90,298 research outputs found
Deconvolution of ASCA X-ray data: I. Spectral-imaging method
In this paper we describe a self-contained method for performing the
spectral-imaging deconvolution of X-ray data on clusters of galaxies observed
by the ASCA satellite. Spatially-resolved spectral studies of data from this
satellite require such a correction because its optics redistribute photons
over regions which are of comparable size to the angular scales of interest in
clusters. This scattering is a function not only of spatial position but also
energy. To perform a correction for these effects we employ Maximum-Likelihood
deconvolution of the image (within energy bands of 1 keV) to determine the
spatial redistribution, followed by a Monte-Carlo energy reassignment of photon
energies with position to determine the spectral redistribution. We present
tests on simulated cluster data, convolved with the various instrumental
characteristics and the X-ray background, which show that our methodology can
successfully recover a variety of intrinsic temperature profiles in typical
observational circumstances. In Paper-II we apply our spectral-imaging
deconvolution procedure to a large sample of galaxy clusters to determine
temperature profiles, some of which will be used in subsequent mass
determinations, presented in Paper-III.Comment: MNRAS, accepted. Paper and single page postscript copies of each
test's radial profile are available on: http://www-xray.ast.cam.ac.uk/~daw
Understanding Mathematics and Science Advice Networks of Middle School Teachers
We report findings from a research project designed to examine the mathematics and science advice networks of teachers who participated in professional development under the auspices of the NSF-funded Rocky Mountain-Middle School Math and Science Partnership. We provide descriptive statistics of results. Additionally, we reflect on the research process and discuss some of the practical challenges involved
Resolving the virial discrepancy in clusters of galaxies with modified Newtonian dynamics
A sample of 197 X-ray emitting clusters of galaxies is considered in the
context of Milgrom's modified Newtonian dynamics (MOND). It is shown that the
gas mass, extrapolated via an assumed model to a fixed radius of 3 Mpc,
is correlated with the gas temperature as predicted by MOND (). The observed temperatures are generally consistent with the inferred
mass of hot gas; no substantial quantity of additional unseen matter is
required in the context of MOND. However, modified dynamics cannot resolve the
strong lensing discrepancy in those clusters where this phenomenon occurs. The
prediction is that additional baryonic matter may be detected in the central
regions of rich clusters.Comment: Submitted to A&A, 4 pages, 3 figures, A&A macro
The excess burden of cancer in men in the UK
In general men are at significantly greater risk than women from nearly all of the common cancers that occur in both sexes (with the exception of breast cancer) (White 2009, Wilkins 2006, DH 2007). This report will consider the current overall burden of cancer among men in the UK, estimated from the latest statistics, and outline the extent of the differences between the sexes. All figures and calculations reported here are based on data extracted from the Cancer Research UK CancerStats web pages extracted in June 2009 (Cancer Research UK, 2009)
Monte Carlo methods and applications for the nuclear shell model
The shell-model Monte Carlo (SMMC) technique transforms the traditional
nuclear shell-model problem into a path-integral over auxiliary fields. We
describe below the method and its applications to four physics issues:
calculations of sdpf- shell nuclei, a discussion of electron-capture rates in
pf-shell nuclei, exploration of pairing correlations in unstable nuclei, and
level densities in rare earth systems.Comment: Proceedings of the Nuclear Structure '98 conference, Gatlinburg, TN,
10-15 August 199
ROSAT PSPC observations of the outer regions of the Perseus cluster of galaxies
We present an analysis of four off-axis ROSAT PSPC observations of the
Perseus cluster of galaxies (Abell~426). We detect the surface brightness
profile to a radius of 80 arcmin ( Mpc) from the X-ray
peak. The profile is measured in various sectors and in three different energy
bands. Firstly, a colour analysis highlights a slight variation of over
the region, and cool components in the core and in the eastern sector. We apply
the -model to the profiles from different sectors and present a solution
to the, so-called, -problem. The residuals from an azimuthally-averaged
profile highlight extended emission both in the East and in the West, with
estimated luminosities of about 8 and 1 , respectively.
