12,808 research outputs found
Document Retrieval on Repetitive Collections
Document retrieval aims at finding the most important documents where a
pattern appears in a collection of strings. Traditional pattern-matching
techniques yield brute-force document retrieval solutions, which has motivated
the research on tailored indexes that offer near-optimal performance. However,
an experimental study establishing which alternatives are actually better than
brute force, and which perform best depending on the collection
characteristics, has not been carried out. In this paper we address this
shortcoming by exploring the relationship between the nature of the underlying
collection and the performance of current methods. Via extensive experiments we
show that established solutions are often beaten in practice by brute-force
alternatives. We also design new methods that offer superior time/space
trade-offs, particularly on repetitive collections.Comment: Accepted to ESA 2014. Implementation and experiments at
http://www.cs.helsinki.fi/group/suds/rlcsa
Observational Evidence for Small-Scale Mixture of Weak and Strong Fields in the Quiet Sun
Three different maps of the quiet Sun, observed with the Advanced Stokes
Polarimeter (ASP) and the Diffraction-Limited Stokes Polarimeter (DLSP), show
evidence of strong (1700 G) and weak (500 G) fields coexisting
within the resolution element both at network and internetwork locations. The
angular resolution of the observations is of 1\arcsec (ASP) and 0.\arcsec6
(DLSP), respectively. Even at the higher DLSP resolution, a significant
fraction of the network magnetic patches harbor a mixture of strong and weak
fields. Internetwork elements that exhibit kG fields when analyzed with a
single-component atmosphere are also shown to harbor considerable amounts of
weak fields. Only those patches for which a single-component analysis yields
weak fields do not show this mixture of field strengths. Finally, there is a
larger fractional area of weak fields in the convective upflows than in the
downflows.Comment: ApJ, in pres
Dark Energy, scalar-curvature couplings and a critical acceleration scale
We study the effects of coupling a cosmologically rolling scalar field to
higher order curvature terms. We show that when the strong coupling scale of
the theory is on the 10^{-3}-10^{-1}eV range, the model passes all experimental
bounds on the existence of fifth forces even if the field has a mass of the
order of the Hubble scale in vacuum and non-suppressed couplings to SM fields.
The reason is that the coupling to certain curvature invariant acts as an
effective mass that grows in regions of large curvature. This prevents the
field from rolling down its potential near sources and makes its effects on
fifth-force search experiments performed in the laboratory to be observable
only at the sub-mm scale. We obtain the static spherically symmetric solutions
of the theory and show that a long-range force appears but it is turned on only
below a fixed Newtonian acceleration scale of the order of the Hubble constant.
We comment on the possibility of using this feature of the model to alleviate
the CDM small scale crisis and on its possible relation to MOND.Comment: 12 pages, 2 figure
Conservation equation on braneworlds in six dimensions
We study braneworlds in six-dimensional Einstein-Gauss-Bonnet gravity. The
Gauss-Bonnet term is crucial for the equations to be well-posed in six
dimensions when non-trivial matter on the brane is included (the also involved
induced gravity term is not significant for their structure), and the matching
conditions of the braneworld are known. We show that the energy-momentum of the
brane is always conserved, independently of any regular bulk energy-momentum
tensor, contrary to the situation of the five-dimensional case.Comment: References added, minor changes, 3 pages, RevTeX, to app. in Class.
Quant. Gra
Weak Lensing Mass Reconstruction of the Galaxy Cluster Abell 209
Weak lensing applied to deep optical images of clusters of galaxies provides
a powerful tool to reconstruct the distribution of the gravitating mass
associated to these structures. We use the shear signal extracted by an
analysis of deep exposures of a region centered around the galaxy cluster Abell
209, at redshift z=0.2, to derive both a map of the projected mass distribution
and an estimate of the total mass within a characteristic radius. We use a
series of deep archival R-band images from CFHT-12k, covering an area of 0.3
deg^2. We determine the shear of background galaxy images using a new
implementation of the modified Kaiser-Squires-Broadhurst pipeline for shear
determination, which we has been tested against the ``Shear TEsting Program 1
and 2'' simulations. We use mass aperture statistics to produce maps of the 2
dimensional density distribution, and parametric fits using both
Navarro-Frenk-White (NFW) and singular-isothermal-sphere profiles to constrain
the total mass. The projected mass distribution shows a pronounced asymmetry,
with an elongated structure extending from the SE to the NW. This is in general
agreement with the optical distribution previously found by other authors. A
similar elongation was previously detected in the X-ray emission map, and in
the distribution of galaxy colours. The circular NFW mass profile fit gives a
total mass of M_{200} = 7.7^{+4.3}_{-2.7} 10^{14} solar masses inside the
virial radius r_{200} = 1.8\pm 0.3 Mpc. The weak lensing profile reinforces the
evidence for an elongated structure of Abell 209, as previously suggested by
studies of the galaxy distribution and velocities.Comment: accepted by A&A, 15 pages, 11 figure
Un aparato sencillo y de alta precisión para la determinación de áreas especificas y del tamaño de poro de solidos
