3,231 research outputs found
A Fractal Perspective on Scale in Geography
Scale is a fundamental concept that has attracted persistent attention in
geography literature over the past several decades. However, it creates
enormous confusion and frustration, particularly in the context of geographic
information science, because of scale-related issues such as image resolution,
and the modifiable areal unit problem (MAUP). This paper argues that the
confusion and frustration mainly arise from Euclidean geometric thinking, with
which locations, directions, and sizes are considered absolute, and it is time
to reverse this conventional thinking. Hence, we review fractal geometry,
together with its underlying way of thinking, and compare it to Euclidean
geometry. Under the paradigm of Euclidean geometry, everything is measurable,
no matter how big or small. However, geographic features, due to their fractal
nature, are essentially unmeasurable or their sizes depend on scale. For
example, the length of a coastline, the area of a lake, and the slope of a
topographic surface are all scale-dependent. Seen from the perspective of
fractal geometry, many scale issues, such as the MAUP, are inevitable. They
appear unsolvable, but can be dealt with. To effectively deal with
scale-related issues, we introduce topological and scaling analyses based on
street-related concepts such as natural streets, street blocks, and natural
cities. We further contend that spatial heterogeneity, or the fractal nature of
geographic features, is the first and foremost effect of two spatial
properties, because it is general and universal across all scales.
Keywords: Scaling, spatial heterogeneity, conundrum of length, MAUP,
topological analysisComment: 12 pages, 5 figures, and one tabl
Density-matrix renormalisation group approach to quantum impurity problems
A dynamic density-matrix renormalisation group approach to the spectral
properties of quantum impurity problems is presented. The method is
demonstrated on the spectral density of the flat-band symmetric single-impurity
Anderson model. We show that this approach provides the impurity spectral
density for all frequencies and coupling strengths. In particular, Hubbard
satellites at high energy can be obtained with a good resolution. The main
difficulties are the necessary discretisation of the host band hybridised with
the impurity and the resolution of sharp spectral features such as the
Abrikosov-Suhl resonance.Comment: 16 pages, 6 figures, submitted to Journal of Physics: Condensed
Matte
Some strategic national initiatives for the Swedish education in the geodata field
Ponencias, comunicaciones y pósters presentados en el 17th AGILE Conference on Geographic Information Science
"Connecting a Digital Europe through Location and Place", celebrado en la Universitat Jaume I del 3 al 6 de junio de 2014.This paper describes national cooperation in Sweden launched by its universities and authorities, aimed at improving geodata education.
These initiatives have been focused upon providing common access to geodata, the production of teaching materials in Swedish and
organizing annual meetings for teachers. We argue that this type of cooperation is vital to providing high quality education for a poorly
recognized subject in a country with a relatively small population
Supercooling of the disordered vortex lattice in Bi_2Sr_2CaCu_2O_8+d
Time-resolved local induction measurements near to the vortex lattice
order-disorder transition in optimally doped
BiSrCaCuO single crystals shows that the
high-field, disordered phase can be quenched to fields as low as half the
transition field. Over an important range of fields, the electrodynamical
behavior of the vortex system is governed by the co-existence of the two phases
in the sample. We interpret the results in terms of supercooling of the
high-field phase and the possible first order nature of the order-disorder
transition at the ``second peak''.Comment: 4 pages, 3 figures. Submitted to Nature, July 10th, 1999; Rejected
August 8th for lack of broad interest Submitted to Physical Review Letters
September 10th, 199
Long XMM observation of the Narrow-Line Seyfert 1 galaxy IRAS13224-3809: rapid variability, high spin and a soft lag
Results are presented from a 500ks long XMM-Newton observation of the
Narrow-Line Seyfert 1 galaxy IRAS13224-3809. The source is rapidly variable on
timescales down to a few 100s. The spectrum shows strong broad Fe-K and L
emission features which are interpreted as arising from reflection from the
inner parts of an accretion disc around a rapidly spinning black hole. Assuming
a power-law emissivity for the reflected flux and that the innermost radius
corresponds to the innermost stable circular orbit, the black hole spin is
measured to be 0.988 with a statistical precision better than one per cent.
