arXiv subjects

Mohammad Bagher Jahani Poshteh

Publications and source records attributed to Mohammad Bagher Jahani Poshteh.

13 recordsLinked to original sources

arXiv2025

Measuring black hole spin through gravitational lensing of pulsars

We propose a new procedure for measuring the spin of a black hole with an unprecedented accuracy based on the gravitational lensing of millisecond pulsars. We derive the basic equations for lensing by a rotating black hole. We show that the frame dragging effect increases the deflection angle of a light ray co-rotating with the black hole. For the primary (secondary) images the angular positions are larger (smaller) for a rotating black hole by an amount on the order of tens of microarcseconds. The differential time delay of images for the case in which the lens is a rotating black hole is smaller than that in the case of non-rotating lens of the same mass, and it can be larger than a few milliseconds. We show that this quantity offers the possibility of reducing the error of spin measurement to less than one percent if we could measure the differential time delay with accuracy of microseconds. We also study relativistic images that are produced by light rays that rotate around the black hole before reaching the observer. The angular positions of relativistic images on the same side as the primary (secondary) image are a few microarcseconds larger (smaller) if the black hole is rotating. Furthermore, the differential time delay between relativistic images is about twelve orders of magnitude larger in the case of rotating lens.

gr-qc↗

arXiv2024

Particle motion and accretion disk around rotating accelerating black holes

We investigate the motion of test particles and the properties of accretion disks around rotating, accelerating black hole spacetimes with non-zero cosmological constants. To do so, we explore the impact of both rotation and acceleration on massive particle dynamics in motion around these configurations. In this respect, we derive the geodesic equations in the Pleba{ń}ski-Demia{ń}ski spacetime and analyse the characteristics of circular orbits, including the innermost and outermost stable circular orbits. Accordingly, we find that the acceleration parameter significantly influences the radii and angular momenta of these orbits. Further, we examine the precession of non-circular orbits and find that the precession is in the same direction as the black hole rotation regardless of the direction of the black hole spin. We also investigate the thermal spectra of geometrically thin, optically thick accretion disks described by the Novikov-Thorne model. We consider both co-rotating and counter-rotating disks and show that the radiative flux and luminosity are significantly larger for co-rotating disks. We compare the spectral features and luminosity of accretion disks around accelerating black holes with those around non-accelerating Kerr black holes, revealing that acceleration generally reduces the luminosity and flux of emitted light. Our results provide insights into the complex interplay between black hole acceleration, rotation, and the surrounding accretion disk dynamics.

astro-ph.HE↗

arXiv2023

Timelike orbits around accelerating black holes

We study the geodesics of massive particles around accelerating Schwarzschild black hole. We show that the radius of the innermost stable circular orbit increases and the angular momentum of particle at this orbit decreases by increasing the acceleration. Apart from quantitative influence, the acceleration qualitatively changes the physics. We show that in accelerating black hole spacetime there exist an outermost stable circular orbit in flat, de Sitter, and anti de Sitter background. Investigations of radial geodesics show that the acceleration acts like a repulsive force in the sense that test particles around accelerating black holes can move radially outward, unless there exist a large negative value of cosmological constant in the background to compensate the repulsive force. We also investigate the precession of perihelion of orbits around accelerating black holes. The precession would be larger compared to the non-accelerating case. It is also shown that the precession in anti de Sitter background could be opposite to the particle motion.

gr-qc↗

arXiv2023

Distinguishing a Slowly Accelerating Black Hole by Differential Time Delays of Images

Accelerating supermassive black holes, connected to cosmic strings, could contribute to structure formation and get captured by galaxies if their velocities are small. This would mean that the acceleration of these black holes is small too. Such a slow acceleration has no significant effect on the shadow of such supermassive black holes. We also show that, for slowly accelerating black holes, the angular position of images in the gravitational lensing effects do not change significantly. We propose a method to observe the acceleration of these black holes through the gravitational lensing. The method is based on the observation that differential time delays associated with the images are substantially different with respect to the case of non-accelerating black holes.

gr-qc↗

arXiv2022

Lensing Signatures of a Slowly-Accelerated Black Hole

Accelerating black holes, connected to cosmic strings could evolve to supermassive black holes. However, if they are going to take part in structure formation and resides at the center of galaxies, their acceleration should be small. This slow acceleration does not change the shadow or image position in gravitational lensing effect significantly. However we show that the time delay associated to these images change significantly. This is in contrast with when the theory governing the strong gravitational field around the black hole is different from general relativity, where not only the differential time delays but the angular position of images would be different. We conclude that, if the observed angular position of images are compatible with the prediction of general relativity, a possible deviation in the differential time delays between the observed values and those predicted by general relativity, could be due to the acceleration of the black hole.

gr-qc↗

arXiv2021

A new nonlinear electrodynamics and electrically charged regular black holes

A regular static, spherically symmetric electrically charged black hole solution of general relativity coupled to a new theory for nonlinear electrodynamics is presented. This theory has the interesting feature that, at far distances from the black hole, in the weak field limit, the theory reduces to Maxwell Lagrangian with Heisenberg-Euler correction term of quantum electrodynamics. The singular center of the black hole is replaced by flat, de Sitter, or anti de Sitter space, if the spacetime in which the black hole is embedded is asymptotically flat, de Sitter, or anti de Sitter, respectively. Requiring the correspondence to Heisenberg-Euler Lagrangian at large distances, in the weak field limit, we find that (\textit{i}) a minimum mass is required for the formation of an event horizon for the regular static, spherically symmetric solution of the theory, and, (\textit{ii}) the mass of the solution must be quantized. We also study the basic thermodynamic properties of the black hole solution and show that they are qualitatively similar to those of Reissner-Nordström black hole.

