12,649 research outputs found
The Lx-T Relation and Intracluster Gas Fractions of X-ray Clusters
We re-examine the X-ray luminosity-temperature relation using a nearly
homogeneous data set of 24 clusters selected for statistically accurate
temperature measurements and absence of strong cooling flows. The data exhibit
a remarkably tight power-law relation between bolometric luminosity and
temperature with a slope 2.88 \pm 0.15. With reasonable assumptions regarding
cluster structure, we infer an upper limit on fractional variations in the
intracluster gas fraction <(\delta\fgas/\fgas)^2)^1/2 \le 15%. Imaging data
from the literature are employed to determine absolute values of fgas within
spheres encompassing density contrast 500 and 200 with respect to the critical
density. Comparing binding mass estimates based on the virial theorem (VT) and
the hydrostatic, betamodel (BM), we find a temperature-dependent discrepancy in
fgas between the two methods caused by sytematic variation of the outer slope
parameter beta with temperature. There is evidence that cool clusters have a
lower mean gas fraction that hot clusters, but it is not possible to assess the
statistical significance of this effect in the present dataset. The temperature
dependance of the ICM density structure, coupled with the increase of the gas
fraction with T in the VT aproach, explains the steepening of the Lx-T
relation. The small variation about the mean gas fraction within this majority
sub-population of clusters presents an important constraint for theories of
galaxy formation and supports arguments against an Einstein-deSitter universe
based on the population mean gas fraction and primordial nucleosynthesis. The
apparent trend of lower gas fractions and more extended atmospheres in low T
systems are consistent with expectations of models incorporating the effects of
galactic winds on the ICM. ABRIDGEDComment: 11 pages, 4 figures, uses mn.sty and epsf.sty, accepted for
publication in MNRAS; minor modifications: discussion added on CF LX (Sec.
3.1);comparison with Allen & Fabian L-T results (Sec.3.1 & Sec.4.4);
statistics precised (3.1), discussion clarified (Sec. 2.2,Sec. 4.4); slight
mistake in the r-T and M-T relation calibration corrected and thus fgas in
Fig.3, Fig 4, Tab 2 slightly change
Spatial Disaggregation of Agricultural Production Data
In this paper we develop a dynamic data-consistent way for estimating agricultural land use choices at a disaggregate level (district-level), using more aggregate data (regional-level). The disaggregation procedure requires two steps. The first step consists in specifying and estimating a dynamic model of land use at the regional level. In the second step, we disaggregate outcomes of the aggregate model using maximum entropy (ME). The ME disaggregation procedure is applied to a sample of California data. The sample includes 6 districts located in Central Valley and 8 possible crops, namely: Alfalfa, Cotton, Field, Grain, Melons, Tomatoes, Vegetables and Subtropical. The disaggregation procedure enables the recovery of land use at the district-level with an out-sample prediction error of 16%. This result shows that the micro behavior, inferred from aggregate data with our disaggregation approach, seems to be consistent with observed behavior.Disaggregation, Bayesian method, Maximum entropy, Land use, Production Economics, C11, C44, Q12,
Statistical Network Analysis for Functional MRI: Summary Networks and Group Comparisons
Comparing weighted networks in neuroscience is hard, because the topological
properties of a given network are necessarily dependent on the number of edges
of that network. This problem arises in the analysis of both weighted and
unweighted networks. The term density is often used in this context, in order
to refer to the mean edge weight of a weighted network, or to the number of
edges in an unweighted one. Comparing families of networks is therefore
statistically difficult because differences in topology are necessarily
associated with differences in density. In this review paper, we consider this
problem from two different perspectives, which include (i) the construction of
summary networks, such as how to compute and visualize the mean network from a
sample of network-valued data points; and (ii) how to test for topological
differences, when two families of networks also exhibit significant differences
in density. In the first instance, we show that the issue of summarizing a
family of networks can be conducted by adopting a mass-univariate approach,
which produces a statistical parametric network (SPN). In the second part of
this review, we then highlight the inherent problems associated with the
comparison of topological functions of families of networks that differ in
density. In particular, we show that a wide range of topological summaries,
such as global efficiency and network modularity are highly sensitive to
differences in density. Moreover, these problems are not restricted to
unweighted metrics, as we demonstrate that the same issues remain present when
considering the weighted versions of these metrics. We conclude by encouraging
caution, when reporting such statistical comparisons, and by emphasizing the
importance of constructing summary networks.Comment: 16 pages, 5 figure
A multiscale regularized restoration algorithm for XMM-Newton data
We introduce a new multiscale restoration algorithm for images with few
photons counts and its use for denoising XMM data. We use a thresholding of the
wavelet space so as to remove the noise contribution at each scale while
preserving the multiscale information of the signal. Contrary to other
algorithms the signal restoration process is the same whatever the signal to
noise ratio is. Thresholds according to a Poisson noise process are indeed
computed analytically at each scale thanks to the use of the unnormalized Haar
wavelet transform. Promising preliminary results are obtained on X-ray data for
Abell 2163 with the computation of a temperature map.Comment: To appear in the Proceedings of `Galaxy Clusters and the High
Redshift Universe Observed in X-rays', XXIth Moriond Astrophysics Meeting
(March 2001), Eds. Doris Neumann et a
Accurate numerical potential and field in razor-thin axisymmetric discs
We demonstrate the high accuracy of the density splitting method to compute
the gravitational potential and field in the plane of razor-thin, axially
symmetric discs, as preliminarily outlined in Pierens & Hure (2004).
