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

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

Thick domain walls around a black hole

We discuss the gravitationally interacting system of a thick domain wall and a black hole. We numerically solve the scalar field equation in the Schwarzschild spacetime and obtain a sequence of static axi-symmetric solutions representing thick domain walls. We find that, for the walls near the horizon, the Nambu--Goto approximation is no longer valid.Comment: 18 pages, 11 figures, one reference adde

A tale of two sit-bones: The cyclist’s ischial hygroma (Perineal nodular induration)

The ischial hygroma, also known as a perineal nodular induration, is a relatively rare and mostly cycling-specific injury that is often incorrectly diagnosed and managed. Here two cases with divergent managements are described to highlight the spectrum of treatment available to manage this condition. The presentation, assessment and management of two cases of perineal nodular induration are discussed.The management options, namely surgical excision vs conservative management, with saddle pressure mapping highlight that there is no single optimal method and that a multidisciplinary approach should be applied to treat these injuries successfully. Perineal nodular induration should be investigated appropriately to exclude less benign causes of perineal masses. Conservative management and surgical excision can both be successful. Clinicians should be familiar with the assessment and management of this relatively rare but debilitating condition in competitive cyclists.Keywords: cycling, perineal injur

Magnetically Driven Accretion in the Kerr Metric III: Unbound Outflows

We have carried out fully relativistic numerical simulations of accretion disks in the Kerr metric. In this paper we focus on the unbound outflows that emerge self-consistently from the accretion flow. These outflows are found in the axial funnel region and consist of two components: a hot, fast, tenuous outflow in the axial funnel proper, and a colder, slower, denser jet along the funnel wall. Although a rotating black hole is not required to produce these unbound outflows, their strength is enhanced by black hole spin. The funnel-wall jet is excluded from the axial funnel due to elevated angular momentum, and is also pressure-confined by a magnetized corona. The tenuous funnel outflow accounts for a significant fraction of the energy transported to large distances in the higher-spin simulations. We compare the outflows observed in our simulations with those seen in other simulations.Comment: 33 pages, 8 figures, ApJ submitte

Thick Domain Walls and Charged Dilaton Black Holes

We study a black hole domain wall system in dilaton gravity which is the low-energy limit of the superstring theory. We solve numerically equations of motion for real self-interacting scalar field and justify the existence of static axisymmetric field configuration representing the thick domain wall in the background of a charged dilaton black hole. It was also confirmed that the extreme dilaton black hole always expelled the domain wall.Comment: 10 pages, 8 figures; to be published in Phys. Rev. D1

Magnetically Driven Accretion Flows in the Kerr Metric I: Models and Overall Structure

This is the first in a series of papers that investigate the properties of accretion flows in the Kerr metric through three-dimensional, general relativistic magnetohydrodynamic simulations of tori with a near-Keplerian initial angular velocity profile. We study four models with increasing black hole spin, from a/M=0 to 0.998, for which the structural parameters of the initial tori are maintained nearly constant. The subsequent accretion flows arise self-consistently from stresses and turbulence created by the magnetorotational instability. We investigate the overall evolution and the late-time global structure in the resulting non-radiative accretion flows, including the magnetic fields within the disks, the properties of the flow in the plunging region, and the flux of conserved quantities into the black hole. Independent of black hole spin, the global structure is described in terms of five regions: the main disk body, the coronal envelope, the inner disk, consisting of an inner torus and plunging region, an evacuated axial funnel, and a bi-conical outflow confined to the corona-funnel boundary. We find evidence for lower accretion rates, stronger funnel-wall outflows, and increased stress in the near hole region with increasing black hole spin.Comment: 29 pages, 15 figures, version of paper with high-resolution figures and links to animations available at http://www.astro.virginia.edu/~jd5v/KD_movies.ht

Increase in childhood asthma admissions in an urbanising population

Objective. In South Africa, rapid urbanisation has increased the risk of childhood asthma. This report reviews the pattern of asthma admissions to the Paediatric Department of Ga-Rankuwa Hospital, South Africa, from 1986 to 1996. Design. Inpatient admission data were reviewed for 1986- 1996. A detailed analysis of the records of asthma patients admitted between 1992 and 1996 was done. Outpatient data were reviewed from 1992.Setting. Ga-Rankuwa Hospital, situated on the border of the Gauteng and orth West provinces of South Africa and serving a large black population in various stages of urbanisation.Main outcome measures. Trends in admission numbers and demographic characteristics.Results. Asthma admissions were 2.5 times higher in 1996 than 1986. The greatest increase in admissions was in the 1 - 47-month age group. The male to female ratio was 1.5:1. More patients came from urban than from rural areas. Admissions peaked during the summer. Re-admissions occurred most frequently within 3 months of the first admission.Conclusion. Paediatric asthma admissions have shown an increase in the past decade. This may be associated with changes in the environment of the community. There is a need for preventive programmes for asthma at community and national level