Browsing by Author "Azreg-Ainou, Mustapha"

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Astrophysical flows near f (T) gravity black holes
(2016) Ahmed, Ayyesha K.; Azreg-Ainou, Mustapha; Bahamonde, Sebastian; Capozziello, Salvatore; Jamil, Mubasher; 27257404
In 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.
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-1598-2021
In 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.
Charged de Sitter-like black holes: quintessence-dependent enthalpy and new extreme solutions
(2015) Azreg-Ainou, Mustapha
We consider Reissner-Nordstrom black holes surrounded by quintessence where both a non-extremal event horizon and a cosmological horizon exist besides an inner horizon (-1 = omega < - 1/3). We determine new extreme black hole solutions that generalize the Nariai horizon to asymptotically de Sitter-like solutions for any order relation between the squares of the charge q(2) and the mass parameter M-2 provided q(2) remains smaller than some limit, which is larger than M-2. In the limit case q(2) = 9 omega M-2(2)/(9.2 - 1), we derive the general expression of the extreme cosmo-black-hole, where the three horizons merge, and we discuss some of its properties. We also show that the endpoint of the evaporation process is independent of any order relation between q(2) and M-2. The Teitelboim energy and the Padmanabhan energy are related by a nonlinear expression and are shown to correspond to different ensembles. We also determine the enthalpy H of the event horizon, as well as the effective thermodynamic volume which is the conjugate variable of the negative quintessential pressure, and show that in general the mass parameter and the Teitelboim energy are different from the enthalpy and internal energy; only in the cosmological case, that is, for Reissner-Nordstrom-de Sitter black hole we have H = M. Generalized Smarr formulas are also derived. It is concluded that the internal energy has a universal expression for all static charged black holes, with possibly a variable mass parameter, but it is not a suitable thermodynamic potential for static-black-hole thermodynamics if M is constant. It is also shown that the reverse isoperimetric inequality holds. We generalize the results to the case of the Reissner-Nordstrom-de Sitter black hole surrounded by quintessence with two physical constants yielding two thermodynamic volumes.
Constraining the generalized uncertainty principle through black hole shadow, S2 star orbit, and quasiperiodic oscillations
(2022) Jusufi, Kimet; Azreg-Ainou, Mustapha; Jamil, Mubasher; Zhu, Tao
In 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).
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, Cosimo
In 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.
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.
Cyclic and heteroclinic flows near general static spherically symmetric black holes
(2016) Ahmed, Ayyesha K.; Azreg-Ainou, Mustapha; Faizal, Mir; Jamil, Mubasher
We 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.
Cyclic and heteroclinic flows near general static spherically symmetric black holes: semi-cyclic flows - addendum and corrigendum
(2017) Azreg-Ainou, Mustapha; 0000-0002-3244-7195; AAZ-1598-2021
We present new accretion solutions of a polytropic perfect fluid onto an f(R)-gravity de Sitter-like black hole. We consider two f(R)-gravity models and obtain finite-period cyclic flows oscillating between the event and cosmological horizons as well as semi-cyclic critical flows executing a two-way motion from and back to the same horizon. Besides the generalizations and new solutions presented in this work, a corrigendum to Eur. Phys. J. C (2016) 76:280 is provided.
Cylindrically symmetric static n-dimensional (un)charged (anti-)de Sitter black holes in generic f(T) gravity
(2020) Azreg-Ainou, Mustapha
Given a generic function f(T) we construct in closed forms cylindrically symmetric static n-dimensional uncharged and charged de Sitter and anti-de Sitter solutions (including black holes, wormholes and possibly other regular solutions) in f(T) gravity. Applications to some known models are considered. Copyright (C) EPLA, 2020
Dynamical and static solutions to R=0 scalar-tensor theory
(2020) Azreg-Ainou, Mustapha
We consider the most cosmologically interesting and relevant case of scalar-tensor theory (STT) and derive new normal and phantom, dynamical and static, solutions. We determine the Bianchi I Kasner exponents and show that the dynamical solutions are heteroclinic orbits connecting two singularities. Approaching the singularities, a purely transverse expansion (no radial expansion or collapse) may occur. Copyright (C) EPLA, 2020
Epicyclic oscillations of charged particles in stationary solutions immersed in a magnetic field with application to the Kerr-Newman black hole
(2019) Azreg-Ainou, Mustapha; 0000-0002-3244-7195; R-1759-2019
We consider a stationary metric immersed in a uniform magnetic field and determine the general expressions for the epicyclic frequencies of charged particles. Applications to the Kerr-Newman black hole are reached of physical consequences and reveal some new effects among which are the existence of radially and vertically stable circular orbits in the region enclosed by the event horizon and the so-called "innermost" stable circular orbit (ISCO) in the plane of symmetry.
