75 research outputs found
Time delays for 11 gravitationally lensed quasars revisited
We test the robustness of published time delays for 11 lensed quasars by
using two techniques to measure time shifts in their light curves.
We chose to use two fundamentally different techniques to determine time
delays in gravitationally lensed quasars: a method based on fitting a numerical
model and another one derived from the minimum dispersion method introduced by
Pelt and collaborators. To analyse our sample in a homogeneous way and avoid
bias caused by the choice of the method used, we apply both methods to 11
different lensed systems for which delays have been published: JVAS B0218+357,
SBS 0909+523, RX J0911+0551, FBQS J0951+2635, HE 1104-1805, PG 1115+080, JVAS
B1422+231, SBS 1520+530, CLASS B1600+434, CLASS B1608+656, and HE 2149-2745
Time delays for three double lenses, JVAS B0218+357, HE 1104-1805, and CLASS
B1600+434, as well as the quadruply lensed quasar CLASS B1608+656 are confirmed
within the error bars. We correct the delay for SBS 1520+530. For PG 1115+080
and RX J0911+0551, the existence of a second solution on top of the published
delay is revealed. The time delays in four systems, SBS 0909+523, FBQS
J0951+2635, JVAS B1422+231, and HE 2149-2745 prove to be less reliable than
previously claimed.
If we wish to derive an estimate of H_0 based on time delays in
gravitationally lensed quasars, we need to obtain more robust light curves for
most of these systems in order to achieve a higher accuracy and robustness on
the time delays
Hubble constant and dark energy inferred from free-form determined time delay distances
Time delays between multiple images of lensed sources can probe the geometry
of the universe. We propose a novel method based on free-form modelling of
gravitational lenses to estimate time-delay distances and, in turn,
cosmological parameters. This approach does not suffer from the degeneracy
between the steepness of the profile and the cosmological parameters. We apply
the method to 18 systems having time delay measurements and find
H_0=69+-6(stat.)+-4(syst.) km s^{-1}Mpc^{-1}. In combination with WMAP9, the
constraints on dark energy are Omega_w=0.68+-0.05 and w=-0.86+-0.17 in a flat
model with constant equation-of-state.Comment: 6 pages; accepted for publication on MNRA
Time-domain studies of gravitationally lensed quasars (GLQs)
We present the overview and current results of an ongoing optical/NIR
monitoring of seven GLQs with the 2-m Liverpool Robotic Telescope. The
photometric data over the first seven years of this programme (2005-2011) are
leading to high-quality light curves, which in turn are being used as key tools
for different standard and novel studies. While brightness records of
non-lensed distant quasars may contain unrecognized extrinsic variations, one
can disentangle intrinsic from extrinsic signal in certain GLQs. Thus, some
GLQs in our sample allow us to assess their extrinsic and intrinsic variations,
as well as to discuss the origin of both kinds of fluctuations. We also
demonstrate the usefulness of GLQ time-domain data to obtain successful
reverberation maps of inner regions of accretion disks around distant
supermassive black holes, and to estimate redshifts of distant lensing
galaxies.Comment: 3 pages, 1 figure, 1 table, New Horizons in Time Domain Astronomy,
Proceedings IAU Symposium 285, R.E.M. Griffin, R.J. Hanisch and R. Seaman,
eds. (2012
ASTErIsM - Application of topometric clustering algorithms in automatic galaxy detection and classification
We present a study on galaxy detection and shape classification using
topometric clustering algorithms. We first use the DBSCAN algorithm to extract,
from CCD frames, groups of adjacent pixels with significant fluxes and we then
apply the DENCLUE algorithm to separate the contributions of overlapping
sources. The DENCLUE separation is based on the localization of pattern of
local maxima, through an iterative algorithm which associates each pixel to the
closest local maximum. Our main classification goal is to take apart elliptical
from spiral galaxies. We introduce new sets of features derived from the
computation of geometrical invariant moments of the pixel group shape and from
the statistics of the spatial distribution of the DENCLUE local maxima
patterns. Ellipticals are characterized by a single group of local maxima,
related to the galaxy core, while spiral galaxies have additional ones related
to segments of spiral arms. We use two different supervised ensemble
classification algorithms, Random Forest, and Gradient Boosting. Using a sample
of ~ 24000 galaxies taken from the Galaxy Zoo 2 main sample with spectroscopic
redshifts, and we test our classification against the Galaxy Zoo 2 catalog. We
find that features extracted from our pipeline give on average an accuracy of ~
93%, when testing on a test set with a size of 20% of our full data set, with
features deriving from the angular distribution of density attractor ranking at
the top of the discrimination power.Comment: 20 pages, 13 Figures, 8 Tables, Accepted for publication in the
Monthly Notices of the Royal Astronomical Societ
Microlensing variability in time-delay quasars
We have searched for microlensing variability in the light curves of five
gravitationally lensed quasars with well-determined time delays: SBS 1520+530,
FBQ 0951+2635, RX J0911+0551, B1600+434 and HE 2149-2745. By comparing the
light curve of the leading image with a suitably time offset light curve of a
trailing image we find that two (SBS 1520+530 and FBQ 0951+2635) out of the
five quasars have significant long-term (years) and short-term (100 days)
brightness variations that may be attributed to microlensing.The short-term
variations may be due to nanolenses, relativistic hot or cold spots in the
quasar accretion disks, or coherent microlensing at large optical depth.Comment: 12 pages, 5 figures, uses natbib.sty and aa.cl
The PCA Lens-Finder: application to CFHTLS
We present the results of a new search for galaxy-scale strong lensing
systems in CFHTLS Wide. Our lens-finding technique involves a preselection of
potential lens galaxies, applying simple cuts in size and magnitude. We then
perform a Principal Component Analysis of the galaxy images, ensuring a clean
removal of the light profile. Lensed features are searched for in the residual
images using the clustering topometric algorithm DBSCAN. We find 1098 lens
candidates that we inspect visually, leading to a cleaned sample of 109 new
lens candidates. Using realistic image simulations we estimate the completeness
of our sample and show that it is independent of source surface brightness,
Einstein ring size (image separation) or lens redshift. We compare the
properties of our sample to previous lens searches in CFHTLS. Including the
present search, the total number of lenses found in CFHTLS amounts to 678,
which corresponds to ~4 lenses per square degree down to i=24.8. This is
equivalent to ~ 60.000 lenses in total in a survey as wide as Euclid, but at
the CFHTLS resolution and depth.Comment: 21 pages, 12 figures, accepted for publication on A&
Bayesian approach to gravitational lens model selection: constraining H_0 with a selected sample of strong lenses
Bayesian model selection methods provide a self-consistent probabilistic
framework to test the validity of competing scenarios given a set of data. We
present a case study application to strong gravitational lens parametric
models. Our goal is to select a homogeneous lens subsample suitable for
cosmological parameter inference. To this end we apply a Bayes factor analysis
to a synthetic catalog of 500 lenses with power-law potential and external
shear. For simplicity we focus on double-image lenses (the largest fraction of
lens in the simulated sample) and select a subsample for which astrometry and
time-delays provide strong evidence for a simple power-law model description.
