19 research outputs found
The localization of single pulse in VLBI observation
In our previous work, we propose a cross spectrum based method to extract
single pulse signals from RFI contaminated data, which is originated from
geodetic VLBI postprocessing. This method fully utilizes fringe phase
information of the cross spectrum and hence maximizes signal power, however the
localization was not discussed in that work yet. As the continuation of that
work, in this paper, we further study how to localize single pulses using
astrometric solving method. Assuming that the burst is a point source, we
derive the burst position by solving a set of linear equations given the
relation between residual delay and offset to a priori position. We find that
the single pulse localization results given by both astrometric solving and
radio imaging are consistent within 3 sigma level. Therefore we claim that it
is possible to derive the position of a single pulse with reasonable precision
based on only 3 or even 2 baselines with 4 milliseconds integration. The
combination of cross spectrum based detection and the localization proposed in
this work then provide a thorough solution for searching single pulse in VLBI
observation. According to our calculation, our pipeline gives comparable
accuracy as radio imaging pipeline. Moreover, the computational cost of our
pipeline is much smaller, which makes it more practical for FRB search in
regular VLBI observation. The pipeline is now publicly available and we name it
as "VOLKS", which is the acronym of "VLBI Observation for frb Localization Keen
Searcher".Comment: 11 pages, 4 figures, 3 tables, accepted for publication in A
Cosmological constraints from the capture of non-Gaussianity in Weak Lensing data
Weak gravitational lensing has become a common tool to constrain the
cosmological model. The majority of the methods to derive constraints on
cosmological parameters use second-order statistics of the cosmic shear.
Despite their success, second-order statistics are not optimal and degeneracies
between some parameters remain. Tighter constraints can be obtained if
second-order statistics are combined with a statistic that is efficient to
capture non-Gaussianity. In this paper, we search for such a statistical tool
and we show that there is additional information to be extracted from
statistical analysis of the convergence maps beyond what can be obtained from
statistical analysis of the shear field. For this purpose, we have carried out
a large number of cosmological simulations along the {\sigma}8-{\Omega}m
degeneracy, and we have considered three different statistics commonly used for
non-Gaussian features characterization: skewness, kurtosis and peak count. To
be able to investigate non-Gaussianity directly in the shear field we have used
the aperture mass definition of these three statistics for different scales.
Then, the results have been compared with the results obtained with the same
statistics estimated in the convergence maps at the same scales. First, we show
that shear statistics give similar constraints to those given by convergence
statistics, if the same scale is considered. In addition, we find that the peak
count statistic is the best to capture non-Gaussianities in the weak lensing
field and to break the {\sigma}8-{\Omega}m degeneracy. We show that this
statistical analysis should be conducted in the convergence maps: first,
because there exist fast algorithms to compute the convergence map for
different scales, and secondly because it offers the opportunity to denoise the
reconstructed convergence map, which improves non-Gaussian features extraction.Comment: Accepted for publication in MNRAS (11 pages, 5 figures, 9 tables
A bias in cosmic shear from galaxy selection: results from ray-tracing simulations
We identify and study a previously unknown systematic effect on cosmic shear
measurements, caused by the selection of galaxies used for shape measurement,
in particular the rejection of close (blended) galaxy pairs. We use ray-tracing
simulations based on the Millennium Simulation and a semi-analytical model of
galaxy formation to create realistic galaxy catalogues. From these, we quantify
the bias in the shear correlation functions by comparing measurements made from
galaxy catalogues with and without removal of close pairs. A likelihood
analysis is used to quantify the resulting shift in estimates of cosmological
parameters. The filtering of objects with close neighbours (a) changes the
redshift distribution of the galaxies used for correlation function
measurements, and (b) correlates the number density of sources in the
background with the density field in the foreground. This leads to a
scale-dependent bias of the correlation function of several percent,
translating into biases of cosmological parameters of similar amplitude. This
makes this new systematic effect potentially harmful for upcoming and planned
cosmic shear surveys. As a remedy, we propose and test a weighting scheme that
can significantly reduce the bias.Comment: 9 pages, 9 figures, version accepted for publication in Astronomy &
Astrophysic
FASTLens (FAst STatistics for weak Lensing) : Fast method for Weak Lensing Statistics and map making
With increasingly large data sets, weak lensing measurements are able to
measure cosmological parameters with ever greater precision. However this
increased accuracy also places greater demands on the statistical tools used to
extract the available information. To date, the majority of lensing analyses
use the two point-statistics of the cosmic shear field. These can either be
studied directly using the two-point correlation function, or in Fourier space,
using the power spectrum. But analyzing weak lensing data inevitably involves
the masking out of regions or example to remove bright stars from the field.
