7,218 research outputs found
Criteria for the experimental observation of multi-dimensional optical solitons in saturable media
Criteria for experimental observation of multi-dimensional optical solitons
in media with saturable refractive nonlinearities are developed. The criteria
are applied to actual material parameters (characterizing the cubic
self-focusing and quintic self-defocusing nonlinearities, two-photon loss, and
optical-damage threshold) for various glasses. This way, we identify operation
windows for soliton formation in these glasses. It is found that two-photon
absorption sets stringent limits on the windows. We conclude that, while a
well-defined window of parameters exists for two-dimensional solitons (spatial
or spatiotemporal), for their three-dimensional spatiotemporal counterparts
such a window \emph{does not} exist, due to the nonlinear loss in glasses.Comment: 8 pages, to appear in Phys. Rev.
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
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
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
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
Chandra Observation of the Cluster Environment of a WAT Radio Source in Abell 1446
Wide-angle tail (WAT) radio sources are often found in the centers of galaxy
clusters where intracluster medium (ICM) ram pressure may bend the lobes into
their characteristic C-shape. We examine the low redshift (z=0.1035) cluster
Abell 1446, host to the WAT radio source 1159+583. The cluster exhibits
possible evidence for a small-scale cluster-subcluster merger as a cause of the
WAT radio source morphology. This evidence includes the presence of temperature
and pressure substructure along the line that bisects the WAT as well as a
possible wake of stripped interstellar material or a disrupted cool core to the
southeast of the host galaxy. A filament to the north may represent cool,
infalling gas that's contributing to the WAT bending while spectroscopically
determined redshifts of member galaxies may indicate some component of a merger
occurring along the line-of-sight. The WAT model of high flow velocity and low
lobe density is examined as another scenario for the bending of 1159+583. It
has been argued that such a model would allow the ram pressure due to the
galaxy's slow motion through the ICM to shape the WAT source. A temperature
profile shows that the cluster is isothermal (kT= 4.0 keV) in a series of
annuli reaching a radius of 400 kpc. There is no evidence of an ongoing cooling
flow. Temperature, abundance, pressure, density, and mass profiles, as well as
two-dimensional maps of temperature and pressure are presented.Comment: 40 AASTeX pages including 15 postscript figures; accepted for
publication in Ap
Individual score validity and student effort in higher education assessment
This study explored the use of the five invalidity flags plus a new sixth flag based on self-reported effort. Participants were 155 entering first-year university students who were measured during an orientation week and again 18 months later. The instruments were a faculty-developed test of oral communications skills with 40 four-option multiple-choice items and a self-reported measure of test-taking motivation (Student Opinion Survey; Sundre, 1999 adapted from Wolf and Smith, 1995). Results indicated that the Flags explored in this study generalized well to university students. There was a moderate correlation between Response Time Effort and Effort as measured by the Student Opinion Scale, suggesting there was a relationship not captured by the dichotomized flags
An Energetic AGN Outburst Powered by a Rapidly Spinning Supermassive Black Hole or an Accreting Ultramassive Black Hole
Powering the 10^62 erg nuclear outburst in the MS0735.6+7421 cluster central
galaxy by accretion implies that its supermassive black hole (SMBH) grew by
~6x10^8 solar masses over the past 100 Myr. We place upper limits on the amount
of cold gas and star formation near the nucleus of <10^9 solar masses and <2
solar masses per year, respectively. These limits imply that an implausibly
large fraction of the preexisting cold gas in the bulge must have been consumed
by its SMBH at the rate of ~3-5 solar masses per year while leaving no trace of
star formation. Such a high accretion rate would be difficult to maintain by
stellar accretion or the Bondi mechanism, unless the black hole mass approaches
10^11 solar masses. Its feeble nuclear luminosities in the UV, I, and X-ray
bands compared to its enormous mechanical power are inconsistent with rapid
accretion onto a ~5x10^9 solar mass black hole. We suggest instead that the AGN
outburst is powered by a rapidly-spinning black hole. A maximally-spinning,
10^9 solar mass black hole contains enough rotational energy, ~10^62 erg, to
quench a cooling flow over its lifetime and to contribute significantly to the
excess entropy found in the hot atmospheres of groups and clusters. Two modes
of AGN feedback may be quenching star formation in elliptical galaxies centered
in cooling halos at late times. An accretion mode that operates in gas-rich
systems, and a spin mode operating at modest accretion rates. The spin
conjecture may be avoided in MS0735 by appealing to Bondi accretion onto a
central black hole whose mass greatly exceeds 10^10 solar mass. The host
galaxy's unusually large, 3.8 kpc stellar core radius (light deficit) may
witness the presence of an ultramassive black hole.Comment: Accepted for publication in ApJ. Modifications: adopted slightly
higher black hole mass using Lauer's M_SMBH vs L_bulge relation and adjusted
related quantities; considered more seriously the consequences of a
ultramassive black hole, motivated by new Kormendy & Bender paper published
after our submission; other modifications per referee comments by Ruszkowsk
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