4,928 research outputs found
Where is the Inner Edge of an Accretion Disk Around a Black Hole?
What is meant by the "inner edge" of an accretion disk around a black hole
depends on the property that defines the edge. We discuss four such definitions
using data from recent high-resolution numerical simulations. These are: the
"turbulence edge", where flux-freezing becomes more important than turbulence
in determining the magnetic field structure; the "stress edge", where plunging
matter loses dynamical contact with the outer accretion flow; the "reflection
edge", the smallest radius capable of producing significant X-ray reflection
features; and the "radiation edge", the innermost place from which significant
luminosity emerges. All these edges are dependent on the accretion rate and are
non-axisymmetric and time-variable. Although all are generally located in the
vicinity of the marginally stable orbit, significant displacements can occur,
and data interpretations placing the disk edge precisely at this point can be
misleading. If observations are to be used successfully as diagnostics of
accretion in strong gravity, the models used to interpret them must take
careful account of these distinctions.Comment: accepted by Ap.J., 26 p
Hydrodynamic Simulations of Oscillating Shock Waves in a Sub-Keplerian Accretion Flow Around Black Holes
We study the accretion processes on a black hole by numerical simulation. We
use a grid based finite difference code for this purpose. We scan the parameter
space spanned by the specific energy and the angular momentum and compare the
time-dependent solutions with those obtained from theoretical considerations.
We found several important results (a) The time dependent flow behaves close to
a constant height model flow in the pre-shock region and a flow with vertical
equilibrium in the post-shock region. (c) The infall time scale in the
post-shock region is several times higher than the free-fall time scale. (b)
There are two discontinuities in the flow, one being just outside of the inner
sonic point. Turbulence plays a major role in determining the locations of
these discontinuities. (d) The two discontinuities oscillate with two different
frequencies and behave as a coupled harmonic oscillator. A Fourier analysis of
the variation of the outer shock location indicates higher power at the lower
frequency and lower power at the higher frequency. The opposite is true when
the analysis of the inner shock is made. These behaviours will have
implications in the spectral and timing properties of black hole candidates.Comment: 19 pages, 13 figures, 1 Table MNRAS (In press
Seed kikuyu performs well
The kikuya variety, Whittet, which can be established from seed has performed well in a trial at Denmark
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
Electron Acceleration around the Supermassive Black Hole at the Galactic Center
The recent detection of variable infrared emission from Sagittarius A*,
combined with its previously observed flare activity in X-rays, provides
compelling evidence that at least a portion of this object's emission is
produced by nonthermal electrons. We show here that acceleration of electrons
by plasma wave turbulence in hot gases near the black hole's event horizon can
account both for Sagittarius A*'s mm and shorter wavelengths emission in the
quiescent state, and for the infrared and X-ray flares, induced either via an
enhancement of the mass accretion rate onto the black hole or by a
reorganization of the magnetic field coupled to the accretion gas. The
acceleration model proposed here produces distinct flare spectra that may be
compared with future coordinated multi-wavelength observations. We further
suggest that the diffusion of high energy electrons away from the acceleration
site toward larger radii might be able to account for the observed
characteristics of Sagittarius A*'s emission at cm and longer wavelengths.Comment: 13 pages, 2 figures and 1 table, submitted to ApJ
What's the point of knowing how?
Why is it useful to talk and think about knowledge-how? Using Edward Craig’s discussion of the function of the concepts of knowledge and knowledge-how as a jumping off point, this paper argues that considering this question can offer us new angles on the debate about knowledge-how. We consider two candidate functions for the concept of knowledge-how: pooling capacities, and mutual reliance. Craig makes the case for pooling capacities, which connects knowledge-how to our need to pool practical capacities. I argue that the evidence is much more equivocal. My suggested diagnosis is that the concept of knowledge-how plays both functions, meaning that the concept of knowledge-how is inconsistent, and that the debate about knowledge-how is at least partly a metalinguistic negotiation. In closing, I suggest a way to revise the philosophical concept of knowledge how
Nitrogen fertilisers for pasture production
At current prices nitrogen fertiliser is not an economic substitute for good clover pastures, but it can supplement the system by increasing nitrogen supply at peak growth periods. It can also provide useful increases in pasture production at critical feed times. Nitrogen usage for pasture production is still in the experimental stages and small areas should be tried before large-scale application
A feasibility study for advanced technology integration for general aviation
An investigation was conducted to identify candidate technologies and specific developments which offer greatest promise for improving safety, fuel efficiency, performance, and utility of general aviation airplanes. Interviews were conducted with general aviation airframe and systems manufacturers and NASA research centers. The following technologies were evaluated for use in airplane design tradeoff studies conducted during the study: avionics, aerodynamics, configurations, structures, flight controls, and propulsion. Based on industry interviews and design tradeoff studies, several recommendations were made for further high payoff research. The most attractive technologies for use by the general aviation industry appear to be advanced engines, composite materials, natural laminar flow airfoils, and advanced integrated avionics systems. The integration of these technologies in airplane design can yield significant increases in speeds, ranges, and payloads over present aircraft with 40 percent to 50 percent reductions in fuel used
Airbag system and method for facilitating emergency egress from an aircraft
An airbag system for elevating the fuselage of an aircraft off a landing surface a sufficient degree to allow for emergency egress of passengers and crew through ventral emergency exit doors. An airbag assembly made up of a plurality of independent airbags is disposed within the aircraft. When activated, the airbag system deploys the airbags external of the aircraft that elevate the fuselage of the aircraft a sufficient degree to allow for utilizing the ventral emergency exit doors on the fuselage to enable evacuating the passengers and crew. An activation mechanism is connected to the inflation.devices associated with each of the airbags. The activation mechanism generates an electrical signal which activates the inflation devices, which in turn fill the airbags with a compressed fluid, thus expanding the airbags and lifting the fuselage. A crew member initiates the activation of the airbag system through one or more switches
EVALUATING CHANGES IN TRAINING MANAGER’S ROLE PERCEPTIONS: A CASE STUDY OF TRHE NUCLEAR POWER INDUSTRY
Organizations are increasingly recognizing the importance of moving from training to performance improvement in Human Resource Development (HRD). Business organizations need to dramatically alter the roles of training managers to ensure that they move towards a performance improvement process, and to ensure that potential solutions are linked to organizational and individual performance problems. As part of a program for nuclear power managers at a major university, we evaluated the changes in the perceptions of twenty-one training managers from around the United States. The evaluation data lead us to conclusions regarding the changing role of training managers from strictly training to that of performance consultant and the challenges manger’s face as they try to build partnering relationships
- …