From static to rotating to conformal static solutions: rotating imperfect fluid wormholes with(out) electric or magnetic field
(2014) Azreg-Ainou, Mustapha
We derive a shortcut stationary metric formula for generating imperfect fluid rotating solutions, in Boyer-Lindquist coordinates, from spherically symmetric static ones. We explore the properties of the curvature scalar and stress-energy tensor for all types of rotating regular solutions we can generate without restricting ourselves to specific examples of regular solutions (regular black holes or wormholes). We show through examples how it is generally possible to generate an imperfect fluid regular rotating solution via radial coordinate transformations. We derive rotating wormholes that are modeled as imperfect fluids and discuss their physical properties. These are independent on the way the stress-energy tensor is interpreted. A solution modeling an imperfect fluid rotating loop black hole is briefly discussed. We then specialize to the recently discussed stable exotic dust Ellis wormhole as emerged in a source-free radial electric or magnetic field, and we generate its, conjecturally stable, rotating counterpart. This turns out to be an exotic imperfect fluid wormhole, and we determine the stress-energy tensor of both the imperfect fluid and the electric or magnetic field.
Geometrothermodynamics: comments, criticisms, and support
(2014) Azreg-Ainou, Mustapha
We write explicitly the Euler identity and the Gibbs-Duhem relation for thermodynamic potentials that are not homogeneous first-order functions of their natural extensive variables. We apply the rules to the theory of geometrothermodynamics and show how the use of the natural extensive variables, instead of the modified ones, leads to misleading results. We further reveal some other ambiguities and inconsistencies in the theory and we make new suggestions.
Gyroscope precession frequency analysis of a five-dimensional charged rotating Kaluza-Klein black hole
(2020) Azreg-Ainou, Mustapha; Jamil, Mubasher; Lin, Kai; 0000-0001-9662-1546; 0000-0002-3244-7195; X-3974-2019
We study the spin precession frequency of a test gyroscope attached to a stationary observer in the five-dimensional rotating Kaluza-Klein black hole (RKKBH). We derive the conditions under which the test gyroscope moves along a timelike trajectory in this geometry, and the regions where the spin precession frequency diverges. The magnitude of the gyroscope precession frequency around the KK black hole diverges at two spatial locations outside the event horizon. However, in the static case, the behavior of the Lense-Thirring frequency of a gyroscope around the KK black hole is similar to the ordinary Schwarzschild black hole. Since a rotating Kaluza-Klein black hole is a generalization of the Kerr-Newman black hole, we present two mass-independent schemes to distinguish these two spacetimes.
Hamiltonian Formulation Of Relativistic Magnetohydrodynamic Accretion On A General Spherically Symmetric And Static Black Hole: Quantum Effects On Shock States
(EUROPEAN PHYSICAL JOURNAL C, 2024-11-19) Azreg-Ainou, Mustapha; Jamil, Mubasher; Noda, Sousuke
In this paper, our aim is to extend our earlier work [Ahmed et al. in Eur. Phys. J. C 76:280, 2016], investigating an axisymmetric plasma flow with angular momentum onto a spherical black hole. To accomplish that goal, we focus on the case in which the ideal magnetohydrodynamic approximation is valid, utilizing certain conservation laws which arise from particular symmetries of the system. After formulating a Hamiltonian of the physical system, we solve the Hamilton equations and look for critical solutions of (both in and out) flows. Reflecting the difference from the Schwarzschild spacetime, the positions of sonic points (fast magnetosonic point, slow magnetosonic point, Alfv & eacute;n point) are altered. We explore several kinds of flows including critical, non-critical, global, magnetically arrested and shock induced. Lastly, we analyze the shock states near a specific quantum corrected Schwarzschild black hole and determine that quantum effects do not favor shock states by pushing the shock location outward.
High-pressure effects on the benzene pre-crystallization metastable states
(2019) Azreg-Ainou, Mustapha; Ibrahimoglu, Beycan; 0000-0002-3244-7195; 31376047
We report new results on the liquid to solid phase transition of benzene. We determine experimentally and investigate the properties of a number of parameters of the benzene metastable state under different pressures (from 0.1 up to 2200atm). It is shown that the supercooling, pressure drop, incubation period, time of abrupt transition from the metastable state to the crystalline state, and time of isothermal freezing all decrease as the external pressure increases, then they all vanish at 2200atm and 356K which may mark the end-point of metastability. Quadratic interpolation formulas for these parameters are provided. The densities and molar heat capacities of supercooled benzene under different pressures have been calculated too.
