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Search Results: 1 - 10 of 121 matches for " Shohreh Abdolrahimi "
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On the Evolution of Binary Neutron Stars
Shohreh Abdolrahimi
Physics , 2009,
Abstract: 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.
Velocity Effects on an Accelerated Unruh-DeWitt Detector
Shohreh Abdolrahimi
Physics , 2013, DOI: 10.1088/0264-9381/31/13/135009
Abstract: We analyze the response of an Unruh-DeWitt detector moving along an unbounded spatial trajectory in a two-dimensional spatial plane with constant independent magnitudes of both the four-acceleration and of a timelike proper time derivative of the four-accelration. In a Fermi-Walker frame moving with the detector, the direction of the acceleration rotates at a constant rate around a great circle. This is the motion of a charge in a uniform electric field when in the frame of the charge there is both an electric and a magnetic field. We compare the response of this detector to a detector moving with constant velocity in a thermal bath of the corresponding temperature for non-relativistic velocities, and in two regimes: ultraviolet and infrared. In infrared regime, the detector in the Minkowski space-time moving along the spatially two-dimensional trajectory should move with a higher speed to keep up with the same excitation rate of the inertial detector in a thermal bath. In ultraviolet regime, the dominant modification in the response of this detector compared to the black body spectrum of Unruh radiation is the same as the dominant modification perceived by a detector moving with constant velocity in a thermal bath.
Analysis of the Fisher solution
Shohreh Abdolrahimi,Andrey A. Shoom
Physics , 2009, DOI: 10.1103/PhysRevD.81.024035
Abstract: 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."
Geometric Properties of Static EMdL Horizons
Shohreh Abdolrahimi,Andrey A. Shoom
Physics , 2011, DOI: 10.1103/PhysRevD.83.104023
Abstract: 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.
Distorted Five-dimensional Electrically Charged Black Holes
Shohreh Abdolrahimi,Andrey A. Shoom
Physics , 2013, DOI: 10.1103/PhysRevD.89.024040
Abstract: We derive and study distorted, five-dimensional, electrically charged, non-extremal black holes on the example of a static and "axisymmetric" black hole distorted by external, electrically neutral matter.The solution satisfies Einstein-Maxwell equations which admits an $\mathbb{R}^1\times U(1)\times U(1)$ isometry group. The inner horizon remains regular if the distortion fields are finite and smooth at the outer horizon. There exists a certain duality transformation between the inner and the outer horizon surfaces which links surface gravity, electrostatic potential, and space-time curvature invariants calculated at the black hole horizons. The product of the inner and outer horizon areas depends only on the black hole's electric charge and the geometric mean of the areas is the upper (lower) limit for the inner (outer) horizon area. The horizon areas, electrostatic potential, and surface gravity satisfy the Smarr formula. We formulated the zeroth and the first laws of mechanics and thermodynamics of the distorted black hole and found a correspondence between the global and local forms of the first law. To illustrate the effect of distortion we consider the dipole-monopole and quadrupole-quadrupole distortion fields. The relative change in the Kretschamnn scalar due to the distortion is greater at the outer horizon than at the inner one. Calculating the maximal proper time of free fall from the outer to the inner horizons we show that the distortion can noticeably change the black hole interior. The change depends on type and strength of distortion fields. In particular, due to the types of distortion fields considered here the black hole horizons can either come arbitrarily close to or move far from each other.
Distorted Local Shadows
Shohreh Abdolrahimi,Robert B. Mann,Christos Tzounis
Physics , 2015, DOI: 10.1103/PhysRevD.91.084052
Abstract: We introduce the notion of a local shadow for a black hole and determine its shape for the particular case of a distorted Schwarzschild black hole. Considering the lowest-order even and odd multiple moments, we compute the relation between the deformations of the shadow of a Schwarzschild black hole and the distortion multiple moments. For the range of values of multiple moments that we consider, the horizon is deformed much less than its corresponding shadow, suggesting the horizon is more `rigid'. Quite unexpectedly we find that a prolate distortion of the horizon gives rise to an oblate distortion of the shadow, and vice-versa.
Interior of a Charged Distorted Black Hole
Shohreh Abdolrahimi,Valeri P. Frolov,Andrey A. Shoom
Physics , 2009, DOI: 10.1103/PhysRevD.80.024011
Abstract: 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.
Distorted 5-dimensional vacuum black hole
Shohreh Abdolrahimi,Andrey A. Shoom,Don N. Page
Physics , 2010, DOI: 10.1103/PhysRevD.82.124039
Abstract: 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.
Spectral Methods in General Relativity and Large Randall-Sundrum II Black Holes
Shohreh Abdolrahimi,Céline Catto?n,Don N. Page,Shima Yaghoobpour-Tari
Physics , 2012, DOI: 10.1088/1475-7516/2013/06/039
Abstract: Using a novel numerical spectral method, we have found solutions for large static Randall-Sundrum II (RSII) black holes by perturbing a numerical AdS_5-CFT_4 solution to the Einstein equation with a negative cosmological constant Lambda that is asymptotically conformal to the Schwarzschild metric. We used a numerical spectral method independent of the Ricci-DeTurck-flow method used by Figueras, Lucietti, and Wiseman for a similar numerical solution. We have compared our black-hole solution to the one Figueras and Wiseman have derived by perturbing their numerical AdS_5-CFT_4 solution, showing that our solution agrees closely with theirs. We have also deduced the new results that to first order in 1/(-\Lambda M^2), the Hawking temperature and entropy of an RSII static black hole have the same values as the Schwarzschild metric with the same mass, but the horizon area is increased by about 4.7/(-\Lambda).
Large Randall-Sundrum II Black Holes
Shohreh Abdolrahimi,Céline Catto?n,Don N. Page,Shima Yaghoobpour-Tari
Physics , 2012, DOI: 10.1016/j.physletb.2013.02.034
Abstract: Using a novel numerical spectral method, we have constructed an AdS_5-CFT_4 solution to the Einstein equation with a negative cosmological constant Lambda that is asymptotically conformal to the Schwarzschild metric. This method is independent of the Ricci-DeTurck-flow method used by Figueras, Lucietti, and Wiseman. We have perturbed the solution to get large static black hole solutions to the Randall-Sundrum II (RSII) braneworld model. Our solution agrees closely with that of Figueras et al. and also allows us to deduce the new results that to first order in 1/(-Lambda M^2), the Hawking temperature and entropy of an RSII static black hole have the same values as the Schwarzschild metric with the same mass, but the horizon area is increased by about 4.7/(-Lambda).
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