arXiv subjects

Joachim Wambsganss

Publications and source records attributed to Joachim Wambsganss.

At least 55 records · Page 3Linked to original sources

arXiv2003

Astrometric Microlensing of Quasars : Dependence on surface mass density and external shear

A small fraction of all quasars are strongly lensed and multiply imaged, with usually a galaxy acting as the main lens. Some, maybe all of these quasars are also affected by microlensing, the effects of stellar mass objects in the lensing galaxy. Stellar microlensing not only has photometric effects (magnitude fluctuations of the quasar images), it also affects the observed position of the images. This astrometric effect was first explored by Lewis and Ibata (1998): the position of the quasar - i.e. the center-of-light of the many microimages - can shift by tens of microarcseconds due to the relatively sudden (dis-)appearance of a pair of microimages when a caustic is being crossed. We explore this effect quantitatively for different values of the surface mass density and external shear covering most of the known multiple quasar systems. We show examples of microlens-induced quasar motion and the corresponding light curves for different quasar sizes. We evaluate statistically the occurrence of large shifts in angular position and their correlation with apparent brightness fluctuations. We also show statistical relations between positional offsets and time from random starting points. As the amplitude of the astrometric offset depends on the source size, astrometric microlensing signatures of quasars - combined with the photometric variations - will provide very good constraints on the size of quasars as a function of wavelength. We predict that such signatures will be detectable for realistic microlensing scenarios with near future technology in the infrared/optical (Keck-Interferometry, VLTI, SIM, GAIA). Such detections will show that not even high redshift quasars define a 'fixed' coordinate system.

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arXiv2003

The dark matter content of lensing galaxies at 1.5 R_e

Many gravitationally lensed quasars exhibit flux ratio "anomalies" that cannot be explained under the hypothesis that the lensing potential is smooth on scales smaller than one kpc. Micro-lensing by stars is a natural source of granularity in the lens potential. The character of the expected fluctuations due to micro-lensing depends sensitively on the relative surface densities of micro-lenses (stars) and smoothly distributed (dark) matter. Observations of flux ratios may therefore be used to infer the ratio of stellar to dark matter along the line of sight -- typically at impact parameters 1.5 times the half light radius. Several recently discovered systems have anomalies that would seem to be explained by micro-lensing only by demanding that 70-90% of the matter along the line of sight be smoothly distributed.

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arXiv2003

Giant Arc Statistics In Concordance With A Concordance LCDM Universe

The frequency of giant arcs - highly distorted and strongly gravitationally lensed background galaxies - is a powerful test for cosmological models. Previous comparisons of arc statistics for the currently favored concordance cosmological model (a flat lambda-dominated universe) with observations have shown an apparently large discrepancy. We present here new ray-shooting results, based on a high resolution (1024^3 particles in a 320 Mpc/h box) large-scale structure simulation normalized to the WMAP observations. We follow light rays through a pseudo-3D matter distribution approximated by up to 38 lens planes, and evaluate the occurrence of arcs for various source redshifts. We find that the frequency of strongly lensed background sources is a steep function of source redshift: the optical depth for giant arcs increases by a factor of five when the background sources are moved from redshift z_s = 1.0 to z_s = 1.5. This is a consequence of a moderate decrease of the critical surface mass density for lensing, combined with the very steep cluster mass function at the high mass end. Our results are consistent with those of Bartelmann et al. (1998) if we - as they did - restrict all sources to be exactly at z_s = 1. But if we allow for a more realistic distribution of source redshifts extending to or beyond z_s > 1.5, the apparent discrepancy vanishes: the frequency of arcs is increased by about a factor of ten as compared to previous estimates, and results in roughly one arc per 20 square degrees over the sky. This prediction for an LCDM model is then in good agreement with the observed frequency of arcs. Hence we consider the ``missing arc'' problem for a concordance LCDM cosmology to be solved.

