1,318 research outputs found
GLM permutation - nonparametric inference for arbitrary general linear models
Introduction: Permutation methods are finding growing use in neuroimag-
ing data analyses (e.g. randomise in FSL, SnPM in SPM,
XBAMM/BAMM/CAMBA, etc). These methods provide ex-
act control of false positives, make only weak assumptions, and
allow nonstandard types of statistics (e.g. smoothed variance t-
test). With fast and inexpensive computing, there would seem
few reasons not to use nonparametric methods.
A significant limitation of these methods, however, is the lack of
flexibility with respect to the experimental design and nuisance
variables. Each specific design dictates the type of exchange-
ability of null data, and hence how to permute. Nuisance effects
(e.g. age) render data non-exchangeable even when the effect of
interest is null. Hence, even something as simple as ANCOVA
has no exact permutation test.
Recently there has been an active literature on approximate–
but accurate–permutation tests for 2-variable regression, one
effect of interest, one nuisance (see review by Anderson &
Robinson [1]). Here we extend and evaluate these methods
for use with an arbitrary General Linear Model (GLM)
Are All Static Black Hole Solutions Spherically Symmetric?
The static black hole solutions to the Einstein-Maxwell equations are all
spherically symmetric, as are many of the recently discovered black hole
solutions in theories of gravity coupled to other forms of matter. However,
counterexamples demonstrating that static black holes need not be spherically
symmetric exist in theories, such as the standard electroweak model, with
electrically charged massive vector fields. In such theories, a magnetically
charged Reissner-Nordstrom solution with sufficiently small horizon radius is
unstable against the development of a nonzero vector field outside the horizon.
General arguments show that, for generic values of the magnetic charge, this
field cannot be spherically symmetric. Explicit construction of the solution
shows that it in fact has no rotational symmetry at all.Comment: 6 pages, plain TeX. Submitted to GRF Essay Competitio
Imaging the spotty surface of Betelgeuse in the H band
This paper reports on H-band interferometric observations of Betelgeuse made
at the three-telescope interferometer IOTA. We image Betelgeuse and its
asymmetries to understand the spatial variation of the photosphere, including
its diameter, limb darkening, effective temperature, surrounding brightness,
and bright (or dark) star spots. We used different theoretical simulations of
the photosphere and dusty environment to model the visibility data. We made
images with parametric modeling and two image reconstruction algorithms: MIRA
and WISARD. We measure an average limb-darkened diameter of 44.28 +/- 0.15 mas
with linear and quadratic models and a Rosseland diameter of 45.03 +/- 0.12 mas
with a MARCS model. These measurements lead us to derive an updated effective
temperature of 3600 +/- 66 K. We detect a fully-resolved environment to which
the silicate dust shell is likely to contribute. By using two imaging
reconstruction algorithms, we unveiled two bright spots on the surface of
Betelgeuse. One spot has a diameter of about 11 mas and accounts for about 8.5%
of the total flux. The second one is unresolved (diameter < 9 mas) with 4.5% of
the total flux. Resolved images of Betelgeuse in the H band are asymmetric at
the level of a few percent. The MOLsphere is not detected in this wavelength
range. The amount of measured limb-darkening is in good agreement with model
predictions. The two spots imaged at the surface of the star are potential
signatures of convective cells.Comment: 10 pages, 10 figures, accepted for publication in A&A, references
adde
Direct observation of substitutional Ga after ion implantation in Ge by means of extended x-ray absorption fine structure
We present an experimental lattice location study of Ga atoms in Ge after ion implantation at elevated temperature (250°C). Using extended x-rayabsorption fine structure (EXAFS) experiments and a dedicated sample preparation method, we have studied the lattice location of Ga atoms in Ge with a concentration ranging from 0.5 at. % down to 0.005 at. %. At Ga concentrations ≤0.05 at.%, all Ga dopants are substitutional directly after ion implantation, without the need for post-implantation thermal annealing. At higher Ga concentrations, a reduction in the EXAFS amplitude is observed, indicating that a fraction of the Ga atoms is located in a defective environment. The local strain induced by the Ga atoms in the Ge matrix is independent of the Ga concentration and extends only to the first nearest neighbor Ge shell, where a 1% contraction in bond length has been measured, in agreement with density functional theory calculations.We acknowledge the support from the Research Foundation
Flanders, the epi-team from imec, the KU Leuven
GOA 09/06 project, the IUAP program P6/42 and the Australian
Research Council. S.C. acknowledges support from
OCAS NV by an OCAS-endowed chair at Ghent University
Resolving asymmetries along the pulsation cycle of the Mira star X Hya
The mass-loss process in Mira stars probably occurs in an asymmetric way
where dust can form in inhomogeneous circumstellar molecular clumps. Following
asymmetries along the pulsation cycle can give us clues about these mass-loss
processes. We imaged the Mira star X Hya and its environnement at different
epochs to follow the evolution of the morphology in the continuum and in the
molecular bands. We observed X Hya with AMBER in J-H-K at low resolution at two
epochs. We modelled squared visibilities with geometrical and physical models.
We also present imaging reconstruction results obtained with MiRA and based on
the physical a priori images. We report on the angular scale change of X Hya
between the two epochs. 1D CODEX profiles allowed us to understand and model
the spectral variation of squared visibilities and constrain the stellar
parameters. Reconstructed model-dependent images enabled us to reproduce
closure phase signals and the azimuthal dependence of squared visibilities.
They show evidence for material inhomogeneities located in the immediate
environment of the star.Comment: Accepted for publication in A&A, 17 pages, 16 figure
Spectral energy distributions of quasars selected in the mid-infrared
We present preliminary results on fitting of SEDs to 142 z>1 quasars selected
in the mid-infrared. Our quasar selection finds objects ranging in extinction
from highly obscured, type-2 quasars, through more lightly reddened type-1
quasars and normal type-1s. We find a weak tendency for the objects with the
highest far-infrared emission to be obscured quasars, but no bulk systematic
offset between the far-infrared properties of dusty and normal quasars as might
be expected in the most naive evolutionary schemes. The hosts of the type-2
quasars have stellar masses comparable to those of radio galaxies at similar
redshifts. Many of the type-1s, and possibly one of the type-2s require a very
hot dust component in addition to the normal torus emission.Comment: 4 pages, 2 figures, to appear in the proceedings of The Spectral
Energy Distribution of Galaxies, Preston, September 2011, eds R.J. Tuffs &
C.C. Popesc
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