Systematic uncertainties are discussed. A soft X-ray lag of 100s confirms this
scenario. The bulk of the power-law continuum source is located at a radius of
2-3 gravitational radii.Comment: 7 pages, 14 figures, submitted to MNRA
Obesity and Low-Grade Inflammation Increase Plasma Follistatin-Like 3 in Humans
Background. Rodent models suggest that follistatin-like 3 (fstl3) is associated with diabetes and obesity. In humans, plasma fstl3 is reduced with gestational diabetes. In vitro, TNF-induces fstl3 secretion, which suggests a link to inflammation. Objective. To elucidate the association between plasma fstl3 and obesity, insulin resistance, and low-grade inflammation in humans. Study Design. Plasma fstl3 levels were determined in a cross-sectional study including three groups: patients with type 2 diabetes, impaired glucose tolerance, and healthy controls. In addition, lipopolysaccharide (LPS), TNF-, or interleukin-6 (IL-6) as well as a hyperinsulinemic euglycemic clamp were used to examine if plasma fstl3 was acutely regulated in humans. Results. Plasma fstl3 was increased in obese subjects independent of glycemic state. Moreover, plasma fstl3 was positively correlated with fat mass, plasma leptin, fasting insulin, and HOMA B and negatively with HOMA S. Furthermore plasma fstl3 correlated positively with plasma TNF-and IL-6 levels. Infusion of LPS and TNF-, but not IL-6 and insulin, increased plasma fstl3 in humans. Conclusion. Plasma fstl3 is increased in obese subjects and associated with fat mass and low-grade inflammation. Furthermore, TNF-increased plasma fstl3, suggesting that TNF-is one of the inflammatory drivers of increased systemic levels of fstl3
A highly-ionized absorber in the X-ray binary 4U 1323-62: a new explanation for the dipping phenomenon
We report the detection of narrow Fe XXV and Fe XXVI X-ray absorption lines
at 6.68 +/- 0.04 keV and 6.97 +/- 0.05 keV in the persistent emission of the
dipping low-mass X-ray binary 4U 1323-62 during a 2003 January XMM-Newton
observation. These features are superposed on a broad emission feature centered
on 6.6 {+0.1}{-0.2} keV. During dipping intervals the equivalent width of the
Fe XXV feature increases while that of the Fe XXVI feature decreases,
consistent with the presence of less strongly ionized material in the
line-of-sight. As observed previously, the changes in the 1.0-10 keV spectrum
during dips are inconsistent with a simple increase in absorption by cool
material. However, the changes in both the absorption features and the
continuum can be modeled by variations in the properties of an ionized
absorber. No partial covering of any component of the spectrum, and hence no
extended corona, are required. From persistent to deep dipping the
photo-ionization parameter, Xi, expressed in erg cm s^{-1}, decreases from
log(Xi) of 3.9 +/- 0.1 to log(Xi) of 3.13 +/- 0.07, while the equivalent
hydrogen column density of the ionized absorber increases from (3.8 +/- 0.4)
10^{22} atoms cm^{-2} to (37 +/- 2) 10^{22} atoms cm^{-2}. Since highly-ionized
absorption features are seen from many other dip sources, this mechanism may
also explain the overall changes in X-ray spectrum observed during dipping
intervals from these systems.Comment: 16 pages, major improvements following referee's report, most figures
and tables changed, one figure added, accepted for publication by Astronomy
and Astrophysic
The X-ray luminous cluster underlying the bright radio-quiet quasar H1821+643
We present a Chandra observation of the only low redshift, z=0.299, galaxy
cluster to contain a highly luminous radio-quiet quasar, H1821+643. By
simulating the quasar PSF, we subtract the quasar contribution from the cluster
core and determine the physical properties of the cluster gas down to 3 arcsec
(15 kpc) from the point source. The temperature of the cluster gas decreases
from 9.0\pm0.5 keV down to 1.3\pm0.2 keV in the centre, with a short central
radiative cooling time of 1.0\pm0.1 Gyr, typical of a strong cool-core cluster.
The X-ray morphology in the central 100 kpc shows extended spurs of emission
from the core, a small radio cavity and a weak shock or cold front forming a
semi-circular edge at 15 arcsec radius. The quasar bolometric luminosity was
estimated to be 2 x 10^{47} erg per sec, requiring a mass accretion rate of 40
Msolar per yr, which corresponds to half the Eddington accretion rate. We
explore possible accretion mechanisms for this object and determine that Bondi
accretion, when boosted by Compton cooling of the accretion material, could
provide a significant source of the fuel for this outburst. We consider
H1821+643 in the context of a unified AGN accretion model and, by comparing
H1821+643 with a sample of galaxy clusters, we show that the quasar has not
significantly affected the large-scale cluster gas properties.Comment: 20 pages, 19 figures, accepted by MNRA
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