8,525 research outputs found
An adaptive perception-based image preprocessing method
The aim of this paper is to introduce an adaptive preprocessing procedure based on human perception in order to increase the performance of some standard image processing techniques. Specifically, image frequency content has been weighted by the corresponding value of the contrast sensitivity function, in agreement with the sensitiveness of human eye to the different image frequencies and contrasts. The 2D Rational dilation wavelet transform has been employed for representing image frequencies. In fact, it provides an adaptive and flexible multiresolution framework, enabling an
easy and straightforward adaptation to the image frequency content. Preliminary experimental results show that the proposed preprocessing allows us to increase the performance of some standard image enhancement algorithms in terms of visual quality and often also in terms of PSNR
Quark-antiquark potential from a deformed AdS/QCD
In this work we calculate the static limit of the energy for a
quark-antiquark pair from the Nambu-Goto action using a holographic approach
with a deformed AdS space, with warp factor .
From this energy we derive the Cornell potential for the quark-antiquark
interaction. We also find a range of values for our parameters which fits
exactly the Cornell potential parameters. In particular, setting the zero
energy of the Cornell potential at 0.33 fermi, we find that GeV
and .Comment: 15 pages, 2 figures. V3: Abstract changed, typos corrected, text
improved, new appendix, figures 1 and 2 slightly different, figure 2 now
presents explicitly our result for the Cornell Potential, results unchanged."
To appear in AHE
Orbital alignment and star-spot properties in the WASP-52 planetary system
We report 13 high-precision light curves of eight transits of the exoplanet WASP-52 b, obtained by using four medium-class telescopes, through different filters, and adopting the defocussing technique. One transit was recorded simultaneously from two different observatories and another one from the same site but with two different instruments, including a multiband camera. Anomalies were clearly detected in five light curves and modelled as star-spots occulted by the planet during the transit events. We fitted the clean light curves with the jktebop code, and those with the anomalies with the prism+gemc codes in order to simultaneously model the photometric parameters of the transits and the position, size and contrast of each star-spot. We used these new light curves and some from the literature to revise the physical properties of the WASP-52 system. Star-spots with similar characteristics were detected in four transits over a period of 43 d. In the hypothesis that we are dealing with the same star-spot, periodically occulted by the transiting planet, we estimated the projected orbital obliquity of WASP-52 b to be λ = 3∘.8.∘ ± 8.∘4. We also determined the true orbital obliquity, ψ = 20° ± 50°, which is, although very uncertain, the first measurement of ψ purely from star-spot crossings. We finally assembled an optical transmission spectrum of the planet and searched for variations of its radius as a function of wavelength. Our analysis suggests a flat transmission spectrum within the experimental uncertainties
SCC: A Service Centered Calculus
We seek for a small set of primitives that might serve as a basis for formalising and programming service oriented applications over global computers. As an outcome of this study we introduce here SCC, a process calculus that features explicit notions of service definition, service invocation and session handling. Our proposal has been influenced by Orc, a programming model for structured orchestration of services, but the SCC’s session handling mechanism allows for the definition of structured interaction protocols, more complex than the basic request-response provided by Orc. We present syntax and operational semantics of SCC and a number of simple but nontrivial programming examples that demonstrate flexibility of the chosen set of primitives. A few encodings are also provided to relate our proposal with existing ones
The Speciality Index as invariant indicator in the BKL Mixmaster Dynamics
The speciality index, which has been mainly used in Numerical Relativity for
studying gravitational waves phenomena as an indicator of the special or
non-special Petrov type character of a spacetime, is applied here in the
context of Mixmaster cosmology, using the Belinski-Khalatnikov-Lifshitz map.
