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

The Effect of Transaction Size on Off-the-Run Treasury Prices

A price pressure effect is implied by segmentation in the market for a security. An empirical property of a segmented market is that the price of the security is sensitive to supply and demand conditions for that specific security, absent changes in risk and absent any new information. This paper examines intra-day trading data from the inter-dealer broker market for U.S. Treasury securities and finds that there is a price pressure effect in the off-the-run Treasury market. Thus, securities that would appear to be very close substitutes, i.e., on-the-run and off-the-run Treasury bonds, behave as if there is some degree of market segmentation. There have been several studies of price pressure in the equity market and Treasury bill market but this is the first study of the off-the-run Treasury note and bond market to investigate a price pressure effect using intra-day data. It is also the first study to analyze price pressure through matched pairs of securities that differ only in liquidity and with high frequency data.

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

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

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

A Numerical Method for General Relativistic Magnetohydrodynamics

This paper describes the development and testing of a general relativistic magnetohydrodynamic (GRMHD) code to study ideal MHD in the fixed background of a Kerr black hole. The code is a direct extension of the hydrodynamic code of Hawley, Smarr, and Wilson, and uses Evans and Hawley constrained transport (CT) to evolve the magnetic fields. Two categories of test cases were undertaken. A one dimensional version of the code (Minkowski metric) was used to verify code performance in the special relativistic limit. The tests include Alfv\'en wave propagation, fast and slow magnetosonic shocks, rarefaction waves, and both relativistic and non-relativistic shock tubes. A series of one- and two-dimensional tests were also carried out in the Kerr metric: magnetized Bondi inflow, a magnetized inflow test due to Gammie, and two-dimensional magnetized constant-$l$ tori that are subject to the magnetorotational instability.Comment: 37 pages, 14 figures, submitted to ApJ. Animations can be viewed at http://www.astro.virginia.edu/~jd5v/grmhd/grmhd.htm

Global General Relativistic Magnetohydrodynamic Simulations of Accretion Tori

This paper presents an initial survey of the properties of accretion flows in the Kerr metric from three-dimensional, general relativistic magnetohydrodynamic simulations of accretion tori. We consider three fiducial models of tori around rotating, both prograde and retrograde, and nonrotating black holes; these three fiducial models are also contrasted with axisymmetric simulations and a pseudo-Newtonian simulation with equivalent initial conditions to delineate the limitations of these approximations.Comment: Submitted to ApJ. 30 pages, 21 figures. Animations and high-resolution version of figures available at http://www.astro.virginia.edu/~jd5

The use of the road to health card in monitoring child health

Objectives: The Road to Health Chart (RTHC) provides a simple, cheap, practical and convenient method of monitoring child health. The RTHC could assist with vaccine compliance and early identification of growth faltering, to improve general health. The purpose of the study was to assess whether the RTHCs were adequately completed and interpreted at primary, secondary and tertiary care levels in South Africa. Methods: The study was carried out at a primary, secondary and tertiary care centre. A questionnaire was administered to 100 subjects from each; seeking demographic information, whether the RTHC was brought along, and if not why it was not brought. Results: Most children were brought by their mothers. The RTHC was not brought to 48% of the consultations; of those, about 72% thought that bringing the RTHC along was not necessary. Health workers seldom asked to see the RTHC at the primary and secondary care settings, but 50% of them did so at Ga-Rankuwa Hospital (p = 0,002). In only 8 cases overall were the children below the 3rd percentile of weight-for-age. Approximately 20% had incomplete immunisations. Conclusions: Many parents believe that the RTHC is only required for Well-baby-clinic visits, not for consultations. The RTHC is not often asked for at consultations; the fact that this is more often done at the tertiary care centre may be that much of the service is supplied by paediatricians-in-training. Health workers should ask to see the RTHC, in order for mothers to understand the importance of the information. The study showed that the RTHC is not used to its full potential. South African Family Practice Vol. 49 (1) 2007: pp. 1