We fit several models to the surface brightness profile, including the one
obtained from the Navarro, Frenk and White (1995) potential. We obtain the best
fit with the gas distribution described by a power law in the inner, cooling
region and a -model for the extended emission. Through the best-fit
results and the constraints from the deprojection of the surface brightness
profiles, we define the radius where the overdensity inside the cluster is 200
times the critical value, , at Mpc. Within Mpc (), the total mass in the Perseus cluster is
and its gas fraction is about 30 per cent.Comment: 21 pages, 23 figures; accepted for publication in MNRAS; also
available at http://www-xray.ast.cam.ac.uk/~settori/paper.htm
Use of implicit graph for recommending relevant videos: a simulated evaluation
In this paper, we propose a model for exploiting community based usage information for video retrieval. Implicit usage information from a pool of past users could be a valuable source to address the difficulties caused due to the semantic gap problem. We propose a graph-based implicit feedback model in which all the usage information can be represented. A number of recommendation algorithms were suggested and experimented. A simulated user evaluation is conducted on the TREC VID collection and the results are presented. Analyzing the results we found some common characteristics on the best performing algorithms, which could indicate the best way of exploiting this type of usage information
Deconvolution of ASCA X-ray data: II. Radial temperature and metallicity profiles for 106 galaxy clusters
In Paper-I we presented a methodology to recover the spatial variations of
properties of the intracluster gas from ASCA X-ray satellite observations of
galaxy clusters. We verified the correctness of this procedure by applying it
to simulated cluster datasets which we had subjected to the various
contaminants common in ASCA data. In this paper we present the results which we
obtain when we apply this method to real galaxy cluster observations. We
determine broad-band temperature and cooling-flow mass-deposition rates for the
106 clusters in our sample, and obtain temperature, abundance and emissivity
profiles (i.e. at least two annular bins) for 98 of these clusters. We find
that 90 percent of these temperature profiles are consistent with isothermality
at the 3-sigma confidence level. This conflicts with the prevalence of
steeply-declining cluster temperature profiles found by Markevitch et al.
(1998) from a sample of 30 clusters. In Paper-III (in preparation) we utilise
our temperature and emissivity profiles to determine radial hydrostatic-mass
properties for a subsample of the clusters presented in this paper.Comment: MNRAS, accpeted. Postscript copy of paper and individual postscript
files for plots in Appendix B can be obtained from:
http://www-xray.ast.cam.ac.uk/~da
The classical double copy for Taub-NUT spacetime
The double copy is a much-studied relationship between gauge theory and
gravity amplitudes. Recently, this was generalised to an infinite family of
classical solutions to Einstein's equations, namely stationary Kerr-Schild
geometries. In this paper, we extend this to the Taub-NUT solution in gravity,
which has a double Kerr-Schild form. The single copy of this solution is a
dyon, whose electric and magnetic charges are related to the mass and NUT
charge in the gravity theory. Finally, we find hints that the classical double
copy extends to curved background geometries.Comment: 13 pages, no figures. Minor edits to match journal versio
Competition Between Stripes and Pairing in a t-t'-J Model
As the number of legs n of an n-leg, t-J ladder increases, density matrix
renormalization group calculations have shown that the doped state tends to be
characterized by a static array of domain walls and that pairing correlations
are suppressed. Here we present results for a t-t'-J model in which a diagonal,
single particle, next-near-neighbor hopping t' is introduced. We find that this
can suppress the formation of stripes and, for t' positive, enhance the
d_{x^2-y^2}-like pairing correlations. The effect of t' > 0 is to cause the
stripes to evaporate into pairs and for t' < 0 to evaporate into
quasi-particles. Results for n=4 and 6-leg ladders are discussed.Comment: Four pages, four encapsulated figure
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