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arXiv2002

The Mean Number of Extra Micro-Image Pairs for Macro-Lensed Quasars

When a gravitationally lensed source crosses a caustic, a pair of images is created or destroyed. We calculate the mean number of such pairs of micro-images $ $ for a given macro-image of a gravitationally lensed point source, due to microlensing by the stars of the lensing galaxy. This quantity was calculated by Wambsganss, Witt & Schneider (1992) for the case of zero external shear, $γ=0$, at the location of the macro-image. Since in realistic lens models a non-zero shear is expected to be induced by the lensing galaxy, we extend this calculation to a general value of $γ$. We find a complex behavior of $ $ as a function of $γ$ and the normalized surface mass density in stars $κ_*$. Specifically, we find that at high magnifications, where the average total magnification of the macro-image is $<μ>=|(1-κ_*)^2-γ^2|^{-1}\gg 1$, $ $ becomes correspondingly large, and is proportional to $<μ>$. The ratio $ /<μ>$ is largest near the line $γ=1-κ_*$ where the magnification $<μ>$ becomes infinite, and its maximal value is 0.306. We compare our semi-analytic results for $ $ to the results of numerical simulations and find good agreement. We find that the probability distribution for the number of extra image pairs is reasonably described by a Poisson distribution with a mean value of $ $, and that the width of the macro-image magnification distribution tends to be largest for $ \sim 1$.

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arXiv2002

Quasar Microlensing at High Magnification and the Role of Dark Matter: Enhanced Fluctuations and Suppressed Saddlepoints

Contrary to naive expectation, diluting the stellar component of the lensing galaxy in a highly magnified system with smoothly distributed ``dark'' matter increases rather than decreases the microlensing fluctuations caused by the remaining stars. For a bright pair of images straddling a critical curve, the saddlepoint (of the arrival time surface) is much more strongly affected than the associated minimum. With a mass ratio of smooth matter to microlensing matter of 4:1, a saddlepoint with a macro-magnification of mu = 9.5 will spend half of its time more than a magnitude fainter than predicted. The anomalous flux ratio observed for the close pair of images in MG0414+0534 is a factor of five more likely than computed by Witt, Mao and Schechter if the smooth matter fraction is as high as 93%. The magnification probability histograms for macro-images exhibit distinctly different structure that varies with the smooth matter content, providing a handle on the smooth matter fraction. Enhanced fluctuations can manifest themselves either in the temporal variations of a lightcurve or as flux ratio anomalies in a single epoch snapshot of a multiply imaged system. While the millilensing simulations of Metcalf and Madau also give larger anomalies for saddlepoints than for minima, the effect appears to be less dramatic for extended subhalos than for point masses. Morever, microlensing is distinguishable from millilensing because it will produce noticeable changes in the magnification on a time scale of a decade or less.

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arXiv2002

The Search for Matter with Gravitational Lensing

Gravitational lensing is a powerful tool to detect compact matter on very different mass scales. Of particular importance is the fact that lensing is sensitive to both luminous and dark matter alike. Depending on the mass scale, all lensing effects are used in the search for matter: offset in position, image distortion, magnification, and multiple images. Gravitational lens detections cover three main mass ranges: roughly stellar mass, galaxy mass and galaxy cluster mass scales, i.e. well known classes of objects. Various searches based on different techniques explored the frequency of compact objects over more than 15 orders of magnitude, so far mostly providing null results in mass ranges different from the ones just mentioned. Combined, the lensing results offer some interesting limits on the cosmological frequency of compact objects in the mass interval 10^{-3} <= M/M_odot <= 10^{15}, unfortunately still with some gaps in between. In the near future, further studies along these lines promise to fill the gaps and to push the limits further down, or they might even detect new object classes.

