arXiv subjects

Shohreh Abdolrahimi

Publications and source records attributed to Shohreh Abdolrahimi.

23 records · Page 2Linked to original sources

arXiv2011

Geometric Properties of Static EMdL Horizons

We study non-degenerate and degenerate (extremal) Killing horizons of arbitrary geometry and topology within the Einstein-Maxwell-dilaton model with a Liouville potential (the EMdL model) in d-dimensional (d>=4) static space-times. Using Israel's description of a static space-time, we construct the EMdL equations and the space-time curvature invariants: the Ricci scalar, the square of the Ricci tensor, and the Kretschmann scalar. Assuming that space-time metric functions and the model fields are real analytic functions in the vicinity of a space-time horizon, we study behavior of the space-time metric and the fields near the horizon and derive relations between the space-time curvature invariants calculated on the horizon and geometric invariants of the horizon surface. The derived relations generalize the similar relations known for horizons of static four and 5-dimensional vacuum and 4-dimensional electrovacuum space-times. Our analysis shows that all the extremal horizon surfaces are Einstein spaces. We present necessary conditions for existence of static extremal horizons within the EMdL model.

hep-th↗

arXiv2010

Distorted 5-dimensional vacuum black hole

In this paper we study how the distortion generated by a static and neutral distribution of external matter affects a 5-dimensional Schwarzschild-Tangherlini black hole. A solution representing a particular class of such distorted black holes admits an RxU(1)xU(1) isometry group. We show that there exists a certain duality transformation between the black hole horizon and a stretched singularity surfaces. The space-time near the distorted black hole singularity has the same topology and Kasner exponents as those of a 5-dimensional Schwarzschild-Tangherlini black hole. We calculate the maximal proper time of free fall of a test particle from the distorted black hole horizon to its singularity and find that, depending on the distortion, it can be less, equal to, or greater than that of a Schwarzschild-Tangherlini black hole of the same horizon area. This implies that due to the distortion, the singularity of a Schwarzschild-Tangherlini black hole can come close to its horizon. A relation between the Kretschmann scalar calculated on the horizon of a 5-dimensional static, asymmetric, distorted black hole and the trace of the square of the Ricci tensor of the horizon surface is derived.

gr-qc↗

arXiv2010

Analysis of the Fisher solution

We study the $d$-dimensional Fisher solution which represents a static, spherically symmetric, asymptotically flat spacetime with a massless scalar field. The solution has two parameters, the mass and the "scalar charge." The Fisher solution has a naked curvature singularity which divides the spacetime manifold into two disconnected parts. The part which is asymptotically flat we call the {\em Fisher spacetime}, and another part we call the {\em Fisher universe}. The Schwarzschild-Tangherlini (ST) solution and the Fisher solution belong to the same theory and are dual to each other. The duality transformation acting in the parameter space maps the exterior region of the ST black hole into the Fisher spacetime which has a naked timelike singularity, and interior region of the black hole into the Fisher universe, which is an anisotropic expanding-contracting universe and which has two spacelike singularities representing its "Big Bang" and "Big Crunch". The Big Bang singularity and the singularity of the Fisher spacetime are {\em radially weak} in the sense that a 1-dimensional object moving along a timelike radial geodesic can arrive to the singularities intact. At the vicinity of the singularity the Fisher spacetime of nonzero mass has a region where its Misner-Sharp energy is negative. The Fisher universe has a marginally trapped surface corresponding to the state of its maximal expansion in the angular directions. These results and derived relations between geometric quantities of the Fisher spacetime, the Fisher universe, and the ST black hole may suggest that the massless scalar field transforms the black hole event horizon into the naked radially weak disjoint singularities of the Fisher spacetime and the Fisher universe which are "dual to the horizon."

gr-qc↗

arXiv2009

Interior of a Charged Distorted Black Hole

We study interior of a charged, non-rotating distorted black hole. We consider static and axisymmetric black holes, and focus on a special case when an electrically charged distorted solution is obtained by the Harrison-Ernst transformation from an uncharged one. We demonstrate that the Cauchy horizon of such black hole remains regular, provided the distortion is regular at the event horizon. The shape and the inner geometry of both the outer and inner (Cauchy) horizons are studied. We demonstrate that there exists a duality between the properties of the horizons. Proper time of a free fall of a test particle moving in the interior of the distorted black hole along the symmetry axis is calculated. We also study the property of the curvature in the inner domain between the horizons. Simple relations between the 4D curvature invariants and the Gaussian curvature of the outer and inner horizon surfaces are found.

gr-qc↗

arXiv2009

On the Evolution of Binary Neutron Stars

In this paper we investigate the evolution of binary neutron stars, namely, their magnetic field, spin, and orbital evolution. The core of a neutron star is considered to be a superfluid, superconductor type II. Flux expulsion of the magnetic field out of the core of a single neutron star has been discussed by previous authors. However, the evolution of the core magnetic field is substantially different for a binary neutron star. While for a single neutron star the fluxoids of the proton superconductor always move outward through the core, in a binary neutron star in the accretion phase fluxoids move back into the core. The subsequent increase of the core magnetic field results in the increase of the surface magnetic field. We ask weather within the framework of this model the formation of millisecond pulsars (MSPs) is possible. We show that despite the increase of the core magnetic field, MSPs are formed in this model. The evolution of the neutron star with various orbital periods, magnetic fields, spin periods, and other parameters are numerically investigated. The equation of state of the neutron star, initial orbital period of the binary, and other parameters of the binary have substantial effects on the evolution of period vs. magnetic field.

astro-ph.SR↗