arXiv subjects

Tayebeh Tahamtan

Publications and source records attributed to Tayebeh Tahamtan.

8 recordsLinked to original sources

arXiv2026

Higher-Dimensional Slowly Rotating Black Holes in Nonlinear Electrodynamics

We study slowly rotating, nonlinearly charged black holes in arbitrary spacetime dimension $d \geq 4$, using a generalized Lense--Thirring ansatz. Working to linear order in the rotation parameters, we solve the Einstein--nonlinear electrodynamics (NLE) field equations for restricted theories ${\cal L}({\cal S})$ depending on single invariant. We provide an extension of the four-dimensional "No Go theorem" proven in ArXiv:2203.01919 and demonstrate the uniqueness of Maxwell theory in $d \geq 4$. Similar to four dimensions, it is proven that the standard rotation generating trick fails to produce full rotating solutions for any nontrivial NLE in higher dimensions. We identify a family of NLE theories admitting slowly rotating solutions with vector potentials encoding rotation effect in Maxwell-like manner; in four dimensions this reproduces the previously found RegMax theory, while in $d\geq4$ we obtain the corresponding electric potential in closed form and the associated Lagrangian implicitly as a function of the radial coordinate.

gr-qc↗

arXiv2026

Bertotti-Robinson and Bonnor-Melvin universes in nonlinear electrodynamics

We review the status of Birkhoff's theorem in the presence of nonlinear electrodynamics (NLE) - extending the analysis to the case without asymptotic flatness. This leads to the Bertotti-Robinson-type (direct product) geometry with generally unequal radii for its $AdS_{2}$ and $S_{2}$ factors, determined by a given NLE model. As can be expected, such a geometry can also be recovered from a near-horizon limit of the corresponding extremal NLE charged black hole (if it exists). These extremal black holes are shown to be linearly stable for specific NLE models, unlike in the Maxwell-$Λ$ case where unequal radii also arise in near-horizon geometry. Regular particle-like models are constructed by replacing the interior of these black holes with corresponding Bertotti-Robinson-type geometry. We also revisit the NLE generalization of the Bonnor-Melvin universe, describing a regular axisymmetric configuration of magnetic field lines in gravito-magnetic equilibrium. Explicit examples are derived for the Maxwell, Born-Infeld, RegMax, and Frolov-Hayward theories of electrodynamics.

gr-qc↗

arXiv2024

Regularized Conformal Electrodynamics: Novel C-metric in (2+1) Dimensions

Conformal electrodynamics is a particularly interesting example of power Maxwell non-linear electrodynamics, designed to possess conformal symmetry in all dimensions. In this paper, we propose a regularized version of Conformal electrodynamics, minimally regularizing the field of a point charge at the origin by breaking the conformal invariance of the theory with a dimensionfull "Born-Infeld-like" parameter. In four dimensions the new theory reduces to the recently studied Regularized Maxwell electrodynamics, distinguished by its "Maxwell-like" solutions for accelerated and slowly rotating black hole spacetimes. Focusing on three dimensions, we show that the new theory shares many of the properties of its four-dimensional cousin, including the existence of the charged C-metric solution (currently unknown in the Maxwell theory).

gr-qc↗

arXiv2023

Solutions and basic properties of regularized Maxwell theory

The regularized Maxwell theory is a recently discovered theory of non-linear electrodynamics that admits many important gravitating solutions within the Einstein theory. Namely, it was originally derived as the unique non-linear electrodynamics (that depends only on the field invariant $F_{μν}F^{μν}$) whose radiative solutions can be found in the Robinson--Trautman class. At the same time, it is the only electrodynamics of this type (apart from Maxwell) whose slowly rotating solutions are fully characterized by the electrostatic potential. In this paper, after discussing the basic properties of the regularized Maxwell theory, we concentrate on its spherical electric solutions. These not only provide `the simplest' regularization of point electric field and its self-energy, but also feature complex thermodynamic behavior (in both canonical and grandcanonical ensembles) and admit an unprecedented phase diagram with multiple first-order, second-order, and zeroth-order phase transitions. Among other notable solutions, we construct a novel C-metric describing accelerated AdS black holes in the regularized Maxwell theory. We also present a generalization of the regularized Maxwell Lagrangian applicable to magnetic solutions, and find the corresponding spherical, slowly rotating, and weakly NUT charged solutions.