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arXiv2002

Gravitational microlensing of gamma-ray blazars

We present a detailed study of the effects of gravitational microlensing on compact and distant $γ$-ray blazars. These objects have $γ$-ray emitting regions which are small enough as to be affected by microlensing effects produced by stars lying in intermediate galaxies. We analyze the temporal evolution of the gamma-ray magnification for sources moving in a caustic pattern field, where the combined effects of thousands of stars are taken into account using a numerical technique. We propose that some of the unidentified $γ$-ray sources (particularly some of those lying at high galactic latitude whose gamma-ray statistical properties are very similar to detected $γ$-ray blazars) are indeed the result of gravitational lensing magnification of background undetected Active Galactic Nuclei (AGNs).

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arXiv2000

Gravitational Lensing - A brief review

Gravitational lensing has established itself as a valuable tool in many astrophysical fields in the roughly 20 years of its existence as an observational science. In this brief review, first the basics of lensing are introduced. Then it is shown that the two propagation effects, gravitational lensing and interstellar scintillation, have a number of properties in common. Finally, various lensing phenomena are discussed with emphasis on recent observations.

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arXiv2000

Microlensing results from APO monitoring of the double quasar Q0957+561A,B between 1995 and 1998

If the halo of the lensing galaxy 0957+561 is made of massive compact objects (MACHOs), they must affect the lightcurves of the quasar images Q0957+561 A and B differently. We search for this microlensing effect in the double quasar by comparing monitoring data for the two images A and B - obtained with the 3.5m Apache Point Observatory from 1995 to 1998 - with intensive numerical simulations. This way we test whether the halo of the lensing galaxy can be made of MACHOs of various masses. We can exclude a halo entirely made out of MACHOs with masses between 10e(-6) Msun and 10e(-2) Msun for quasar sizes of less than 3*10e(14)cm (h = 0.6), hereby extending previous limits upwards by one order of magnitude.

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arXiv2000

Microlensing of Quasars

There are two possible causes of variability in gravitationally lensed quasars: intrinsic fluctuations of the quasar and ``microlensing'' by compact objects along the line of sight. If disentangled from each other, microlens-induced variability can be used to study two cosmological issues of great interest: the size and brightness profile of quasars on one hand, and the distribution of compact (dark) matter along the line of sight on the other. Here we present a summary of recent observational evidence for quasar microlensing as well as of theoretical progress in the field. Particular emphasis is given to the questions which microlensing can address regarding the search for dark matter, both in the halos of lensing galaxies and in a cosmologically distributed form. A discussion of desired observations and required theoretical studies is presented as a conclusion/outlook.

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arXiv2000

Cosmological Microlensing

Variability in gravitationally lensed quasars can be due to intrinsic fluctuations of the quasar or due to ``microlensing'' by compact objects along the line of sight. If disentangled from each other, microlens-induced variability can be used to study two cosmological issues of great interest, the size and brightness profile of quasars on one hand, and the distribution of compact (dark) matter along the line of sight on the other. In particular, multi-waveband observations are useful for this goal. In this review recent theoretical progress as well as observational evidence for quasar microlensing over the last few years will be summarized. Comparison with numerical simulations will show where we stand. Particular emphasis will be given to the questions microlensing can address regarding the search for dark matter, both in the halos of lensing galaxies and in a cosmologically distributed form. A discussion of desired observations and required theoretical studies will be given as a conclusion/outlook.

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arXiv1999

The Gravitationally Lensed Quasar Q2237+0305 in X-rays: ROSAT/HRI Detection of the "Einstein Cross"

We report the first detection of the gravitationally lensed quasar Q2237+0305 in X-rays. With a ROSAT/HRI exposure of 53 ksec taken in Nov./Dec. 1997, we found a count rate of 0.006 counts per second for the combined four images. This corresponds to an X-ray flux of $2.2\times10^{-13} erg/cm^2$/sec and an X-ray luminosity of $4.2\times10^{45}$erg/sec (in the ROSAT energy window 0.1-2.4 keV). The ROSAT/HRI detector is not able to resolve spatially the four quasar images (maximum separation 1.8 arcsec). The analysis is based on about 330 source photons. The signal is consistent with no variability, but with low significance. This detection is promising in view of the upcoming X-ray missions with higher spatial/spectral resolution and/or collecting power (Chandra X-ray Observatory, XMM and ASTRO-E).