Se describe el principio del funcionamiento de un equipo que permite determinar las isotermas de adsorción de gases por sólidos a bajas temperaturas y se ilustra la forma de calcular el área específica y la distribución de tamaño de poro de los sólidos a partir de los datos de adsorción. Se utilizaron catalizadores de níquel metálico y de cromito de cobre; como adsórbate se empleó nitrógeno y los estudios de adsorción se realizaron a la temperatura del nitrógeno líquido
Unusual Stokes V profiles during flaring activity of a delta sunspot
We analyze a set of full Stokes profile observations of the flaring active
region NOAA 10808 recorded with the Vector-Spectromagnetograph (VSM) of the
SOLIS facility. We aim to quantify transient and permanent changes in the
magnetic field and velocity field. The results are put in context with MDI
magnetograms and reconstructed RHESSI X-ray images. We find signs of
restructuring of the photospheric magnetic field during the flare close to the
polarity inversion line (PIL) at the flaring site. At two locations in the
umbra we encounter strong fields (approx. 3 kG), as inferred from the Stokes I
profiles which, however, exhibit a low polarization signal. During the flare we
observe in addition asymmetric Stokes V profiles at one of these sites. The
asymmetric Stokes V profiles appear co-spatial and co-temporal with a strong
apparent polarity reversal observed in MDI-magnetograms and a chromospheric
hard X-ray source. The two-component atmosphere fits of the asymmetric Stokes
profiles result in line-of-sight velocity differences in the range of approx.
12km/s to 14 km/s between the two components in the photosphere. Another
possibility is that local atmospheric heating is causing the observed
asymmetric Stokes V profile shape. In either case our analysis shows that a
very localized patch of approx. 5 arcsec in the photospheric umbra, co-spatial
with a flare footpoint, exhibits a sub-resolution fine structure.Comment: 13 pages, 10 figures, 1 tabl
Spheres, Deficit Angles and the Cosmological Constant
We consider compactifications of six dimensional gravity in four dimensional
Minkowski or de Sitter space times a two dimensional sphere, S^2. As has been
recently pointed out, it is possible to introduce 3-branes in these backgrounds
with arbitrary tension without affecting the effective four dimensional
cosmological constant, since its only effect is to induce a deficit angle in
the sphere. We show that if a monopole like configuration of a 6D U(1) gauge
field is used to produce the spontaneous compactification of the two extra
dimensions in a sphere a fine tuning between brane and bulk parameters is
reintroduced once the quantization condition for the gauge field is taken into
account, so the 4D cosmological constant depends on the brane tension. This
problem is absent if instead of the monopole we consider a four form field
strength in the bulk to obtain the required energy-momentum tensor. Also,
making use of the four form field, we generalize the solution to an arbitrary
number of dimensions (\ge 6), keeping always four noncompact dimensions and
compactifying the rest in a n-dimensional sphere. We show that a (n+1)-brane
with arbitrary tension can be introduced in this background without affecting
the effective 4D cosmological constant.Comment: 14 pages, LaTe
Instability of brane cosmological solutions with flux compactifications
We discuss the stability of the higher-dimensional de Sitter (dS) brane
solutions with two-dimensional internal space in the Einstein-Maxwel theory. We
show that an instability appears in the scalar-type perturbations with respect
to the dS spacetime. We derive a differential relation which has the very
similar structure to the ordinary laws of thermodynamics as an extension of the
work for the six-dimensional model [20]. In this relation, the area of dS
horizon (integrated over the two internal dimensions) exactly behaves as the
thermodynamical entropy. The dynamically unstable solutions are in the
thermodynamically unstable branch. An unstable dS compactification either
evolves toward a stable configuration or two-dimensional internal space is
decompactified. These dS brane solutions are equivalent to the accelerating
cosmological solutions in the six-dimensional Einstein-Maxwell-dilaton theory
via dimensional reduction. Thus, if the seed higher-dimensional solution is
unstable, the corresponding six-dimensional solution is also unstable. From the
effective four-dimensional point of view, a cosmological evolution from an
unstable cosmological solution in higher dimensions may be seen as a process of
the transition from the initial cosmological inflation to the current dark
energy dominated Universe.Comment: 11 pages, 3 figures, references added, to appear in CQ
Magnetic properties of photospheric regions having very low magnetic flux
The magnetic properties of the quiet Sun are investigated using a novel
inversion code, FATIMA, based on the Principal Component Analysis of the
observed Stokes profiles. The stability and relatively low noise sensitivity of
this inversion procedure allows for the systematic inversion of large data sets
with very weak polarization signal. Its application to quiet Sun observations
of network and internetwork regions reveals that a significant fraction of the
quiet Sun contains kilogauss fields (usually with very small filling factors)
and confirms that the pixels with weak polarization account for most of the
magnetic flux. Mixed polarities in the resolution element are also found to
occur more likely as the polarization weakens.Comment: To apapear in ApJ. 39 pages, 12 figures (2 of them are color figures
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