormal modes (QNMs). We find that the shadow curvature radius of the GUP-modified Kerr black hole is bigger compared to the Kerr vacuum solution and increases linearly monotonically with the increase of the GUP parameter. We then investigate the characteristic points of intrinsic curvature of the shadow from a topological point of view to calculate the angular size for these curvature radii of the shadow. To this end, we have used the EHT data for the M87* black hole to constrain the upper limits of the GUP parameter and our result is beta < 10(95). Finally, we have explored the connection between the shadow radius and the scalar/electromagnetic/gravitational QNMs. Using the orbit of S2 star we have obtained a bound for the GUP parameter beta < 10(87). The GUP-modified Kerr black hole is also used to provide perfect curve fitting of the particle oscillation upper and lower frequencies to the observed frequencies for three microquasars and to restrict the values of the correction parameter in the metric of the modified black hole to very reasonable bound beta < 10(77).Item Constraints on Barrow Entropy from M87* and S2 Star Observations(2022) Jusufi, Kimet; Azreg-Ainou, Mustapha; Jamil, Mubasher; Saridakis, Emmanuel N.We use data from M87* central black hole shadow, as well as from the S2 star observations, in order to extract constraints on Barrow entropy. The latter is a modified entropy arising from quantum-gravitational effects on the black hole horizon, quantified by the new parameter & UDelta;. Such a change in entropy leads to a change in temperature, as well as to the properties of the black hole and its shadow. We investigate the photon sphere and the shadow of a black hole with Barrow entropy, and assuming a simple model for infalling and radiating gas we estimate the corresponding intensity. Furthermore, we use the radius in order to extract the real part of the quasinormal modes, and for completeness we investigate the spherical accretion of matter onto the black hole, focusing on isothermal and polytropic test fluids. We extract the allowed parameter region, and by applying a Monte-Carlo-Markov Chains analysis we find that & UDelta;& SIME; 0.0036(-0.0145)(+0.0792). Hence, our results place the upper bound & UDelta;& LSIM;0.0828 at 1 sigma, a constraint that is less strong than the Big Bang Nucleosynthesis one, but significantly stronger than the late-time cosmological constraints.Item Axion-plasmon or magnetized plasma effect on an observable shadow and gravitational lensing of a Schwarzschild black hole(2021) Atamurotov, Farruh; Jusufi, Kimet; Jamil, Mubasher; Abdujabbarov, Ahmadjon; Azreg-Ainou, Mustapha; AAZ-1598-2021In this paper, we study the influence of the axion-plasmon, as proposed in [Phys. Rev. Lett. 120, 181803 (2018)] on the optical properties of the Schwarzschild black hole. Our aim is to provide a test to detect the effects of a fixed axion background using black holes. To accomplish our goal, we explore the effect of the axion-plasmon coupling on the motion of photons around the Schwarzschild black hole and check the possibility of observing those effects upon the black hole shadow, the gravitational deflection angle, Einstein rings and shadow images obtained by radially infalling gas on a black hole within a plasma medium. We find that these quantities are indeed affected by the axion-plasmon coupling parameters which consequently generalize some of the well-known results in the literature. It is shown that the size of the black hole shadow decreases with increasing axion-plasmon if observed from a sufficiently large distance.Item X-ray reflection spectroscopy with Kaluza-Klein black holes(2020) Zhu, Jiachen; Abdikamalov, Askar B.; Ayzenberg, Dimitry; Azreg-Ainou, Mustapha; Bambi, Cosimo; Jamil, Mubasher; Nampalliwar, Sourabh; Tripathi, Ashutosh; Zhou, MengleiKaluza-Klein theory is a popular alternative theory of gravity, with both non-rotating and rotating black hole solutions known. This allows for the possibility that the theory could be observationally tested. We present a model which calculates the reflection spectrum of a black hole accretion disk system, where the black hole is described by a rotating solution of the Kaluza-Klein theory. We also use this model to analyze X-ray data from the stella-mass black hole in GRS 1915+105 and provide constraints on the free parameters of the Kaluza-Klein black holes.Item Cyclic and heteroclinic flows near general static spherically symmetric black holes(2016) Ahmed, Ayyesha K.; Azreg-Ainou, Mustapha; Faizal, Mir; Jamil, MubasherWe investigate the Michel-type accretion onto a static spherically symmetric black hole. Using a Hamiltonian dynamical approach, we show that the standard method employed for tackling the accretion problem has masked some properties of the fluid flow. We determine new analytical solutions that are neither transonic nor supersonic as the fluid approaches the horizon(s); rather, they remain subsonic for all values of the radial coordinate. Moreover, the three-velocity vanishes and the pressure diverges on the horizon(s), resulting in a flow-out of the fluid under the effect of its own pressure. This is in favor of the earlier prediction that pressure-dominant regions form near the horizon. This result does not depend on the form of the metric and it applies to a neighborhood of any horizon where the time coordinate is timelike. For anti-de Sitter-like f(R) black holes we discuss the stability of the critical flow and determine separatrix heteroclinic orbits. For de Sitter-like f(R) black holes, we construct polytropic cyclic, non-homoclinic, physical flows connecting the two horizons. These flows become non-relativistic for Hamiltonian values higher than the critical value, allowing for a good estimate of the proper period of the flow.Item Astrophysical flows near f (T) gravity black holes(2016) Ahmed, Ayyesha K.; Azreg-Ainou, Mustapha; Bahamonde, Sebastian; Capozziello, Salvatore; Jamil, Mubasher; 27257404In this paper, we study the accretion process for fluids flowing near a black hole in the context of f (T) teleparallel gravity. Specifically, by performing a dynamical analysis by a Hamiltonian system, we are able to find the sonic points. After that, we consider different isothermal test fluids in order to study the accretion process when they are falling onto the black hole. We find that these flows can be classified according to the equation of state and the black hole features. Results are compared in f (T) and f (R) gravity.Item Strong gravitational lensing by a charged Kiselev black hole(2017) Azreg, Ainou Mustapha; Bahamonde, Sebastian; Jamil, Mubasher; 0000-0002-3244-7195; R-1759-2019; AAZ-1598-2021We study the gravitational lensing scenario where the lens is a spherically symmetric charged black hole (BH) surrounded by quintessence matter. The null geodesic equations in the curved background of the black hole are derived. The resulting trajectory equation is solved analytically via perturbation and series methods for a special choice of parameters, and the distance of the closest approach to black hole is calculated. We also derive the lens equation giving the bending angle of light in the curved background. In the strong field approximation, the solution of the lens equation is also obtained for all values of the quintessence parameter w(q). For all w(q), we show that there are no stable closed null orbits and that corrections to the deflection angle for the Reissner-Nordstrom black hole when the observer and the source are at large, but finite, distances from the lens do not depend on the charge up to the inverse of the distances squared. A part of the present work, analyzed, however, with a different approach, is the extension of Younas et al. (Phys Rev D 92:084042, 2015) where the uncharged case has been treated.