1,343 research outputs found
Evidence for self-interaction of charge distribution in charge-coupled devices
Charge-coupled devices (CCDs) are widely used in astronomy to carry out a
variety of measurements, such as for flux or shape of astrophysical objects.
The data reduction procedures almost always assume that ther esponse of a given
pixel to illumination is independent of the content of the neighboring pixels.
We show evidence that this simple picture is not exact for several CCD sensors.
Namely, we provide evidence that localized distributions of charges (resulting
from star illumination or laboratory luminous spots) tend to broaden linearly
with increasing brightness by up to a few percent over the whole dynamic range.
We propose a physical explanation for this "brighter-fatter" effect, which
implies that flatfields do not exactly follow Poisson statistics: the variance
of flatfields grows less rapidly than their average, and neighboring pixels
show covariances, which increase similarly to the square of the flatfield
average. These covariances decay rapidly with pixel separation. We observe the
expected departure from Poisson statistics of flatfields on CCD devices and
show that the observed effects are compatible with Coulomb forces induced by
stored charges that deflect forthcoming charges. We extract the strength of the
deflections from the correlations of flatfield images and derive the evolution
of star shapes with increasing flux. We show for three types of sensors that
within statistical uncertainties,our proposed method properly bridges
statistical properties of flatfields and the brighter-fatter effect
A Three-Flavor, Lorentz-Violating Solution to the LSND Anomaly
We investigate whether postulating the existence of Lorentz-violating,
CPT-conserving interactions allows three-neutrino solutions to the LSND anomaly
that are also consistent with all other neutrino data. We show that
Lorentz-violating interactions that couple only to one of the active neutrinos
have the right properties to explain all the data. The details of the data make
this solution unattractive. We find, for example, that a highly non-trivial
energy dependence of the Lorentz-violating interactions is required.Comment: 15 pages, two eps figures. V2 - Minor modification
Spin Effects in High Energy Fragmentation Processes
Recent measurements, in particular those on polarization and spin
alignment of vector mesons in annihilation at LEP, and those on the
azimuthal asymmetry at HERA, have attracted much attention on the spin effects
in high energy fragmentation processes. In this talk, we make a brief
introduction to the different topics studied in this connection and a short
summary of the available data. After that, we present a short summary of the
main theoretical results that we obtained in studying these different topics.
The talk was mainly based on the publications [5-9] which have been finished in
collaboration with C.Boros, Liu Chun-xiu and Xu Qing-hua.Comment: Plenary talk given at the 3rd Circum-Pan-Pacifc Symposium on High
Energy Spin Physics, October 2001, 8 pages, 4 figure
Anti-Lambda polarization in high energy pp collisions with polarized beam
We study the polarization of the anti-Lambda particle in polarized high
energy pp collisions at large transverse momenta. The anti-Lambda polarization
is found to be sensitive to the polarization of the anti-strange sea of the
nucleon. We make predictions using different parameterizations of the polarized
quark distribution functions. The results show that the measurement of
longitudinal anti-Lambda polarization can distinguish different
parameterizations, and that similar measurements in the transversely polarized
case can give some insights into the transversity distribution of the
anti-strange sea of nucleon.Comment: 11 pages, 4 figure
Can luminosity distance measurements probe the equation of state of dark energy
Distance measurements to Type Ia supernovae (SNe Ia) at cosmological
distances indicate that the Universe is accelerating and that a large fraction
of the critical energy density exists in a component with negative pressure.
Various hypotheses on the nature of this ``dark energy'' can be tested via
their prediction for the equation of state of this component. If the dark
energy is due to a scalar field, its equation of state will in general vary
with time and is related to the potential of the field. We review the intrinsic
degeneracies of luminosity distance measurements and compute the expected
accuracies that can be obtained for the equation of state parameter from a
realistic high statistic SNe Ia experiment.Comment: 12 pages, 3 Postscript figures, use epsfig, amssymb, amsmath.
Submitted to Physics Letters.
Constraining the CDM and Galileon models with recent cosmological data
The Galileon theory belongs to the class of modified gravity models that can
explain the late-time accelerated expansion of the Universe. In previous works,
cosmological constraints on the Galileon model were derived, both in the
uncoupled case and with a disformal coupling of the Galileon field to matter.
