295 research outputs found
X-ray and Radio Variability of M31*, The Andromeda Galaxy Nuclear Supermassive Black Hole
We confirm our earlier tentative detection of M31* in X-rays and measure its
light-curve and spectrum. Observations in 2004-2005 find M31* rather quiescent
in the X-ray and radio. However, X-ray observations in 2006-2007 and radio
observations in 2002 show M31* to be highly variable at times. A separate
variable X-ray source is found near P1, the brighter of the two optical nuclei.
The apparent angular Bondi radius of M31* is the largest of any black hole, and
large enough to be well resolved with Chandra. The diffuse emission within this
Bondi radius is found to have an X-ray temperature ~0.3 keV and density 0.1
cm-3, indistinguishable from the hot gas in the surrounding regions of the
bulge given the statistics allowed by the current observations. The X-ray
source at the location of M31* is consistent with a point source and a power
law spectrum with energy slope 0.9+/-0.2. Our identification of this X-ray
source with M31* is based solely on positional coincidence.Comment: 25 pages, 8 figures, submitted to Ap
Chiral Symmetry Breaking in Quenched Massive Strong-Coupling QED
We present results from a study of subtractive renormalization of the fermion
propagator Dyson-Schwinger equation (DSE) in massive strong-coupling quenched
QED. Results are compared for three different fermion-photon proper vertex
{\it Ans\"{a}tze\/}: bare , minimal Ball-Chiu, and
Curtis-Pennington. The procedure is straightforward to implement and
numerically stable. This is the first study in which this technique is used and
it should prove useful in future DSE studies, whenever renormalization is
required in numerical work.Comment: REVTEX 3.0, 15 pages plus 7 uuencoded PostScript figure
Coulomb excitation of the mirror pair
Background: Electric-quadrupole () strengths relate to the underlying
quadrupole deformation of a nucleus and present a challenge for many nuclear
theories. Mirror nuclei in the vicinity of the line of represent a
convenient laboratory for testing deficiencies in such models, making use of
the isospin-symmetry of the systems. Purpose: Uncertainties associated with
literature strengths in \textsuperscript{23}Mg are some of the largest in
nuclei in the -shell. The purpose of the
present work is to improve the precision with which these values are known, to
enable better comparison with theoretical models. Methods: Coulomb-excitation
measurements of Mg and Na were performed at the TRIUMF-ISAC
facility using the TIGRESS spectrometer. They were used to determine the
matrix elements of mixed / transitions. Results: Reduced
transition strengths, , were extracted for \textsuperscript{23}Mg and
\textsuperscript{23}Na. Their precision was improved by factors of
approximately six for both isotopes, while agreeing within uncertainties with
previous measurements. Conclusions: A comparison was made with both shell-model
and {\it ab initio} valence-space in-medium similarity renormalization group
calculations. Valence-space in-medium similarity-renormalization-group
calculations were found to underpredict the absolute strength - in
agreement with previous studies
Invasion and Persistence of Infectious Agents in Fragmented Host Populations
One of the important questions in understanding infectious diseases and their prevention and control is how infectious agents can invade and become endemic in a host population. A ubiquitous feature of natural populations is that they are spatially fragmented, resulting in relatively homogeneous local populations inhabiting patches connected by the migration of hosts. Such fragmented population structures are studied extensively with metapopulation models. Being able to define and calculate an indicator for the success of invasion and persistence of an infectious agent is essential for obtaining general qualitative insights into infection dynamics, for the comparison of prevention and control scenarios, and for quantitative insights into specific systems. For homogeneous populations, the basic reproduction ratio plays this role. For metapopulations, defining such an ‘invasion indicator’ is not straightforward. Some indicators have been defined for specific situations, e.g., the household reproduction number . However, these existing indicators often fail to account for host demography and especially host migration. Here we show how to calculate a more broadly applicable indicator for the invasion and persistence of infectious agents in a host metapopulation of equally connected patches, for a wide range of possible epidemiological models. A strong feature of our method is that it explicitly accounts for host demography and host migration. Using a simple compartmental system as an example, we illustrate how can be calculated and expressed in terms of the key determinants of epidemiological dynamics
Dynamical chiral symmetry breaking and confinement with an infrared-vanishing gluon propagator?