Observational Tests Of Asymptotically Flat r2 Spacetimes
(Başkent Üniversitesi Mühendislik Fakültesi, 2024-04-07) Zhu, Tao; Nguyen, Hoang Ky; Azreg-Ainou, Mustapha; Jamil, Mubasher
A novel class of Buchdahl-inspired metrics with closed-form expressions was recently obtained based on Buchdahl's seminal work on searching for static, spherically symmetric metrics in R-2 gravity in vacuo. Buchdahl-inspired spacetimes provide an interesting framework for testing predictions of R(2 )ravity models against observations. To test these Buchdahl-inspired spacetimes, we consider observational constraints imposed on the deviation parameter, which characterizes the deviation of the asymptotically flat Buchdahl-inspired metric from the Schwarzschild spacetime. We utilize several recent solar system experiments and observations of the S2 star in the galactic center and the black hole shadow. By calculating the effects of Buchdahl-inspired spacetimes on astronomical observations both within and outside of the solar system, including the deflection angle of light by the Sun, gravitational time delay, perihelion advance, shadow, and geodetic precession, we determine observational constraints on the corresponding deviation parameters by comparing theoretical predictions with the most recent observations. Among these constraints, we find that the tightest one comes from the Cassini mission's measurement of gravitational time delay.
On 'rotating charged AdS solutions in quadratic f(T) gravity': new rotating solutions
(2020) Azreg-Ainou, Mustapha; 0000-0002-3244-7195
We show that there are two or more procedures to generalize the known four-dimensional transformation, aiming to generate cylindrically rotating charged exact solutions, to higher dimensional spacetimes. In the one procedure, presented in Eur. Phys. J. C (2019) 79:668, one uses a non-trivial, non-diagonal, Minkowskian metric (eta) over bari j to derive complicated rotating solutions. In the other procedure, discussed in this work, one selects a diagonal Minkowskian metric.i j to derive much simpler and appealing rotating solutions. We also show that if (g mu.,.i j) is a rotating solution then ( (g) over bar mu., (eta) over bar. i j) is a rotating solution too with similar geometrical properties, provided (eta) over bar i j and.i j are related by a symmetric matrix R: (eta) over bar i j = eta ik Rk j.
Orbital mechanics and quasiperiodic oscillation resonances of black holes in Einstein-AEther theory
(2020) Azreg-Ainou, Mustapha; Chen, Zihang; Deng, Bojun; Jamil, Mubasher; Zhu, Tao; Wu, Qiang; Lim, Yen-Kheng
In this paper, we study the motion of test particles around two exact charged black hole solutions in Einstein-AEther theory. Specifically, we first consider the quasiperiodic oscillations (QPOs) and their resonances generated by the particle moving in the Einstein-AEther black hole and then turn to study the periodic orbits of the massive particles. For QPOs, we drop the usually adopted assumptions nu(U) = nu(theta), nu(L) = nu(r), and nu(U)/nu(L) = 3/2 with nu(U) (nu(L)) and nu(r) (nu(theta)) being the upper (lower) frequency of QPOs and radial (vertical) epicyclic frequency of the orbiting particles, respectively. Instead, we put-forward a new working ansatz for which the Keplerian radius is much closer to that of the innermost stable circular orbit and explore in detail the effects of the aether field on the frequencies of QPOs. We then realize good curves for the frequencies of QPOs, which fit to data of three microquasars very well by ignoring any effects of rotation and magnetic fields. The innermost stable circular orbits (isco) of timelike particles are also analyzed, and we find the isco radius increases with increasing c(13) for the first type black hole while decreases with increasing c(14) for the second one. We also obtain several periodic orbits and find that they share similar taxonomy schemes as the periodic equatorial orbits in the Schwarzschild/Kerr metrics, in addition to exact solutions for certain choices of the Einstein-AE ther parameters. The equations for null geodesics are also briefly considered, where we study circular photon orbits and bending angles for gravitational lensing.
Quasinormal modes, quasiperiodic oscillations, and the shadow of rotating regular black holes in nonminimally coupled Einstein-Yang-Mills theory
(2021) Jusufi, Kimet; Azreg-Ainou, Mustapha; Jamil, Mubasher; Wei, Shao-Wen; Wu, Qiang; Wang, Anzhong; AAZ-1598-2021
In this paper, we obtain an effective metric describing a regular and rotating magnetic black hole (BH) solution with a Yang-Mills electromagnetic source in Einstein-Yang-Mills (EYM) theory using the Newman-Janis (NJ) algorithm via the noncomplexification radial coordinate procedure. We then study the BH shadow and the quasinormal modes (QNMs) for massless scalar and electromagnetic fields and the quasiperiodic oscillations (QPOs). To this end, we also study the embedding diagram for the rotating EYM BH. The energy conditions, shadow curvature radius, topology, and the dynamical evolution of scalar and electromagnetic perturbations using the time domain integration method are investigated. We show that the shadow radius decreases by increasing the magnetic charge, while the real part of QNMs of scalar and electromagnetic fields increases by increasing the magnetic charge. This result is consistent with the inverse relation between the shadow radius and the real part of QNMs. In addition, we have studied observational constraints on the EYM parameter. via frequency analysis of QPOs and the EHT data of shadow cast by the M87 central black hole. We also find that the decaying rate of the EYM BH is slower than that of the neutral and ends up with a tail. We argue that the rotating EYM black hole can be distinguished from the Kerr-Newman black hole with a magnetic charge based on the difference between the angular diameters of their shadows.