There, we showed that these models agree with the most recent cosmological
data. In this work, we used updated cosmological data sets to derive new
constraints on Galileon models, including the case of a constant conformal
Galileon coupling to matter. We also explored the tracker solution of the
uncoupled Galileon model. After updating our data sets, especially with the
latest \textit{Planck} data and BAO measurements, we fitted the cosmological
parameters of the CDM and Galileon models. The same analysis framework
as in our previous papers was used to derive cosmological constraints, using
precise measurements of cosmological distances and of the cosmic structure
growth rate. We showed that all tested Galileon models are as compatible with
cosmological data as the CDM model. This means that present
cosmological data are not accurate enough to distinguish clearly between both
theories. Among the different Galileon models, we found that a conformal
coupling is not favoured, contrary to the disformal coupling which is preferred
at the level over the uncoupled case. The tracker solution of the
uncoupled Galileon model is also highly disfavoured due to large tensions with
supernovae and \textit{Planck}+BAO data. However, outside of the tracker
solution, the general uncoupled Galileon model, as well as the general
disformally coupled Galileon model, remain the most promising Galileon
scenarios to confront with future cosmological data. Finally, we also discuss
constraints coming from Lunar Laser Ranging experiment and gravitational wave
speed of propagation.Comment: 22 pages, 17 figures, published version in A&
Photometry of supernovae in an image series : methods and application to the Supernova Legacy Survey (SNLS)
We present a technique to measure lightcurves of time-variable point sources
on a spatially structured background from imaging data. The technique was
developed to measure light curves of SNLS supernovae in order to infer their
distances. This photometry technique performs simultaneous PSF photometry at
the same sky position on an image series. We describe two implementations of
the method: one that resamples images before measuring fluxes, and one which
does not. In both instances, we sketch the key algorithms involved and present
the validation using semi-artificial sources introduced in real images in order
to assess the accuracy of the supernova flux measurements relative to that of
surrounding stars. We describe the methods required to anchor these PSF fluxes
to calibrated aperture catalogs, in order to derive SN magnitudes. We find a
marginally significant bias of 2 mmag of the after-resampling method, and no
bias at the mmag accuracy for the non-resampling method. Given surrounding star
magnitudes, we determine the systematic uncertainty of SN magnitudes to be less
than 1.5 mmag, which represents about one third of the current photometric
calibration uncertainty affecting SN measurements. The SN photometry delivers
several by-products: bright star PSF flux mea- surements which have a
repeatability of about 0.6%, as for aperture measurements; we measure relative
astrometric positions with a noise floor of 2.4 mas for a single-image bright
star measurement; we show that in all bands of the MegaCam instrument, stars
exhibit a profile linearly broadening with flux by about 0.5% over the whole
brightness range.Comment: Accepted for publication in A&A. 20 page
Experimental constraints on the uncoupled Galileon model from SNLS3 data and other cosmological probes
The Galileon model is a modified gravity theory that may provide an
explanation for the accelerated expansion of the Universe. This model does not
suffer from instabilities or ghost problems (normally associated with
higher-order derivative theories), restores local General Relativity -- thanks
to the Vainshtein screening effect -- and predicts late time acceleration of
the expansion. In this paper, we derive a new definition of the Galileon
parameters that allows us to avoid having to choose initial conditions for the
Galileon field, and then test this model against precise measurements of the
cosmological distances and the rate of growth of cosmic structures. We observe
a small tension between the constraints set by growth data and those from
distances. However, we find that the Galileon model remains consistent with
current observations and is still competitive with the \Lambda CDM model,
contrary to what was concluded in recent publications.Comment: 19 pages, 15 figures, accepted to Astronomy and Astrophysic
Supernovae and the Nature of the Dark Energy
The use of Type Ia supernovae as calibrated standard candles is one of the
most powerful tools to study the expansion history of the universe and thereby
its energy components. While the analysis of some ~50 supernovae at redshifts
around z~0.5 have provided strong evidence for an energy component with
negative pressure, ``dark energy'', more data is needed to enable an accurate
estimate of the amount and nature of this energy. This might be accomplished by
a dedicated space telescope, the SuperNova / Acceleration Probe (2000; SNAP),
which aims at collecting a large number of supernovae with z<2.
In this paper we assess the ability of the SNAP mission to determine various
properties of the ``dark energy.'' To exemplify, we expect SNAP, if operated
for three years to study Type Ia supernovae, to be able to determine the
parameters in a linear equation of state w(z)=w0 + w1 z to within a statistical
uncertainty of +-0.04 for w0 and +0.15,-0.17 for w1 assuming that the universe
is known to be flat and an independent high precision (sigma_{Omega_m}=0.015)
measurement of the mass density Omega_m, is used to constrain the fit. An
additional improvement can be obtained if a large number of low-z, as well as
high-z, supernovae are included in the sample.Comment: 13 pages, submitted to A&
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