General Relativistic Magnetohydrodynamic Simulations of Black Hole Accretion Disks

Observations are providing increasingly detailed quantitative information about the accretion flows that power such high energy systems as X-ray binaries and active galactic nuclei. Analytic models of such systems must rely on assumptions such as regular flow geometry and a simple, parameterized stress. Global numerical simulations offer a way to investigate the basic physical dynamics of accretion flows without these assumptions. For black hole accretion studies one solves the equations of general relativistic magnetohydrodynamics. Magnetic fields are of fundamental importance to the structure and evolution of accretion disks because magnetic turbulence is the source of the anomalous stress that drives accretion. We have developed a three-dimensional general relativistic magnetohydrodynamic simulation code to evolve time-dependent accretion systems self-consistently. Recent global simulations of black hole accretion disks suggest that the generic structure of the accretion flow is usefully divided into five regimes: the main disk, the inner disk, the corona, the evacuated funnel, and the funnel wall jet. The properties of each of these regions are summarized.Comment: invited review at the conference "Stellar-mass, Intermediate-mass, and Supermassive Black Holes", held in Kyoto, Japan, Octorber 28-31, 2003, to be published in Progress of Theoretical Physics Supplemen

Gravitational Capture of Cosmic Strings by a Black Hole

The gravitational interaction of an infinitely long cosmic string with a Schwarzschild black hole is studied. We consider a straight string that is initially at a great distance and moving at some initial velocity v (0 < v < c) towards the black hole. The equations of motion of the string are solved numerically to obtain the dependence of the capture impact parameter on the initial velocity.Comment: 15 pages, 2 figures, submitted to Phys Rev

Scattering of a Long Cosmic String by a Rotating Black Hole

The scattering of a straight, infinitely long string by a rotating black hole is considered. We assume that a string is moving with velocity v and that initially the string is parallel to the axis of rotation of the black hole. We demonstrate that as a result of scattering, the string is displaced in the direction perpendicular to the velocity by an amount kappa(v,b), where b is the impact parameter. The late-time solution is represented by a kink and anti-kink, propagating in opposite directions at the speed of light, and leaving behind them the string in a new ``phase''. We present the results of the numerical study of the string scattering and their comparison with the weak-field approximation, valid where the impact parameter is large, b/M >> 1, and also with the scattering by a non-rotating black hole which was studied in earlier works.Comment: 27 pages, 14 figures, to be published in Classical and Quantum Gravit

Lipohypertrophy - a complication of insulin injections

No Abstract.Journal of the Society for Endocrinology, Metabolism and Diabetes of South Africa Vol. 11 (2) 2002: pp. 64-6

Afrikaans as sangtaal*

Eerstens: Sang, wat een van die onderafdelings vorm van die breë begrip „musiek”, wat op sy beurt weer een van die vertakkings is van die nog breër begrip „kuns”. Aan definisies van kuns en meer spesifiek van musiek is daar geen gebrek nie. Laat ons vir die huidige volstaan met die volgende: Musiek is die sinvolle rangskikking van musikale klanke in ’n sinvolle ritmiese verband. Sang, as een van die middele waarmee musiek gemaak word, maak gebruik van die menslike stem wat daartoe in staat is om verskillende toonhoogtes voort te bring in ’n vinniger of stadiger opeenvolging met mekaar. Die menslike stem kan slegs een toonhoogte op ’n gegewe moment voortbring, en die derde groot element van musiek nl. harmonie (die ander twee is melodie en ritme en is implisiet in die bogenoemde definisie) kan alleen dán na vore kom wanneer een stem in kombinasie met een of meer ander stemme of instrumente aan- gewend word

Stroke in the young

CITATION: De Villiers, R. V. P., February, E. & De Villiers, M. L. 2003. Stroke in the young. South African Medical Journal, 93(11):836-837.The original publication is available at http://www.samj.org.za[No abstract available]Publisher’s versio

The poor children of the poor: Coping with diabetes control in a resource-poor setting

Background. Coping with diabetes control is difficult for newly diagnosed and experienced patients alike. Children with diabetes face severe challenges, as they may not yet have attained the necessary cognitive, fine motor or psychosocial skills required for performance of the tasks required from the diabetic patient. Most therefore require some adult assistance.Objectives. To establish whether paediatric diabetic patients are adequately supported by their families in terms of giving insulin injections and doing home blood glucose monitoring (HBGM), and whether insulin and the necessary equipment are appropriately stored in their homes.Methods. Patients attending a paediatric diabetes clinic were interviewed. The data collected included demographic variables, type of insulin, measurement of insulin doses, administration of insulin, and blood glucose monitoring tests.Results. Twenty-five subjects were interviewed: 18 measured the insulin themselves, five mothers and one aunt did so, and in one case the mother and patient did so together. The four children aged ≤10 years had their insulin measured by their mothers, but one had to administer the injection himself. Eight of the nine children aged 11 - 15 years measured and administered the insulin themselves; in four cases the doses were checked by an adult. The mothers of four children did the fingerpricks, and eight children were helped with measuring the results. Only two children aged 11 - 15 years had their doses checked by an adult.Conclusion. Adult assistance with regard to both insulin injections and HBGM is rarely forthcoming. The children seem not to besufficiently supported by their families

Primary Sjogren's syndrome associated with inappropriate antidiuretic hormone secretion A case report

A patient in whom primary Sjogren's syndrome and inappropriate antiduretic hormone secretion were associated is reported. This is the first report of such an association. The possible pathophysiological mechanisms are discussed and vasculitis proposed as the underlying pathogenetic mechanism

Application of the Cubed-Sphere Grid to Tilted Black-Hole Accretion Disks

In recent work we presented the first results of global general relativistic magnetohydrodynamic (GRMHD) simulations of tilted (or misaligned) accretion disks around rotating black holes. The simulated tilted disks showed dramatic differences from comparable untilted disks, such as asymmetrical accretion onto the hole through opposing "plunging streams" and global precession of the disk powered by a torque provided by the black hole. However, those simulations used a traditional spherical-polar grid that was purposefully underresolved along the pole, which prevented us from assessing the behavior of any jets that may have been associated with the tilted disks. To address this shortcoming we have added a block-structured "cubed-sphere" grid option to the Cosmos++ GRMHD code, which will allow us to simultaneously resolve the disk and polar regions. Here we present our implementation of this grid and the results of a small suite of validation tests intended to demonstrate that the new grid performs as expected. The most important test in this work is a comparison of identical tilted disks, one evolved using our spherical-polar grid and the other with the cubed-sphere grid. We also demonstrate an interesting dependence of the early-time evolution of our disks on their orientation with respect to the grid alignment. This dependence arises from the differing treatment of current sheets within the disks, especially whether they are aligned with symmetry planes of the grid or not.Comment: 15 pages, 11 figures, submitted to Ap