hep-th↗

arXiv2021

Thermodynamics of $z=4$ Hořava-Lifshitz black holes

Thermodynamics of $z=4$ Hořava-Lifshitz black holes in 3+1 dimensions is studied in extended phase space. By using the scaling argument we find the Smarr relation and the first law for the black hole solutions of $z=4$ Hořava-Lifshitz gravity. We find that it is necessary to take into account the variation of dimensionful parameters of the theory in the first law. We find that the reverse isoperimetric inequality can be violated for spherical, flat, and hyperbolic horizons and in all such cases we have black holes for which the specific heat at constant pressure and volume are positive. This provides a counterexample to a recent conjecture stating that black holes violating the reverse isoperimetric inequality are thermodynamically unstable. We find for $z=4$ Hořava-Lifshitz black holes with hyperbolic horizons that there are two critical points: one showing Van der Waals behavior, the other reverse Van der Waals behavior.

hep-th↗

arXiv2021

Black hole pair production on cosmic strings in the presence of a background magnetic field

We investigate the pair creation of magnetically charged black holes on a cosmic string in the presence of a background magnetic field. The string may either break or fray to produce a pair of accelerating black holes described by Ernst metric. By using the instanton action we obtain the rate of such production. For large values of background magnetic field the production of large black holes is probable. Comparing our results with the case of black hole pair creation in magnetic field with no string, we show that fraying/breaking of the cosmic string can substantially enhance the production rate. We also obtain the result of monopole-antimonopole pair production on cosmic string in an external magnetic field, using the WKB approximation and compare it with the black hole results. We also provide a heuristic study of black hole pair creation on cosmic string in the presence of a magnetic field in the de Sitter background, although the analog of Ernst metric with a cosmological constant is lacking. Like a cosmic string and/or a background magnetic field, a positive cosmological constant increases the production rate.

hep-th↗

arXiv2019

Gravitational Lensing by Black Holes in Einsteinian Cubic Gravity

We investigate the predictions of Einsteinian Cubic Gravity (ECG) for the lensing effects due to supermassive black holes at the center of Milky Way and other galaxies. Working in the context of spherical symmetry, we obtain the metric function from a continued fraction method and find that both time delays and the angular positions of images considerably deviate from general relativity, as large as milliarcseconds. This suggests that observational tests of ECG are indeed feasible.

gr-qc↗

arXiv2018

Shadows, Signals, and Stability in Einsteinian Cubic Gravity

We conduct a preliminary investigation into the phenomenological implications of Einsteinian cubic gravity (ECG), a 4-dimensional theory of gravity cubic in curvature of interest for its unique formulation and properties. We find an analytic approximation for a spherically symmetric black hole solution to this theory using a continued fraction ansatz. This approximate solution is valid everywhere outside of the horizon and we use it to study the orbit of massive test bodies near a black hole, specifically computing the innermost stable circular orbit. We compute constraints on the ECG coupling parameter imposed by Shapiro time delay. We then compute the shadow of an ECG black hole and find it to be larger than its Einsteinian counterpart in general relativity for the same value of the mass. Applying our results to Sgr A*, we find that departures from general relativity are small but in principle distinguishable.

gr-qc↗

arXiv2017

Phase transition and thermodynamic stability in extended phase space and charged Hořava-Lifshitz black holes

For charged black holes in Horava-Lifshitz gravity, a second order phase transition takes place in extended phase space where the cosmological constant is taken as thermodynamic pressure. We relate the second order nature of phase transition to the fact that the phase transition occurs at a sharp temperature and not over a temperature interval. Once we know the continuity of the first derivatives of the Gibbs free energy, we show that all the Ehrenfest equations are readily satisfied. We study the effect of the perturbation of the cosmological constant as well as the perturbation of the electric charge on thermodynamic stability of Horava-Lifshitz black hole. We also use thermodynamic geometry to study phase transition in extended phase space. We investigate the behavior of scalar curvature of Weinhold, Ruppeiner, and Quevedo metric in extended phase space of charged Horava-Lifshitz black holes. It is checked if these curvatures could reproduce the result of specific heat for the phase transition.

hep-th↗

arXiv2015

Generalized Ehrenfest's Equations and phase transition in Black Holes

We generalize Ehrenfest's equations to systems having two work terms, i.e. systems with three degrees of freedom. For black holes with two work terms we obtain nine equations instead of two to be satisfied at the critical point of a second order phase transition. We finally generalize this method to a system with an arbitrary number of degrees of freedom and found there is $\frac{N(N+1)^{2}}{2}$ equations to be satisfied at the point of a second order phase transition where $N$ is number of work terms in the first law of thermodynamics.

gr-qc↗

arXiv2013

Phase transition, critical behavior, and critical exponents of Myers-Perry black holes

The critical behavior of Myers-Perry black holes with equal angular momenta in even dimensions are studied. We include the corrections beyond the semiclassical approximation on Hawking temperature in the grand canonical ensemble. Having done so, we find that the critical behavior and critical exponents of Myers-Perry black holes correspond to those of a Van der Waals liquid-gas where this analogy holds in any dimension. Also, using Ehrenfest's equations, we calculate the order of the phase transition in the semiclassical approximation for the canonical ensemble and beyond the semiclassical approximation for the grand canonical ensemble near the critical point. Finally, the Ruppeiner curvature formula is used to investigate the thermodynamic geometry of Myers-Perry black holes.

gr-qc↗