Because residual kernels in Poisson integrals are not C^infinity-class
functions, we use a dynamical space mapping in order to increase the efficiency
of advanced quadrature schemes. In terms of accuracy, results are better by
orders of magnitude than for the classical FFT-methods.Comment: 11 pages, 5 color figures, 2 table
Registering and uploading datasets in the generation CP central registry
Poster presented at Generation Challenge Programme 2009 Annual Research Meeting. Bamako (Mali), 20-23 Sep 200
Characterisation of biodiversity in improved rubber agroforests in West-Kalimantan, Indonesia. Real and Potential uses for spontaneous plants
Since the introduction of rubber at the turn of the 20th century smallholders have developed an original complex agroforestry system called jungle rubber in which non selected young rubber trees (seedlings) are managed extensively alongside secondary forest re-growth. The issue of improving smallholder rubber productivity at affordable capital investments and levels of inputs while maintaining the environmental benefits of jungle rubber has been addressed by the Smallholder Rubber Agroforestry Project (SRAP: a joint project run by ICRAF, GAPKINDO and CIRAD). In 1995-1996, 27 trials (with a total of 100 plots) were set up in three provinces in Indonesia to assess the possibility of associating clonal rubber with agroforestry practices under smallholder conditions (Penot, 1997). Two RAS types were selected for this study: RAS n° 1 and n° 3. RAS n° 1 is basically improved jungle rubber using clonal planting material (see a description of RAS types in annexe 1). The rubber trees are in competition with spontaneous vegetation in the inter-row but results show that there are no negative consequences for rubber growth during the immature period. RAS n° 3 was designed for areas infested by Imperata cylindrica, with the establishment of shrubby leguminous cover crops and fast-growing tree species in the inter-rows with the aim of shading out weeds. The other type, RAS n° 2, is based on intercropping clonal rubber with various annual and perennial crops, including fruit and timber trees (Penot et al, 1994). In all cases, RAS have a planting density of 550 clonal rubber trees/ha and a variable number of associated fruit, timber or fast growing shade trees (from 92 to 256/ha). In addition to the RAS experimental plots, âRAS sendiriâ (or âendogenous RASâ) are rubber agroforests improved by farmers without outside assistance. The district of Sanggau in the province of West Kalimantan was identified by SRAP as representative of traditional jungle-rubber-based local farming systems that have developed over the last 90 years. The district of Sanggau is located in the central area of the Kapuas river basin, between 1° N and 0°6' S and 09°8' W and 11°33' E. The district covers 18 302 km2, i.e. 13 % of the province. The trial plots described in this study are located in the villages of Embaong, Engkayu, Kopar, and Trimulia (the last being in the transmigration area). Most soils in the province of West-Kalimantan are acrisoils associated with ferralitic soils. Such soils have relatively good physical characteristics but poor chemical value and become acid. Rubber is widely grown in this area as it can grow in poor soils. The landscape is dominated by logged-over forest, secondary forest and a mosaic of jungle rubber and fallow with secondary forest re-growth. Large scale logging activities took place from 1950s to the 1980s at the expense of primary forest. At present, forested areas are located in hilly or remote areas and are very limited in extent. Oil palm and Acacia mangium plantations developed exponentially in the 1990s increasing the conversion of degraded forest areas into Estates that cultivate perennial crops. The main objective of this study is to assess existing plant biodiversity in RAS systems compared to that of jungle rubber. The second objective is to review the current uses of certain plants and their market potential.
Probing the dark matter profile of hot clusters and the M-T relation with XMM-Newton
We present results based on XMM-Newton observations of a small sample of hot
galaxy clusters. Making a full use of XMM-Newton's spectro-imaging
capabilities, we have extracted the radial temperature profile and gas density
profile, and with this information, calculated the total mass profile of each
cluster (under the assumption of hydrostatic equilibrium and spherical
symmetry). Comparing the individual scaled total mass profiles, we have probed
the Universality of rich cluster mass profiles over a wide range of radii (from
0.01 to 0.7 the virial radius). We have also tested the shape of cluster mass
profiles by comparing with the predicted profiles from numerical simulations of
hierarchical structure formation. We also derived the local mass-temperature
(M-T) scaling relation over a range of temperature going from 4 to 9 keV, that
we compare with theoretical predictions.Comment: 7 pages, 2 figures, Advances in Space Research in press (proceedings
of the COSPAR 2004 Assembly, Paris
The imbalanced antiferromagnet in an optical lattice
We study the rich properties of the imbalanced antiferromagnet in an optical
lattice. We present its phase diagram, discuss spin waves and explore the
emergence of topological excitations in two dimensions, known as merons, which
are responsible for a Kosterlitz-Thouless transition that has never
unambiguously been observed.Comment: 4 pages, 5 figures, RevTe
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