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

Transport of Large Scale Poloidal Flux in Black Hole Accretion

We report on a global, three-dimensional GRMHD simulation of an accretion torus embedded in a large scale vertical magnetic field orbiting a Schwarzschild black hole. This simulation investigates how a large scale vertical field evolves within a turbulent accretion disk and whether global magnetic field configurations suitable for launching jets and winds can develop. We find that a "coronal mechanism" of magnetic flux motion, which operates largely outside the disk body, dominates global flux evolution. In this mechanism, magnetic stresses driven by orbital shear create large-scale half-loops of magnetic field that stretch radially inward and then reconnect, leading to discontinuous jumps in the location of magnetic flux. In contrast, little or no flux is brought in directly by accretion within the disk itself. The coronal mechanism establishes a dipole magnetic field in the evacuated funnel around the orbital axis with a field intensity regulated by a combination of the magnetic and gas pressures in the inner disk. These results prompt a reevaluation of previous descriptions of magnetic flux motion associated with accretion. Local pictures are undercut by the intrinsically global character of magnetic flux. Formulations in terms of an "effective viscosity" competing with an "effective resistivity" are undermined by the nonlinearity of of the magnetic dynamics and the fact that the same turbulence driving mass motion (traditionally identified as "viscosity") can alter magnetic topology.Comment: 45 pages, 17 figures, 1 movie; ApJ accepted; updated version contains several new figures and a movie detailing the operation of the coronal mechanism. The movie and a version of the paper with high resolution figures can be found at http://www.astro.virginia.edu/~krb3u/0906.2784

A near-infrared study of the star forming region RCW 34

We report the results of a near-infrared imaging study of a $7.8 \times 7.8$ arcmin$^2$ region centered on the 6.7 GHz methanol maser associated with the RCW 34 star forming region using the 1.4m IRSF telescope at Sutherland. A total of 1283 objects were detected simultaneously in J, H, and K for an exposure time of 10800 seconds. The J-H, H-K two-colour diagram revealed a strong concentration of more than 700 objects with colours similar to what is expected of reddened classical T Tauri stars. The distribution of the objects on the K {\it vs} J-K colour-magnitude diagram is also suggestive that a significant fraction of the 1283 objects is lower mass pre-main sequence stars. We also present the luminosity function for the subset of about 700 pre-main sequence stars and show that it suggests ongoing star formation activity for about $10^7$ years. An examination of the spatial distribution of the pre-main sequence stars shows that the fainter (older) part of the population is more dispersed over the observed region and the brighter (younger) subset is more concentrated around the position of the O8.5V star. This suggests that the physical effects of the O8.5V star and the two early B-type stars on the remainder of the cloud out of which they formed, could have played a role in the onset of the more recent episode of star formation in RCW 34.Comment: Accepted for publication in The Astronomical Journa

A review of hydatid disease of the brain

No Abstrac

Start-up of a UASB effluent treatment plant on distillery wastewater

Distillery effluent is a contaminated stream with COD values of up to 30 000 mg/l and low pH values of between 3 and 4. The anaerobic biological treatment of distillery effluents is widely applied as an effective step in removing more than 90% of the COD in the effluent stream. This paper reports on the seasonal operation of a UASB treatment plant treating a distillery wastewater stream with particular focus on seasonal start-up conditions after the first process commissioning. The start-up period was typically one week before process stability could be achieved. It is recommended that the loading rate to the plant be controlled between 4 and 8 kg COD/m3·d until the process is stable and COD removal efficiencies remain, on average, higher than 90%. After the start-up period the loading rate applied (4 to 18 kg COD/m3d) did not significantly affect the COD removal efficiency of the plant. High removal efficiencies of higher than 90% were achieved and stop-start operation of the UASB process posed no problem for treatment. WaterSA Vol.28(1) 2002: 63-6

GRMHD prediction of coronal variability in accreting black holes

On the basis of data from an energy-conserving 3D general relativistic MHD simulation, we predict the statistical character of variability in the coronal luminosity from accreting black holes. When the inner boundary of the corona is defined to be the electron scattering photosphere, its location depends only on the mass accretion rate in Eddington units (\dot{M}). Nearly independent of viewing angle and \dot{M}, the power spectrum over the range of frequencies from approximately the orbital frequency at the innermost stable circular orbit (ISCO) to ~100 times lower is well approximated by a power-law with index -2, crudely consistent with the observed power spectra of hard X-ray fluctuations in AGN and the hard states of Galactic binary black holes. The underlying physical driver for variability in the light curve is variations in the accretion rate caused by the chaotic character of MHD turbulence, but the power spectrum of the coronal light output is significantly steeper. Part of this contrast is due to the fact that the mass accretion rate can be significantly modulated by radial epicyclic motions that do not result in dissipation, and therefore do not drive luminosity fluctuations. The other part of this contrast is due to the inward decrease of the characteristic inflow time, which leads to decreasing radial coherence length with increasing fluctuation frequency.Comment: Accepted for publication in ApJ, 35 pages, 11 figures (8 color and 3 greyscale), AASTEX. High-resolution versions can be found at the following links: [PS] http://www.pha.jhu.edu/~scn/papers/grmhd_var.ps [PDF] http://www.pha.jhu.edu/~scn/papers/grmhd_var.pd

