353 research outputs found
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- 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
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
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
Where is the Radiation Edge in Magnetized Black Hole Accretion discs?
General Relativistic (GR) Magnetohydrodynamic (MHD) simulations of black hole
accretion find significant magnetic stresses near and inside the innermost
stable circular orbit (ISCO), suggesting that such flows could radiate in a
manner noticeably different from the prediction of the standard model, which
assumes that there are no stresses in that region. We provide estimates of how
phenomenologically interesting parameters like the ``radiation edge", the
innermost ring of the disc from which substantial thermal radiation escapes to
infinity, may be altered by stresses near the ISCO. These estimates are based
on data from a large number of three-dimensional GRMHD simulations combined
with GR ray-tracing. For slowly spinning black holes (), the radiation
edge lies well inside where the standard model predicts, particularly when the
system is viewed at high inclination. For more rapidly spinning black holes,
the contrast is smaller. At fixed total luminosity, the characteristic
temperature of the accretion flow increases between a factor of over
that predicted by the standard model, whilst at fixed mass accretion rate,
there is a corresponding enhancement of the accretion luminosity which may be
anywhere from tens of percent to order unity. When all these considerations are
combined, we find that, for fixed black hole mass, luminosity, and inclination
angle, our uncertainty in the characteristic temperature of the radiation
reaching distant observers due to uncertainty in dissipation profile (around a
factor of 3) is {\it greater} than the uncertainty due to a complete lack of
knowledge of the black hole's spin (around a factor of 2) and furthermore that
spin estimates based on the stress-free inner boundary condition provide an
upper limit to .Comment: 20 pages, 17 figures, accepted by MNRAS; major changes to original,
including entirely new sections discussing characteristic temperature of
black hole accretion flows and implications for measurements of black hole
spin, along with substantially expanded conclusio
The study of a novel flat-topping resonator for more intense proton beams of better quality from cyclotrons
ABSTRACT
The multi-disciplinary accelerator based facilities at iThemba LABS are used intensively for
nuclear physics experiments, radiotherapy and the production of radioisotopes. To increase the
beam intensity for radioisotope production and to improve the beam quality of the 66 MeV
proton beam, a double-gap horizontal half-wave flat-topping resonator has been developed for
the separated-sector cyclotron to operate at the associated fixed frequency. This type of flattopping
resonator has never before been implemented in a cyclotron and this study is the first
to show that it can be done, featuring a special characteristic not offered by other types. The
resonator is reviewed against the other types of resonators that are already in use at other
institutes around the world.
The flat-topping voltage of the selected type has a sinusoidal half-wave distribution along a
radial line in each of its acceleration gaps with the nodal points located on the injection and
extraction orbits. The flat-topping voltage therefore progressively increases from zero at both
of the two most critical orbits in a cyclotron to a maximum at about halfway between them. As
a result will this resonator, apart from its basic function to reduce the energy spread in the
beam, not decrease the orbit separation at the injection and extraction orbits in the cyclotron,
as is the case with other types of flat-topping resonators. This advantageous feature implies
that the beam pattern in the cyclotron is not affected in the regions of the delicate injection and
extraction components and therefore will these components or the operational control of the
beam not require any modification to accommodate the resonator.
In order to design a resonator that will meet our requirements, the theory of the beam
dynamics and resonator characteristics were studied to ascertain the expected improvement in
beam quality and beam intensity when a flat-topping resonator is implemented with the
cyclotron. All resonator types were considered and studied in terms of their power dissipation,
voltage distribution, harmonic number, space requirements and influence on the beam. The
horizontal half-wave resonator type, with two acceleration gaps, was selected as the most
suitable for our application, because of its preferred electromagnetic characteristics and its
geometric shape that permits the installation inside an existing vacuum chamber through an
existing flange. Initially a half-scale resonator model was build to test the feasibility of such a
resonator and also to verify the calculation methods.
Transmission line methods and numerical field analysis in 3D were applied to determine the
resonator characteristics. In the former method a computer program, POISSON, was used to
calculate curvilinear squares on sections through a triangular-shaped transmission line and in
the latter method a commercial computer program, SOPRANO, was used. SOPRANO is part
of an internationally acclaimed suite of programs and the acquired knowledge and skill to use
this state-of-the-art software for the studying and designing of such and other electromagnetic
devices also put the institute amongst the front-runners in the world.
The calculated characteristics of the half- and full-scale resonator models, the study of the
different electromagnetic modes that resonate in close proximity to the required frequency, the
heat transport modelling and the theory and implementation of the coupling and tuning
devices are all in good agreement with their respective measured results and are reported in
this document.