Possible applications for the associated chaotic dynamics are discussed
Structure formation and quasi-spherical collapse from initial curvature perturbations with numerical relativity simulations
We use numerical relativity simulations to describe the spacetime evolution
during nonlinear structure formation in CDM cosmology. Fully nonlinear
initial conditions are set at an initial redshift , based
directly on the gauge invariant comoving curvature perturbation
commonly used to model early-universe fluctuations. Assigning a simple 3-D
sinusoidal structure to , we then have a lattice of
quasi-spherical over-densities representing idealised dark matter halos
connected through filaments and surrounded by voids. This structure is
implemented in the synchronous-comoving gauge, using a pressureless perfect
fluid (dust) description of CDM, and then it is fully evolved with the Einstein
Toolkit code. With this, we look into whether the Top-Hat spherical and
homogeneous collapse model provides a good description of the collapse of
over-densities. We find that the Top-Hat is an excellent approximation for the
evolution of peaks, where we observe that the shear is negligible and collapse
takes place when the linear density contrast reaches the predicted critical
value . Additionally, we characterise the outward
expansion of the turn-around boundary and show how it depends on the initial
distribution of matter, finding that it is faster in denser directions,
incorporating more and more matter in the infalling region. Using the EBWeyl
code [1] we look at the distribution of the electric and magnetic parts of the
Weyl tensor, finding that they are stronger along and around the filaments,
respectively. We introduce a method to dynamically classify different regions
in Petrov types. With this, we find that the spacetime is of Petrov type I
everywhere, as expected, but we can identify the leading order type, finding a
transition between different types as non-linearity grows, with production of
gravitational waves.Comment: 29 pages, 18 figures, 1 table, 2 appendices, abridged abstract, see
PDF for the full versio
Hard X-ray selected giant radio galaxies - I. The X-ray properties and radio connection
We present the first broad-band X-ray study of the nuclei of 14 hard X-ray
selected giant radio galaxies, based both on the literature and on the analysis
of archival X-ray data from NuSTAR, XMM-Newton, Swift and INTEGRAL. The X-ray
properties of the sources are consistent with an accretion-related X-ray
emission, likely originating from an X-ray corona coupled to a radiatively
efficient accretion flow. We find a correlation between the X-ray luminosity
and the radio core luminosity, consistent with that expected for AGNs powered
by efficient accretion. In most sources, the luminosity of the radio lobes and
the estimated jet power are relatively low compared with the nuclear X-ray
emission. This indicates that either the nucleus is more powerful than in the
past, consistent with a restarting of the central engine, or that the giant
lobes are dimmer due to expansion losses.Comment: 11 pages, 3 figures. Accepted for publication in MNRA
Physical properties and radius variations in the HAT-P-5 planetary system from simultaneous four-colour photometry
The radii of giant planets, as measured from transit observations, may vary
with wavelength due to Rayleigh scattering or variations in opacity. Such an
effect is predicted to be large enough to detect using ground-based
observations at multiple wavelengths. We present defocussed photometry of a
transit in the HAT-P-5 system, obtained simultaneously through Stromgren u,
Gunn g and r, and Johnson I filters. Two more transit events were observed
through a Gunn r filter. We detect a substantially larger planetary radius in
u, but the effect is greater than predicted using theoretical model atmospheres
of gaseous planets. This phenomenon is most likely to be due to systematic
errors present in the u-band photometry, stemming from variations in the
transparency of Earth's atmosphere at these short wavelengths. We use our data
to calculate an improved orbital ephemeris and to refine the measured physical
properties of the system. The planet HAT-P-5b has a mass of 1.06 +/- 0.11 +/-
0.01 Mjup and a radius of 1.252 +/- 0.042 +/- 0.008 Rjup (statistical and
systematic errors respectively), making it slightly larger than expected
according to standard models of coreless gas-giant planets. Its equilibrium
temperature of 1517 +/- 29 K is within 60K of that of the extensively-studied
planet HD 209458b.Comment: Version 2 corrects the accidental omission of one author in the arXiv
metadata. Accepted for publication in MNRAS. 9 pages, 4 figures, 7 tables.
The properties of HAT-P-5 have been added to the Transiting Extrasolar Planet
Catalogue at http://www.astro.keele.ac.uk/~jkt/tepcat
- …