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arXiv1998

Gravitational Lensing in Astronomy

Deflection of light by gravity was predicted by General Relativity and observationaly confirmed in 1919. In the following decades various aspects of the gravitational lens effect were explored theoretically, among them the possibility of multiple or ring-like images of background sources, the use of lensing as a gravitational telescope on very faint and distant objects, and the possibility to determine Hubble's constant with lensing. Only relatively recently gravitational lensing became an observational science after the discovery of the first doubly imaged quasar in 1979. Today lensing is a booming part of astrophysics. In addition to multiply-imaged quasars, a number of other aspects of lensing have been discovered since, e.g. giant luminous arcs, quasar microlensing, Einstein rings, galactic microlensing events, arclets, or weak gravitational lensing. By now literally hundreds of individual gravitational lens phenomena are known. Although still in its childhood, lensing has established itself as a very useful astrophysical tool with some remarkable successes. It has contributed significant new results in areas as different as the cosmological distance scale, the large scale matter distribution in the universe, mass and mass distribution of galaxy clusters, physics of quasars, dark matter in galaxy halos, or galaxy structure.

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arXiv1998

Limits on MACHOs from microlensing in the double quasar Q0957+561

The light curves of the two images of the double quasar Q0957+561 as obtained by Kundic et al. (1997) are almost identical, except for an overall time delay and scaling factor. This allows us to put limits on the amount of microlensing that took place during the time interval corresponding to the monitoring observations. We perform numerical simulations in which we model the microlensing behaviour of the (halo of the) lensing galaxy in the system. We test ``MACHO-masses'' ranging from 10^{-8} to 10^{-1} M_{sun} and quasar sizes from 10^{14} to 3\times 10^{15} cm. Statistically comparing the expected microlensing-induced changes from 100000 simulated light curves over a period of 160 days with the (lack of) observed fluctuations, we can constrain regions in the parameter space of MACHO mass and quasar size with various degrees of confidence. In particular, a halo consisting of objects at the low end of our mass scale can be ruled out with high confidence for a small quasar size. A halo consisting of objects with 10^{-2} or 10^{-1} M_{sun} cannot be ruled out yet, but it should produce MACHO induced fluctuations in future observations. We also test halos with only 50% or 25% of the mass in compact objects; constraints here are a bit less stringent.

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arXiv1997

A Robust Determination of the Time Delay in 0957+561A,B and a Measurement of the Global Value of Hubble's Constant

Photometric monitoring of the gravitational lens system 0957+561A,B in the g and r bands with the Apache Point Observatory (APO) 3.5 m telescope during 1996 shows a sharp g band event in the trailing (B) image light curve at the precise time predicted from the observation of an event during 1995 in the leading (A) image with a delay of 415 days. This success confirms the "short delay," and the lack of any feature at a delay near 540 days rejects the "long delay" for this system, resolving a long-standing controversy. A series of statistical analyses of our light curve data yield a best fit delay of 417 +/- 3 days (95% confidence interval). Recent improvements in the modeling of the lens system (consisting of a galaxy and cluster) allow us to derive a value of the global (at z = 0.36) value of Hubble's constant H_0 using Refsdal's method, a simple and direct distance determination based on securely understood physics and geometry. The result is H_0 = 63 +/- 12 km/s/Mpc (for Omega = 1) where this 95% confidence interval is dominated by remaining lens model uncertainties.

