705 research outputs found
Mass-sheet degeneracy, power-law models and external convergence: Impact on the determination of the Hubble constant from gravitational lensing
The light travel time differences in strong gravitational lensing systems
allows an independent determination of the Hubble constant. This method has
been successfully applied to several lens systems. The formally most precise
measurements are, however, in tension with the recent determination of
from the Planck satellite for a spatially flat six-parameters
cosmology. We reconsider the uncertainties of the method, concerning the mass
profile of the lens galaxies, and show that the formal precision relies on the
assumption that the mass profile is a perfect power law. Simple analytical
arguments and numerical experiments reveal that mass-sheet like transformations
yield significant freedom in choosing the mass profile, even when exquisite
Einstein rings are observed. Furthermore, the characterization of the
environment of the lens does not break that degeneracy which is not physically
linked to extrinsic convergence. We present an illustrative example where the
multiple imaging properties of a composite (baryons + dark matter) lens can be
extremely well reproduced by a power-law model having the same velocity
dispersion, but with predictions for the Hubble constant that deviate by . Hence we conclude that the impact of degeneracies between parametrized
models have been underestimated in current measurements from lensing, and
need to be carefully reconsidered.Comment: Accepted for publication in Astronomy and Astrophysics. Discussion
expanded (MSD and velocity dispersion, MSD and free form lens models, MSD and
multiple source redshifts
Source-position transformation -- an approximate invariance in strong gravitational lensing
The main obstacle for gravitational lensing to determine accurate masses of
deflectors, or to determine precise estimates for the Hubble constant, is the
degeneracy of lensing observables with respect to the mass-sheet transformation
(MST). The MST is a global modification of the mass distribution which leaves
all image positions, shapes and flux ratios invariant, but which changes the
time delay. Here we show that another global transformation of lensing mass
distributions exists which almost leaves image positions and flux ratios
invariant, and of which the MST is a special case. Whereas for axi-symmetric
lenses this source position transformation exactly reproduces all strong
lensing observables, it does so only approximately for more general lens
situations. We provide crude estimates for the accuracy with which the
transformed mass distribution can reproduce the same image positions as the
original lens model, and present an illustrative example of its performance.
This new invariance transformation most likely is the reason why the same
strong lensing information can be accounted for with rather different mass
models.Comment: Submitted to Astronomy and Astrophysics. Comments welcome. 9 page
Imprints of the quasar structure in time-delay light curves: Microlensing-aided reverberation mapping
Owing to the advent of large area photometric surveys, the possibility to use
broad band photometric data, instead of spectra, to measure the size of the
broad line region of active galactic nuclei, has raised a large interest. We
describe here a new method using time-delay lensed quasars where one or several
images are affected by microlensing due to stars in the lensing galaxy. Because
microlensing decreases (or increases) the flux of the continuum compared to the
broad line region, it changes the contrast between these two emission
components. We show that this effect can be used to effectively disentangle the
intrinsic variability of those two regions, offering the opportunity to perform
reverberation mapping based on single band photometric data. Based on simulated
light curves generated using a damped random walk model of quasar variability,
we show that measurement of the size of the broad line region can be achieved
using this method, provided one spectrum has been obtained independently during
the monitoring. This method is complementary to photometric reverberation
mapping and could also be extended to multi-band data. Because the effect
described above produces a variability pattern in difference light curves
between pairs of lensed images which is correlated with the time-lagged
continuum variability, it can potentially produce systematic errors in
measurement of time delays between pairs of lensed images. Simple simulations
indicate that time-delay measurement techniques which use a sufficiently
flexible model for the extrinsic variability are not affected by this effect
and produce accurate time delays.Comment: Accepted for publication in Astronomy and Astrophysic
Optical linear polarization measurements of quasars obtained with the 3.6m telescope at the La Silla Observatory
We report 192 previously unpublished optical linear polarization measurements
of quasars obtained in April 2003, April 2007, and October 2007 with the
European Southern Observatory Faint Object Spectrograph and Camera (EFOSC2)
instrument attached to the 3.6m telescope at the La Silla Observatory. Each
quasar was observed once. Among the 192 quasars, 89 have a polarization degree
, 18 have , and two have .Comment: Accepted for publication in A&
Alignment of quasar polarizations with large-scale structures