Through a likelihood analysis we recover the input value of the Hubble constant
to within 3\sigma statistical uncertainty. We apply this methodology to a
sample of double image lensed quasars. In the case of B1600+434, SBS 1520+530
and SDSS J1650+4251 the Bayes' factor analysis favors a simple power-law model
description with high statistical significance. Assuming a flat \LambdaCDM
cosmology, the combined likelihood data analysis of such systems gives the
Hubble constant H_0=76+15-5 km/s/Mpc having marginalized over the lens model
parameters, the cosmic matter density and consistently propagated the
observational errors on the angular position of the images. The next generation
of cosmic structure surveys will provide larger lens datasets and the method
described here can be particularly useful to select homogeneous lens subsamples
adapted to perform unbiased cosmological parameter inferenceComment: 13 pages; 13 figures; includes Bayesian analysis of a synthetic lens
catalog generated with GRAVLENS, several additional results; matches MNRAS
accepted versio
Cosmography from two-image lens systems: overcoming the lens profile slope degeneracy
The time delays between the multiple images of a strong lens system, together
with a model of the lens mass distribution, allow a one-step measurement of a
cosmological distance, namely, the "time-delay distance" of the lens (D_dt)
that encodes cosmological information. The time-delay distance depends
sensitively on the radial profile slope of the lens mass distribution;
consequently, the lens slope must be accurately constrained for cosmological
studies. We show that the slope cannot be constrained in two-image systems with
single-component compact sources, whereas it can be constrained in systems with
two-component sources provided the separation between the image components can
be measured with milliarcsecond precisions, which is not feasible in most
systems. In contrast, we demonstrate that spatially extended images of the
source galaxy in two-image systems break the radial slope degeneracy and allow
D_dt to be measured with uncertainties of a few percent. Deep and
high-resolution imaging of the lens systems are needed to reveal the extended
arcs, and stable point spread functions are required for our lens modelling
technique. Two-image systems, no longer plagued by the radial profile slope
degeneracy, would augment the sample of useful time-delay lenses by a factor of
~6, providing substantial advances for cosmological studies.Comment: 14 pages, 9 figures, revisions based on referee's comments, accepted
for publication in MNRA
A Sampling Strategy for High-Dimensional Spaces Applied to Free-Form Gravitational Lensing
We present a novel proposal strategy for the Metropolis-Hastings algorithm
designed to efficiently sample general convex polytopes in 100 or more
dimensions. This improves upon previous sampling strategies used for free-form
reconstruction of gravitational lenses, but is general enough to be applied to
other fields. We have written a parallel implementation within the lens
modeling framework GLASS. Testing shows that we are able to produce uniform
uncorrelated random samples which are necessary for exploring the degeneracies
inherent in lens reconstruction.Comment: 10 pages, 9 figures. Accepted for publication in MNRA
Q2237+0305 in X-rays: spectra and variability with XMM-Newton
X-ray observations of gravitationally lensed quasars may allow us to probe
the inner structure of the central engine of a quasar. Observations of
Q2237+0305 (Einstein Cross) in X-rays may be used to constrain the inner
structure of the X-ray emitting source. Here we analyze the XMM-Newton
observation of the quasar in the gravitational lens system Q2237+0305 taken
during 2002. Combined spectra of the four images of the quasar in this system
were extracted and modelled with a power-law model. Statistical analysis was
used to test the variability of the total flux. The total X-ray flux from all
the images of this quadruple gravitational lens system is 6 x 10^{-13}
erg/cm2/s in the range 0.2-10 keV, showing no significant X-ray spectral
variability during almost 42 ks of the observation time. Fitting of the cleaned
source spectrum yields a photon power-law index of Gamma=1.82+0.07/-0.08. The
X-ray lightcurves obtained after background subtraction are compatible with the
hypothesis of a stationary flux from the source.Comment: 6 pages, 7 figures, 4 tables, accepted by A&
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