This study lead the way to have the first-ever double-gap horizontal half-wave flat-topping
resonator in a cyclotron successfully commissioned at iThemba LABS and the first tests with
beam report very stable operation. Accelerator physicists now have another option to utilize for the establishing of flat-topped acceleration voltages
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
Dependence of inner accretion disk stress on parameters: the Schwarzschild case
We explore the parameter dependence of inner disk stress in black hole
accretion by contrasting the results of a number of simulations, all employing
3-d general relativistic MHD in a Schwarzschild spacetime. Five of these
simulations were performed with the intrinsically conservative code HARM3D,
which allows careful regulation of the disk aspect ratio, H/R; our simulations
span a range in H/R from 0.06 to 0.17. We contrast these simulations with two
previously reported simulations in a Schwarzschild spacetime in order to
investigate possible dependence of the inner disk stress on magnetic topology.
In all cases, much care was devoted to technical issues: ensuring adequate
resolution and azimuthal extent, and averaging only over those time-periods
when the accretion flow is in approximate inflow equilibrium. We find that the
time-averaged radial-dependence of fluid-frame electromagnetic stress is almost
completely independent of both disk thickness and poloidal magnetic topology.
It rises smoothly inward at all radii (exhibiting no feature associated with
the ISCO) until just outside the event horizon, where the stress plummets to
zero. Reynolds stress can also be significant near the ISCO and in the plunging
region; the magnitude of this stress, however, depends on both disk thickness
and magnetic topology. The two stresses combine to make the net angular
momentum accreted per unit rest-mass 7-15% less than the angular momentum of
the ISCO.Comment: Accepted for publication in ApJ, 52 pages, 38 figures, AASTEX.
High-resolution versions can be found at the following links:
http://ccrg.rit.edu/~scn/papers/schwarzstress.ps,
http://ccrg.rit.edu/~scn/papers/schwarzstress.pd
The effects of the antenna power pattern uncertainty within a global 21 cm experiment
Experimental 21 cm cosmology aims to detect the formation of the first stars
during the cosmic dawn and the subsequent epoch of reionization by utilizing
the 21 cm hydrogen line transition. While several experiments have published
results that begin to constrain the shape of this signal, a definitive
detection has yet to be achieved. In this paper, we investigate the influence
of uncertain antenna-sky interactions on the possibility of detecting the
signal. This paper aims to define the level of accuracy to which a simulated
antenna beam pattern is required to agree with the actual observing beam
pattern of the antenna to allow for a confident detection of the global 21 cm
signal. By utilising singular value decomposition, we construct a set of
antenna power patterns that incorporate minor, physically motivated variations.
We take the absolute mean averaged difference between the original beam and the
perturbed beam averaged over frequency () to quantifying this
difference, identifying the correlation between and antenna
temperature. To analyse the impact of on making a confident
detection, we utilize the REACH Bayesian analysis pipeline and compare the
Bayesian evidence and root-mean-square error for antenna
beams of different values. Our calculations suggest that achieving
an agreement between the original and perturbed antenna power pattern with
better than -35 dB is necessary for confident detection of the
global 21 cm signal. Furthermore, we discuss potential methods to achieve the
required high level of accuracy within a global 21~cm experiment
Charge-starved, relativistic jets and blazar variability
High energy emission from blazars is thought to arise in a relativistic jet
launched by a supermassive black hole. The emission site must be far from the
hole and the jet relativistic, in order to avoid absorption of the photons. In
extreme cases, rapid variability of the emission suggests that structures of
length-scale smaller than the gravitational radius of the central black hole
are imprinted on the jet as it is launched, and modulate the radiation released
after it has been accelerated to high Lorentz factor. We propose a mechanism
which can account for the acceleration of the jet, and for the rapid
variability of the radiation, based on the propagation characteristics of
large-amplitude waves in charge-starved, polar jets. Using a two-fluid
(electron-positron) description, we find the outflows exhibit a delayed
acceleration phase, that starts at roughly 1pc, where the inertia associated
with the wave currents becomes important. The time-structure imprinted on the
jet at launch modulates photons produced by the accelerating jet provided the
pair multiplicity in the black-hole magnetosphere is sufficiently small,
suggesting that very rapid variability is confined to sources in which the
electromagnetic cascade in the black-hole magnetosphere is not prolific.Comment: 8 pages, 1 figure. Momentum equation corrected. Conclusions
unchanged. Erratum submitted to Ap
- ā¦