Prevalence and characteristics of erectile dysfunction in black and mixed race primary care populations of the Cape Flats and Helderberg Basin area of the Western Cape, South Africa

Objective: To estimate the prevalence of erectile dysfunction (ED) among users of primary care in a Black and Mixed Race urban population in the Western Cape, and to describe any associated health and psychosocial factors. Design: Cross-sectional survey by interviewer administered questionnaire. Setting: Two primary care medical centres, 40km apart, in Cape Town metropolitan area. Serve different ethnic groups, with no cross-contamination between them. Study period: March-June 1999 Patients: 833 Males (35-70 years old) attending these health centres for primary care. Systematic selection of all attendees. Main outcome measures: Prevalence of ED and presence of associated health and psychosocial factors. Describe patient demographics, physical attributes, sexual relationships. Results: Results of 730 males with current sexual partners: Mean ages 48 years (SD:7 years) all; 46 years (SD:9 years) Black group; 51 years (SD: 9 years) Mixed Race group. All degrees of ED prevalence: All 77.1% (95% CI: 74.0-80.2), Black 76.4% (95% CI: 71.8-80.4) and Mixed Race 77.7% (95% CI: 72.8-82.0). Significantly associated diseases: hypertension, diabetes, gastrointestinal and heart disease. Alcohol consumption (younger patients), smoking (older patients) significantly related to ED. Males with ED: more sexual partners than males without ED. More than 90% choose primary care physician/ generalist as primary ED care-giver. Conclusions: ED is very common in both study groups. Primary care workers must be prepared to manage associated risk factors and health implications. ED sufferers in this population may also be at higher risk for sexually transmitted diseases due to multiple sexual partners. (SA Fam Pract 2003;45(1):14-20) Keywords: erectile dysfunction, impotence, prevalence, black

First-year Accounting students’ confidence levels during Covid-19: Evidence over a period of two years at a South African university

The Coronavirus Disease (Covid-19) outbreak has caused serious and continuous concerns globally within the higher education sector. One of these is the confidence levels of students to perform specific academic-related tasks. This study explored first-year accounting students’ confidence levels during Covid-19 over a two-year period at a South African university to determine if any associations exist between students’ gender, age, choice of study, language, and method of study funding, compared to confidence level, whilst applying a quantitative research design. Mixed results were obtained. Areas for further research identified include collaboration with other universities, and to expand the scope of the population, to determine the status of students’ confidence levels for different accountancy-related modules during and post-Covid-19. The study will inform and sensitise lecturers, students and academic institutions regarding the confidence levels of students to perform specific academic-related tasks within accounting education and training under Covid-19 conditions

Rigidly Rotating Strings in Stationary Spacetimes

In this paper we study the motion of a rigidly rotating Nambu-Goto test string in a stationary axisymmetric background spacetime. As special examples we consider the rigid rotation of strings in flat spacetime, where explicit analytic solutions can be obtained, and in the Kerr spacetime where we find an interesting new family of test string solutions. We present a detailed classification of these solutions in the Kerr background.Comment: 19 pages, Latex, 9 figures, revised for publication in Classical and Quantum Gravit

Die devitaliseringsdialektiek van die skool: enkele fundamentele probleme

Historically the school has been subject to a de-/vitalizing power-play fulfilled through the manipulative power of holders of power under the primacy of a specific religious ground motif The result has been that the school is seemingly experiencing an identity crisis because of the incomplete definition of its task of educative teaching, because man/power-holders have historically been wont to regard himself/themselves as the justified determiner of meaning of his/their structures. Subsequently the school's foundational function, its destinational function and its sphere of competence have been defined by way of apostatic ground motifs and secular points of departure. Thus the school has been subjected to a never-ending, self-denying bipolar play of power which can be self-undermining. While no power-play can be driven to its fullest consequences, 1school reforms* have been self-abortive in nature. There is therefore the demand to provide the school with a stable value system which for the Christian is situated in the creation-fall-redemption religious ground motif which places man in a radical Creator-creature relationship