8,158 research outputs found
X-ray Supercavities in the Hydra A Cluster and the Outburst History of the Central Galaxy's Active Nucleus
A 227 ksec Chandra Observatory X-ray image of the hot plasma in the Hydra A
cluster has revealed an extensive cavity system. The system was created by a
continuous outflow or a series of bursts from the nucleus of the central galaxy
over the past 200-500 Myr. The cavities have displaced 10% of the plasma within
a 300 kpc radius of the central galaxy, creating a swiss-cheese-like topology
in the hot gas. The surface brightness decrements are consistent with empty
cavities oriented within 40 degrees of the plane of the sky. The outflow has
deposited upward of 10^61 erg into the cluster gas, most of which was propelled
beyond the inner ~100 kpc cooling region. The supermassive black hole has
accreted at a rate of approximately 0.1-0.25 solar masses per year over this
time frame, which is a small fraction of the Eddington rate of a ~10^9 solar
mass black hole, but is dramatically larger than the Bondi rate. Given the
previous evidence for a circumnuclear disk of cold gas in Hydra A, these
results are consistent with the AGN being powered primarily by infalling cold
gas. The cavity system is shadowed perfectly by 330 MHz radio emission. Such
low frequency synchrotron emission may be an excellent proxy for X-ray cavities
and thus the total energy liberated by the supermassive black hole.Comment: 8 pages, 3 figures; Submitted to ApJ, revised per referee's
suggestion
Crew Resource Management and Its Possible Role in Nursing Risk Management
Crew Resource Management (CRM) was introduced within the aviation industry in the late 1970s after an aircraft ran out of fuel whilst the pilots were trying to solve an undercarriage problem. To reduce such errors and ultimately lower the probability of failure and the severity of risks that occur, training in CRM was rolled out across the whole industry. It has been successful over the last few years in major reductions in the number of crashes and fatalities in the commercial aviation sector. Nursing has similar concerns in that errors can ultimately result in fatalities. There are parallels in the needs and expectations of pilots and nurses to assess risk, reduce risk and deliver reliable and dependable professional services. In this paper the parallels of pilots and nurses demands are compared to assess if the lessons learned in aviation can assist nurses deliver procedures with lower risks. The analysis will draw on the demands and expectations and how they both deal with risk, challenging errors and ensuring that identified risks are not overlooked or ignored. Finally, suggestions of adopting, sharing and benchmarking between these two industries can adopt best practices so that both industries can learn from each other
The Detectability of AGN Cavities in Cooling-Flow Clusters
Chandra X-ray Observatory has revealed X-ray cavities in many nearby cooling
flow clusters. The cavities trace feedback from the central active galactic
nulceus (AGN) on the intracluster medium (ICM), an important ingredient in
stabilizing cooling flows and in the process of galaxy formation and evolution.
But, the prevalence and duty cycle of such AGN outbursts is not well
understood. To this end, we study how the cooling is balanced by the cavity
heating for a complete sample of clusters (the Brightest 55 clusters of
galaxies, hereafter B55). In the B55, we found 33 cooling flow clusters, 20 of
which have detected X-ray bubbles in their ICM. Among the remaining 13, all
except Ophiuchus could have significant cavity power yet remain undetected in
existing images. This implies that the duty cycle of AGN outbursts with
significant heating potential in cooling flow clusters is at least 60 % and
could approach 100 %, but deeper data is required to constrain this further.Comment: 4 pages, 2 figures; to appear in the proceedings of "The Monsters'
Fiery Breath", Madison, Wisconsin 1-5 June 2009, Eds. Sebastian Heinz & Eric
Wilcots; added annotation to the figur
Fully-Coupled Simulation of Cosmic Reionization. I: Numerical Methods and Tests
We describe an extension of the Enzo code to enable fully-coupled radiation
hydrodynamical simulation of inhomogeneous reionization in large cosmological volumes with thousands to millions of point sources. We
solve all dynamical, radiative transfer, thermal, and ionization processes
self-consistently on the same mesh, as opposed to a postprocessing approach
which coarse-grains the radiative transfer. We do, however, employ a simple
subgrid model for star formation which we calibrate to observations. Radiation
transport is done in the grey flux-limited diffusion (FLD) approximation, which
is solved by implicit time integration split off from the gas energy and
ionization equations, which are solved separately. This results in a faster and
more robust scheme for cosmological applications compared to the earlier
method. The FLD equation is solved using the hypre optimally scalable geometric
multigrid solver from LLNL. By treating the ionizing radiation as a grid field
as opposed to rays, our method is scalable with respect to the number of
ionizing sources, limited only by the parallel scaling properties of the
radiation solver. We test the speed and accuracy of our approach on a number of
standard verification and validation tests. We show by direct comparison with
Enzo's adaptive ray tracing method Moray that the well-known inability of FLD
to cast a shadow behind opaque clouds has a minor effect on the evolution of
ionized volume and mass fractions in a reionization simulation validation test.