gr-qc↗

arXiv2022

Slowly rotating black holes in nonlinear electrodynamics

We show how (at least in principle) one can construct electrically and magnetically charged slowly rotating black hole solutions coupled to non-linear electrodynamics (NLE). Our generalized Lense-Thirring ansatz is, apart from the static metric function $f$ and the electrostatic potential $ϕ$ inherited from the corresponding spherical solution, characterized by two new functions $h$ (in the metric) and $ω$ (in the vector potential) encoding the effect of rotation. In the linear Maxwell case, the rotating solutions are completely characterized by static solution, featuring $h=(f-1)/r^2$ and $ω=1$. We show that when the first is imposed, the ansatz is inconsistent with any restricted (see below) NLE but the Maxwell electrodynamics. In particular, this implies that the (standard) Newman-Janis algorithm cannot be used to generate rotating solutions for any restricted non-trivial NLE. We present a few explicit examples of slowly rotating solutions in particular models of NLE, as well as briefly discuss the NLE charged Taub-NUT spacetimes.

gr-qc↗

arXiv2019

Emergence of magnetic null points in electro-vacuum magnetospheres of compact objects: The case of a plunging neutron star

Relativistic effects of compact objects onto electromagnetic fields in their vicinity are investigated using the test-field approximation. In particular, we study the possible emergence of magnetic null points which are astrophysically relevant for the processes of magnetic reconnection. While the magnetic reconnection occurs in the presence of plasma and may lead to violent mass ejection, we show here that strong gravitation of the supermassive black hole may actively support the process by suitably entangling the field lines even in the electro-vacuum description. In this contribution we further discuss the case of a dipole-type magnetic field of the neutron star on the plunging trajectory to the supermassive black hole. While we have previously shown that given model in principle admits the formation of magnetic null points, here we explore whether and where the null points appear for the astrophysically relevant values of the parameters.

astro-ph.HE↗

arXiv2019

Oblique magnetic fields and the role of frame dragging

Magnetic null points can develop near the ergosphere boundary of a rotating black hole by the combined effects of strong gravitational field and the frame-dragging mechanism. The electric component does not vanish in the magnetic null and an efficient process or particle acceleration can occur. The situation is relevant for starving (low-accretion-rate, such as the Milky Way's supermassive black hole) nuclei of some galaxies which exhibit only episodic accretion events. The presence of the magnetic field field of an external origin is an important aspect. We propose that such conditions can develop when a magnetized neutron star approaches the supermassive black hole during late stages of its inspiral motion. The field lines of the neutron star dipole thread the black hole's event horizon and change rapidly their connectivity. We put in comparison the case of a dipole-type magnetic field of a sinking and orbiting star near a non-rotating black hole, and the near-horizon structure of an asymptotically uniform magnetic field of a distant source near a fast-rotating black hole. Although the two cases are qualitatively different from each other, they both develop magnetically neutral null points near the event horizon.

astro-ph.HE↗

arXiv2011

Stable Magnetic Universes Revisited

A regular class of static, cylindrically symmetric pure magnetic field metrics is rederived in a different metric ansatz in all dimensions. Radial, time dependent perturbations show that for dimensions d>3 such spacetimes are stable at both near r\approx0 and large radius r\rightarrow\infty. In a different gauge these stability analysis and similar results were known beforehand. For d=3, however, simultaneous stability requirement at both, near and far radial distances can not be reconciled for time - dependent perturbations. Restricted, numerical geodesics for neutral particles reveal a confinement around the center in the polar plane. Charged, time-like geodesics for d=4 on the other hand are shown numerically to run toward infinity.

gr-qc↗