Masking out the stars is common practice but the gaps in the data need proper
handling. In this paper, we show how an inpainting technique allows us to
properly fill in these gaps with only operations, leading to a new
image from which we can compute straight forwardly and with a very good
accuracy both the pow er spectrum and the bispectrum. We propose then a new
method to compute the bispectrum with a polar FFT algorithm, which has the main
advantage of avoiding any interpolation in the Fourier domain. Finally we
propose a new method for dark matter mass map reconstruction from shear
observations which integrates this new inpainting concept. A range of examples
based on 3D N-body simulations illustrates the results.Comment: Final version accepted by MNRAS. The FASTLens software is available
from the following link : http://irfu.cea.fr/Ast/fastlens.software.ph
Constraining the dark energy dynamics with the cosmic microwave background bispectrum
We consider the influence of the dark energy dynamics at the onset of cosmic
acceleration on the Cosmic Microwave Background (CMB) bispectrum, through the
weak lensing effect induced by structure formation. We study the line of sight
behavior of the contribution to the bispectrum signal at a given angular
multipole : we show that it is non-zero in a narrow interval centered at a
redshift satisfying the relation , where the
wavenumber corresponds to the scale entering the non-linear phase, and is
the cosmological comoving distance. The relevant redshift interval is in the
range 0.1\lsim z\lsim 2 for multipoles 1000\gsim\ell\gsim 100; the signal
amplitude, reflecting the perturbation dynamics, is a function of the
cosmological expansion rate at those epochs, probing the dark energy equation
of state redshift dependence independently on its present value. We provide a
worked example by considering tracking inverse power law and SUGRA Quintessence
scenarios, having sensibly different redshift dynamics and respecting all the
present observational constraints. For scenarios having the same present
equation of state, we find that the effect described above induces a projection
feature which makes the bispectra shifted by several tens of multipoles, about
10 times more than the corresponding effect on the ordinary CMB angular power
spectrum.Comment: 15 pages, 7 figures, matching version accepted by Physical Review D,
one figure improve
GaBoDS: The Garching-Bonn Deep Survey: VI. Cosmic shear analysis
Aims. We present a cosmic shear analysis and data validation of 15 square
degree high-quality R-band data of the Garching-Bonn Deep Survey obtained with
the Wide Field Imager of the MPG/ESO 2.2m telescope. Methods. We measure the
two-point shear correlation functions to calculate the aperture mass
dispersion. Both statistics are used to perform the data quality control.
Combining the cosmic shear signal with a photometric redshift distribution of a
galaxy sub-sample obtained from two square degree of UBVRI-band observations of
the Deep Public Survey we determine constraints for the matter density Omega_m,
the mass power spectrum normalisation sigma_8 and the dark energy density
Omega_Lambda in the magnitude interval R in [21.5,24.5]. In this magnitude
interval the effective number density of source galaxies is n=12.5/sq. arcmin,
and their mean redshift is z_m=0.78. To estimate the posterior likelihood we
employ the Monte Carlo Markov Chain method. Results. Using the aperture mass
dispersion we obtain for the mass power spectrum normalisation sigma_8=0.80 +-
0.10 (1 sigma statistical error) at a fixed matter density Omega_m=0.30
assuming a flat universe with negligible baryon content and marginalising over
the Hubble parameter and the uncertainties in the fitted redshift distribution.Comment: 23 pages, 19 figures, submitted to A&
Galaxy Clustering and Dark Energy
We study the evolution of galaxy clustering in various cosmological models
with quintessence. We investigate how the analytical predictions vary with
change of dark energy equation of state . Comparing these predictions
against available data we discuss to what extent the problems of galaxy biasing
can be modelled. This will be key in constraining the dark energy equation of
state with future galaxy surveys. We use a compilation of various surveys to
study the number density and amplitude of galaxy clustering from observations
of the local universe at to that of the Lyman break galaxies and
Ly- emitters at . We find that there is a degeneracy
between the dark energy equation of state and the way galaxies populate dark
matter haloes; objects are more biased in models with more negative values of
dark energy equation of state .
We conclude that, while future all sky CMB observations will determine
cosmological parameters with unprecedented precision, and cross correlation of
weak lensing experiments and galaxy surveys will provide a cleaner and accurate
picture of bias associated with collapsed objects, the rate of growth of large
scale structure in such surveys can potentially constrain the equation of state
of dark energy and the potential of the scalar field associated with
quintessence. In particular, we show that the abundance and spatial
distribution of galaxy clusters at intermediate redshifts strongly depend on
the dark energy equation of state. When accurate measurement of galaxy
clustering at high-redshit becomes possible, it will provide constraints on
dark energy that are independent and complementary to type Ia supernova
studies.Comment: 12 pages, 7 figures. Slightly modified version published in MNRA
Mars Express tracking and orbit determination trials with Chinese VLBI network
With strong support from European Space Agency (ESA), Shanghai Astronomical Observatory (SHAO) organized a tracking and orbit determination trails using Chinese VLBI Network (CVN) to track Mars Express, the first Mars probe launched by ESA. Using a high-resolution VLBI software correlator and Doppler measurement system developed in-house, two sets of tracking data. VLBI and Doppler, were acquired. The trials represent the first successful foray held in China to track a probe about 360 million kilometers away from the Earth. The tracking data are analyzed using a Mars satellite orbit determination software system developed at SHAO. The results show that the accuracy of 5 s integrated three-way-Doppler data is about 0.3 mm/s, or roughly the same accuracy as ESA's tracking data. Position discrepancies between the Doppler-based orbit solution of 8 h arc-length (about 1 orbital revolution) and ESA's reconstructed orbit are of the order of several hundred meters. In preparing for the Russia-China co-sponsored Mars exploration mission Phobos-Grunt-YingHuo, simulations were carried out to evaluate the achievable orbital accuracy levels and the contributions of VLBI and Doppler data respectively. Results show that Doppler data provide better orbit accuracy, so that for VLBI to be able to provide kilometer level orbit solutions, the accuracy of VLBI measurement needs to be improved by at least one order of magnitude