We illustrate an application of our method to the problem of inhomogeneous
reionization in a 80 Mpc comoving box resolved with Eulerian grid
cells and dark matter particles.Comment: 32 pages, 23 figures. ApJ Supp accepted. New title and substantial
revisions re. v
The powerful outburst in Hercules A
The radio source Hercules A resides at the center of a cooling flow cluster
of galaxies at redshift z = 0.154. A Chandra X-ray image reveals a shock front
in the intracluster medium (ICM) surrounding the radio source, about 160 kpc
from the active galactic nucleus (AGN) that hosts it. The shock has a Mach
number of 1.65, making it the strongest of the cluster-scale shocks driven by
an AGN outburst found so far. The age of the outburst ~5.9e7 y, its energy
about 3e61 erg and its mean power ~1.6e46 erg/s. As for the other large AGN
outbursts in cooling flow clusters, this outburst overwhelms radiative losses
from the ICM of the Hercules A cluster by a factor of ~100. It adds to the case
that AGN outbursts are a significant source of preheating for the ICM. Unless
the mechanical efficiency of the AGN in Hercules A exceeds 10%, the central
black hole must have grown by more than 1.7e8 Msun to power this one outburst.Comment: 4 pages, 5 figures, accepted by ApJ
Microwave curing of carbon-epoxy composites: Penetration depth and material characterisation
Microwave heating has several major advantages over conventional conductive heating when used to cure carbonâepoxy composites, especially in speed of processing. Despite this and many other well-known advantages, microwave heating of carbonâepoxy composites has not taken off in industry, or even academia, due to the problems associated with microwave energy distribution, arcing, tool design and (ultimately) part quality and consistency, thus leading to a large scepticism regarding the technique/technology for heating such type of materials. This paper presents some evidence which suggests that with the correct hardware and operating procedure/methodology, consistent and high quality carbonâepoxy laminates can be produced, with the possibility of scaling up the process, as demonstrated by the micro- and macro-scale mechanical test results. Additionally, the author proposes a methodology to practically measure the maximum microwave penetration depth of a carbonâepoxy composite material
Jet Interactions with the Hot Halos of Clusters and Galaxies
X-ray observations of cavities and shock fronts produced by jets streaming
through hot halos have significantly advanced our understanding of the
energetics and dynamics of extragalactic radio sources. Radio sources at the
centers of clusters have dynamical ages between ten and several hundred million
years. They liberate between 1E58-1E62 erg per outburst, which is enough energy
to regulate cooling of hot halos from galaxies to the richest clusters. Jet
power scales approximately with the radio synchrotron luminosity to the one
half power. However, the synchrotron efficiency varies widely from nearly unity
to one part in 10,000, such that relatively feeble radio source can have
quasar-like mechanical power. The synchrotron ages of cluster radio sources are
decoupled from their dynamical ages, which tend to be factors of several to
orders of magnitude older. Magnetic fields and particles in the lobes tend to
be out of equipartition. The lobes may be maintained by heavy particles (e.g.,
protons), low energy electrons, a hot, diffuse thermal gas, or possibly
magnetic (Poynting) stresses. Sensitive X-ray images of shock fronts and
cavities can be used to study the dynamics of extragalactic radio sources.Comment: 10 pages, 3 figures, invited review, "Extragalactic Jets: Theory and
Observation from Radio to Gamma Ray, held in Girdwood, Alaska, U.S.A. 21-24
May, 2007, minor text changes; one added referenc
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