We have measured the optical linear polarization of quasars belonging to
Gpc-scale quasar groups at redshift z ~ 1.3. Out of 93 quasars observed, 19 are
significantly polarized. We found that quasar polarization vectors are either
parallel or perpendicular to the directions of the large-scale structures to
which they belong. Statistical tests indicate that the probability that this
effect can be attributed to randomly oriented polarization vectors is of the
order of 1%. We also found that quasars with polarization perpendicular to the
host structure preferentially have large emission line widths while objects
with polarization parallel to the host structure preferentially have small
emission line widths. Considering that quasar polarization is usually either
parallel or perpendicular to the accretion disk axis depending on the
inclination with respect to the line of sight, and that broader emission lines
originate from quasars seen at higher inclinations, we conclude that quasar
spin axes are likely parallel to their host large-scale structures.Comment: Accepted for publication in Astronomy and Astrophysic
The different origins of high- and low-ionization broad emission lines revealed by gravitational microlensing in the Einstein cross
We investigate the kinematics and ionization structure of the broad emission
line region of the gravitationally lensed quasar QSO2237+0305 (the Einstein
cross) using differential microlensing in the high- and low-ionization broad
emission lines. We combine visible and near-infrared spectra of the four images
of the lensed quasar and detect a large-amplitude microlensing effect
distorting the high-ionization CIV and low-ionization H line profiles
in image A. While microlensing only magnifies the red wing of the Balmer line,
it symmetrically magnifies the wings of the CIV emission line. Given that the
same microlensing pattern magnifies both the high- and low-ionization broad
emission line regions, these dissimilar distortions of the line profiles
suggest that the high- and low-ionization regions are governed by different
kinematics. Since this quasar is likely viewed at intermediate inclination, we
argue that the differential magnification of the blue and red wings of
H favors a flattened, virialized, low-ionization region whereas the
symmetric microlensing effect measured in CIV can be reproduced by an emission
line formed in a polar wind, without the need of fine-tuned caustic
configurations.Comment: 11 pages, 4 figures, A&
Large scale correlations of quasar polarisation vectors: Hints of extreme scale structures?
A survey measuring quasar polarization vectors has been started in two
regions towards the North and South Galactic Poles. Here, We review the
discovery of significant correlations of orientations of polarization vectors
over huge angular distances. We report new results including a larger sample of
the quasars confirming the existence of coherent orientations at redshifts z>1.Comment: Proc. of Astronomical Polarimetry - Current Status and Future
Directions March 15-19, 2004. Waikoloa Beach Marriott, Hawaii. 5 pages, 3
figure
Mid-infrared microlensing of accretion disc and dusty torus in quasars: effects on flux ratio anomalies
Multiply-imaged quasars and AGNs observed in the mid-infrared (MIR) range are
commonly assumed to be unaffected by the microlensing produced by the stars in
their lensing galaxy. In this paper, we investigate the validity domain of this
assumption. Indeed, that premise disregards microlensing of the accretion disc
in the MIR range, and does not account for recent progress in our knowledge of
the dusty torus. To simulate microlensing, we first built a simplified image of
the quasar composed of an accretion disc, and of a larger ring-like torus. The
mock quasars are then microlensed using an inverse ray-shooting code. We
simulated the wavelength and size dependence of microlensing for different
lensed image types and fraction of compact objects projected in the lens. This
allows us to derive magnification probabilities as a function of wavelength, as
well as to calculate the microlensing-induced deformation of the spectral
energy distribution of the lensed images. We find that microlensing variations
as large as 0.1 mag are very common at 11 microns (observer-frame). The main
signal comes from microlensing of the accretion disc, which may be significant
even when the fraction of flux from the disc is as small as 5 % of the total
flux. We also show that the torus of sources with Lbol <~ 10^45 erg/s is
expected to be noticeably microlensed. Microlensing may thus be used to get
insight into the rest near-infrared inner structure of AGNs. Finally, we
investigate whether microlensing in the mid-infrared can alter the so-called
Rcusp relation that links the fluxes of the lensed images triplet produced when
the source lies close to a cusp macro-caustic. This relation is commonly used
to identify massive (dark-matter) substructures in lensing galaxies. We find
that significant deviations from Rcusp may be expected, which means that
microlensing can explain part of the flux ratio problem.Comment: Updated to match the version published in Astronomy and Astrophysics.
12 pages. Abridged version of the abstract. Microlensing maps and source
profiles used in the simulations are available via CDS -
http://vizier.cfa.harvard.edu/viz-bin/VizieR?-source=J/A+A/553/A5
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