20,994 research outputs found
Is there a propeller neutron star in Cas?
Cas is the prototype of a small population of B0-B1.5 III-V
classical Be (cBe) stars that emit anomalous and hard X-rays with a unique
array of properties. Cas is known to host, like other cBe stars, a
decretion disk and also a low mass companion. Recently Postnov et al. have
posited that this companion is a magnetized rapidly spinning neutron star that
deflects direct gravitational accretion from a stellar/disk wind via the
"propeller mechanism." These authors state that the key X-ray observations are
"remarkably well produced" in this scenario. We reexamine this mechanism in
detail and conclude that there are a number of fatal objections in its
application to the Cas case. Among other considerations these issues
include the prediction under the propeller scenario of a much smaller
population of Cas stars than is observed and the lack of allowance for
observed correlations of X-ray and UV and/or optical properties over a variety
of timescales.Comment: 9 page
Entropy Maximization in the Presence of Higher-Curvature Interactions
Within the context of the entropic principle, we consider the entropy of
supersymmetric black holes in N=2 supergravity theories in four dimensions with
higher-curvature interactions, and we discuss its maximization at points in
moduli space at which an excess of hypermultiplets becomes massless. We find
that the gravitational coupling function F^(1) enhances the maximization at
these points in moduli space. In principle, this enhancement may be modified by
the contribution from higher F^(g)-couplings. We show that this is indeed the
case for the resolved conifold by resorting to the non-perturbative expression
for the topological free energy.Comment: 22 pages, 8 figures, AMS-LaTe
The Digitized Second Palomar Observatory Sky Survey (DPOSS) II: Photometric Calibration
We present the photometric calibration technique for the Digitized Second
Palomar Observatory Sky Survey (DPOSS), used to create seamless catalogs of
calibrated objects over large sky areas. After applying a correction for
telescope vignetting, the extensive plate overlap regions are used to transform
sets of plates onto a common instrumental photometric system. Photometric
transformations to the Gunn gri system for each plate, for stars and galaxies,
are derived using these contiguous stitched areas and an extensive CCD imaging
library obtained for this purpose. We discuss the resulting photometric
accuracy, survey depth, and possible systematic errors.Comment: 25 pages, 13 figures. Accepted to AJ. Some figures shrunk or missing
to limit file size; the full paper is available at
http://www.sdss.jhu.edu/~rrg/science/papers/photometrypaper.ps.g
Performing edge detection by difference of Gaussians using q-Gaussian kernels
In image processing, edge detection is a valuable tool to perform the
extraction of features from an image. This detection reduces the amount of
information to be processed, since the redundant information (considered less
relevant) can be unconsidered. The technique of edge detection consists of
determining the points of a digital image whose intensity changes sharply. This
changes are due to the discontinuities of the orientation on a surface for
example. A well known method of edge detection is the Difference of Gaussians
(DoG). The method consists of subtracting two Gaussians, where a kernel has a
standard deviation smaller than the previous one. The convolution between the
subtraction of kernels and the input image results in the edge detection of
this image. This paper introduces a method of extracting edges using DoG with
kernels based on the q-Gaussian probability distribution, derived from the
q-statistic proposed by Constantino Tsallis. To demonstrate the method's
potential, we compare the introduced method with the traditional DoG using
Gaussians kernels. The results showed that the proposed method can extract
edges with more accurate details.Comment: 5 pages, 5 figures, IC-MSQUARE 201
Black hole entropy functions and attractor equations
The entropy and the attractor equations for static extremal black hole
solutions follow from a variational principle based on an entropy function. In
the general case such an entropy function can be derived from the reduced
action evaluated in a near-horizon geometry. BPS black holes constitute special
solutions of this variational principle, but they can also be derived directly
from a different entropy function based on supersymmetry enhancement at the
horizon. Both functions are consistent with electric/magnetic duality and for
BPS black holes their corresponding OSV-type integrals give identical results
at the semi-classical level. We clarify the relation between the two entropy
functions and the corresponding attractor equations for N=2 supergravity
theories with higher-derivative couplings in four space-time dimensions. We
discuss how non-holomorphic corrections will modify these entropy functions.Comment: 21 pages,LaTeX,minor change
CNA 8170: linhagem de arroz adaptada para o ecossistema terra firme do Estado do Pará.
Cópia de trabalho editado em CD-ROM
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