We study a model Dyson-Schwinger equation for the quark propagator closed
using an {\it Ansatz} for the gluon propagator of the form \mbox{} and two {\it Ans\"{a}tze} for the quark-gluon vertex: the
minimal Ball-Chiu and the modified form suggested by Curtis and Pennington.
Using the quark condensate as an order parameter, we find that there is a
critical value of such that the model does not support dynamical chiral
symmetry breaking for . We discuss and apply a confinement test which
suggests that, for all values of , the quark propagator in the model {\bf is
not} confining. Together these results suggest that this Ansatz for the gluon
propagator is inadequate as a model since it does not yield the expected
behaviour of QCD.Comment: 21 Pages including 4 PostScript figures uuencoded at the end of the
file. Replacement: slight changes of wording and emphasis. ADP-93-215/T133,
ANL-PHY-7599-TH-93, FSU-SCRI-93-108, REVTEX 3.
Isospin symmetry in B(E2) values: Coulomb excitation study of Mg-21
The ~=~ nucleus Mg has been studied by Coulomb
excitation on Pt and Pd targets. A 205.6(1)-keV
-ray transition resulting from the Coulomb excitation of the
ground state to the first excited state in
Mg was observed for the first time. Coulomb excitation cross-section
measurements with both targets and a measurement of the half-life of the
state yield an adopted value of
~=~13.3(4)~W.u. A new excited
state at 1672(1)~keV with tentative assignment was also
identified in Mg. This work demonstrates large difference of the
values between
~=~, ~=~21 mirror nuclei. The difference is investigated in
the shell-model framework employing both isospin conserving and breaking USD
interactions and using modern \textsl{ab initio} nuclear structure
calculations, which have recently become applicable in the shell.Comment: 8 pages, 6 figures, submitted to Phys. Rev. C, Rapid Communicatio
Renormalization and Chiral Symmetry Breaking in Quenched QED in Arbitrary Covariant Gauge
We extend a previous Landau-gauge study of subtractive renormalization of the
fermion propagator Dyson-Schwinger equation (DSE) in strong-coupling, quenched
QED_4 to arbitrary covariant gauges. We use the fermion-photon proper vertex
proposed by Curtis and Pennington with an additional correction term included
to compensate for the small gauge-dependence induced by the ultraviolet
regulator. We discuss the chiral limit and the onset of dynamical chiral
symmetry breaking in the presence of nonperturbative renormalization. We
extract the critical coupling in several different gauges and find evidence of
a small residual gauge-dependence in this quantity.Comment: REVTEX 3.0, 27 pages including 14 Extended Postscript files
comprising 9 figures. Replacement: discussion of chiral limit corrected, and
some minor typographical errors fixed. To appear in Phys. Rev.
The ACTIVE cognitive training trial and predicted medical expenditures
<p>Abstract</p> <p>Background</p> <p>Health care expenditures for older adults are disproportionately high and increasing at both the individual and population levels. We evaluated the effects of the three cognitive training interventions (memory, reasoning, or speed of processing) in the ACTIVE study on changes in predicted medical care expenditures.</p> <p>Methods</p> <p>ACTIVE was a multisite randomized controlled trial of older adults (≥ 65). Five-year follow-up data were available for 1,804 of the 2,802 participants. Propensity score weighting was used to adjust for potential attrition bias. Changes in predicted annual<b/>medical expenditures were calculated at the first and fifth annual follow-up assessments using a new method for translating functional status scores. Multiple linear regression methods were used in this cost-offset analysis.</p> <p>Results</p> <p>At one and five years post-training, annual predicted expenditures declined<b/>by 128 (p = .309), respectively, in the speed of processing treatment group, but there were no statistically significant changes in the memory or reasoning treatment groups compared to the no-contact control group at either period. Statistical adjustment for age, race, education, MMSE scores, ADL and IADL performance scores, EPT scores, chronic condition counts, and the SF-36 PCS and MCS scores at baseline did not alter the one-year (143; p = .250) expenditure declines in the speed of processing treatment group.</p> <p>Conclusion</p> <p>The speed of processing intervention significantly reduced subsequent annual predicted medical care expenditures at the one-year post-baseline comparison, but annual savings were no longer statistically significant at the five-year post-baseline comparison.</p
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