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arXiv1996

Discovering Galactic Planets by Gravitational Microlensing: Magnification Patterns and Light Curves

The current searches for microlensing events towards the galactic bulge can be used to detect planets around the lensing stars. Their effect is a short-term modulation on the smooth lightcurve produced by the main lensing star. Current and planned experiments should be sensitive enough to discover planets ranging from Jupiter mass down to Earth mass. For a successful detection of planets, it is necessary to accurately and frequently monitor a microlensing event photometrically, once it has been "triggered". We present a large variety of two-dimensional magnification distributions for systems consisting of an ordinary star and a planetary companion. We cover planet/star mass ratios from $m_{pl}/M_* = 10^{-5}$ to $ 10^{-3}$. These limits correspond roughly to $M_{Earth}$ and $M_{Jup}$, for a typical lens mass of $M_* = 0.3 M_{\odot}$. We explore a range of star-planet distances, with particular emphasis on the case of "resonant lensing", a situation in which the planet is located at or very near the Einstein ring of the lensing star. We show a wide selection of light curves - one dimensional cuts through the magnification patterns - to illustrate the broad range of possible light curve perturbations caused by planets. The strongest effects are to be expected for caustic crossings. But even tracks passing outside the caustics can have considerable effects on the light curves. The easiest detectable (projected) distance range for the planets is between about 0.6 and 1.6 Einstein radii. Planets in this distance range produce caustics inside the Einstein ring of the star. For a lensing star with a mass of about $0.3 M_{\odot}$ at a distance of 6 kpc and a source at 8 kpc, this corresponds to physical distances between star and planet of about 1 to 3 AU.

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arXiv1996

Testing Cosmological Models by Gravitational Lensing: I. Method and First Applications

Gravitational lensing directly measures mass density fluctuations along the lines of sight to very distant objects. No assumptions need to be made concerning bias, the ratio of fluctuations in galaxy density to mass density. Hence, lensing is a very useful tool to study the universe at low to moderate redshifts. We describe in detail a new method to trace light rays from redshift zero through a three dimensional mass distribution to high redshift. As an example, this method is applied here to a standard cold dark matter universe. We obtain a variety of results, some of them statistical in nature, others from rather detailed case studies of individual ``lines of sight". Among the former are the frequency of multiply imaged quasars, the distribution of separation of the multiple quasars, and the redshift distribution of lenses: all that as a function of quasar redshift. We find effects from very weak lensing to highly magnified multiple images of high redshift objects, which, for extended background sources, (i.e. galaxies), range from slight deformations of the shapes through tangentially aligned arclets up to giant luminous arcs. Different cosmological models differ, increasingly with redshift, in their predictions for the mass (thus gravitational potential) distributions. Our ultimate goal is to apply this method to a number of cosmogonic models and to eliminate some models whose gravitational lensing properties are inconsistent with those observed.

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arXiv1996

Effects of Weak Gravitational Lensing from Large-Scale Structure on the Determination of $q_0$

Weak gravitational lensing by large-scale structure affects the determination of the cosmological deceleration parameter $q_0$. We find that the lensing induced dispersions on truly standard candles are $0.04$ and $0.02$ mag at redshift $z=1$ and $z=0.5$, respectively, in a COBE-normalized cold dark matter universe with $Ω_0=0.40$, $Λ_0=0.6$, $H=65$km/s/Mpc and $σ_8=0.79$. It is shown that one would observe $q_0=-0.44^{+0.17}_{-0.05}$ and $q_0=-0.45^{+0.10}_{-0.03}$ (the errorbars are $2σ$ limits) with standard candles with zero intrinsic dispersion at redshift $z=1$ and $z=0.5$, respectively, compared to the truth of $q_0=-0.40$ in this case, i.e., a 10\% error in $q_0$ will be made. A standard COBE normalized $Ω_0=1$ CDM model would produce three times as much variance and a mixed (hot and cold) dark matter model would lead to an intermediate result. One unique signature of this dispersion effect is its non Gaussianity. Although the lensing induced dispersion at lower redshift is still significantly smaller than the currently best observed (total) dispersion of $0.12$ mag in a sample of type Ia supernovae, selected with the multicolor light curve shape method, it becomes significant at higher redshift. We show that there is an optimal redshift, in the range $z\sim 0.5-2.0$ depending on the amplitude of the intrinsic dispersion of the standard candles, at which $q_0$ can be most accurately determined.

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