25,785 research outputs found
On Multiple Einstein Rings
A number of recent surveys for gravitational lenses have found examples of
double Einstein rings. Here, we investigate analytically the occurrence of
multiple Einstein rings. We prove, under very general assumptions, that at most
one Einstein ring can arise from a mass distribution in a single plane lensing
a single background source. Two or more Einstein rings can therefore only occur
in multi-plane lensing. Surprisingly, we show that it is possible for a single
source to produce more than one Einstein ring. If two point masses (or two
isothermal spheres) in different planes are aligned with observer and source on
the optical axis, we show that there are up to three Einstein rings. We also
discuss the image morphologies for these two models if axisymmetry is broken,
and give the first instances of magnification invariants in the case of two
lens planes.Comment: MNRAS, in press (extra figure included
Generating solutions via sigma-models
We review recent development of solution-generating techniques for four and
five-dimensional Einstein equations coupled to vector and scalar fields. This
includes D=4 Einstein-Maxwell-dilaton-axion theory with multiple vector fields,
D=5 Einstein-Maxwell gravity with the Chern-Simons term (minimal
five-dimensional supergravity), and some other models which attracted attention
in connection with black rings. The method is based on reduction to
three-dimensional gravity coupled sigma-models with symmetric target spaces.
Our recent results open a way to construct the general charged black rings in
five-dimensional supergravity possibly coupled to vector multiplets.Comment: An updated version of the talk given at ICGA8 and published in the
Proceedings. 14 pages, ptpte
Comparison of an X-ray selected sample of massive lensing clusters with the MareNostrum Universe LCDM simulation
A long-standing problem of strong lensing by galaxy clusters regards the
observed high rate of giant gravitational arcs as compared to the predictions
in the framework of the "standard" cosmological model. Recently, few other
inconsistencies between theoretical expectations and observations have been
claimed which regard the large size of the Einstein rings and the high
concentrations of few clusters with strong lensing features. All of these
problems consistently indicate that observed galaxy clusters may be
gravitational lenses stronger than expected. We use clusters extracted from the
MareNostrum Universe to build up mock catalogs of galaxy clusters selected
through their X-ray flux. We use these objects to estimate the probability
distributions of lensing cross sections, Einstein rings, and concentrations for
the sample of 12 MACS clusters at presented in Ebeling et al. (2007)
and discussed in Zitrin et al. (2010). We find that simulated clusters produce
less arcs than observed clusters do. The medians of the
distributions of the Einstein ring sizes differ by between
simulations and observations. We estimate that, due to cluster triaxiality and
orientation biases affecting the lenses with the largest cross sections, the
concentrations of the individual MACS clusters inferred from the lensing
analysis should be up to a factor of larger than expected from the
CDM model. The arc statistics, the Einstein ring, and the
concentration problems in strong lensing clusters are mitigated but not solved
on the basis of our analysis. Nevertheless, due to the lack of redshifts for
most of the multiple image systems used for modeling the MACS clusters, the
results of this work will need to be verified with additional data. The
upcoming CLASH program will provide an ideal sample for extending our
comparison (abridged).Comment: 11 pages, 9 figures, accepted for publication on A&
Light Propagation in Inhomogeneous Universes. IV. Strong Lensing and Environmental Effects
We study the gravitational lensing of high-redshift sources in a LCDM
universe. We have performed a series of ray-tracing experiments, and selected a
subsample of cases of strong lensing (multiple images, arcs, and Einstein
rings). For each case, we identify a massive galaxy that is primarily
responsible for lensing, and studied how the various density inhomogeneities
along the line of sight (other galaxies, background matter) affect the
properties of the image. The matter located near the lensing galaxy, and
physically associated with it, has a small effect. The background matter
increases the magnification by a few percents at most, while nearby galaxies
can increase it by up to about 10 percent. The effect on the image separation
is even smaller. The only significant effect results from the random alignment
of physically unassociated galaxies, which can increase the magnification by
factors of several, create additional images, and turn arcs into rings. We
conclude that the effect of environment on strong lensing in negligible in
general, and might be important only in rare cases. We show that our conclusion
does not depend on the radial density profile of the galaxies responsible for
lensing.Comment: 23 pages, 7 figures (one in color). Accepted for publication in The
Astrophysical Journal. Minor typos correcte
Visual Distortions Near a Neutron Star and Black Hole
The visual distortion effects visible to an observer traveling around and
descending to the surface of an extremely compact star are described.
Specifically, trips to a ``normal" neutron star, a black hole, and an
ultracompact neutron star with extremely high surface gravity, are described.
Concepts such as multiple imaging, red- and blue-shifting, conservation of
surface brightness, the photon sphere, and the existence of multiple Einstein
rings are discussed in terms of what the viewer would see. Computer generated,
general relativistically accurate illustrations highlighting the distortion
effects are presented and discussed. A short movie (VHS) depicting many of
these effects is available to those interested free of charge.Comment: 23 pages, Plain TeX (v. 3.0), figures in American Journal of Physics,
61, 619, 1993, video available upon written (hard copy) request onl
A PCA-based automated finder for galaxy-scale strong lenses
We present an algorithm using Principal Component Analysis (PCA) to subtract
galaxies from imaging data, and also two algorithms to find strong,
galaxy-scale gravitational lenses in the resulting residual image. The combined
method is optimized to find full or partial Einstein rings. Starting from a
pre-selection of potential massive galaxies, we first perform a PCA to build a
set of basis vectors. The galaxy images are reconstructed using the PCA basis
and subtracted from the data. We then filter the residual image with two
different methods. The first uses a curvelet (curved wavelets) filter of the
residual images to enhance any curved/ring feature. The resulting image is
transformed in polar coordinates, centered on the lens galaxy center. In these
coordinates, a ring is turned into a line, allowing us to detect very faint
rings by taking advantage of the integrated signal-to-noise in the ring (a line
in polar coordinates). The second way of analysing the PCA-subtracted images
identifies structures in the residual images and assesses whether they are
lensed images according to their orientation, multiplicity and elongation. We
apply the two methods to a sample of simulated Einstein rings, as they would be
observed with the ESA Euclid satellite in the VIS band. The polar coordinates
transform allows us to reach a completeness of 90% and a purity of 86%, as soon
as the signal-to-noise integrated in the ring is higher than 30, and almost
independent of the size of the Einstein ring. Finally, we show with real data
that our PCA-based galaxy subtraction scheme performs better than traditional
subtraction based on model fitting to the data. Our algorithm can be developed
and improved further using machine learning and dictionary learning methods,
which would extend the capabilities of the method to more complex and diverse
galaxy shapes
Scattering and leapfrogging of vortex rings in a superfluid
The dynamics of vortex ring pairs in the homogeneous nonlinear Schr\"odinger
equation is studied. The generation of numerically-exact solutions of traveling
vortex rings is described and their translational velocity compared to revised
analytic approximations. The scattering behavior of co-axial vortex rings with
opposite charge undergoing collision is numerically investigated for different
scattering angles yielding a surprisingly simple result for its dependence as a
function of the initial vortex ring parameters. We also study the leapfrogging
behavior of co-axial rings with equal charge and compare it with the dynamics
stemming from a modified version of the reduced equations of motion from a
classical fluid model derived using the Biot-Savart law.Comment: 12 pages, 11 figure
- …