Simple Lens Models Research Articles

Analytic Time Delays andH0Estimates for Gravitational Lenses

We study gravitational lens time delays for a general family of lensing potentials, which includes the popular singular isothermal elliptical potential and singular isothermal elliptical density distribution but allows general angular structure. Using a novel approach, we show that the time delay can be cast in a very simple form, depending only on the observed image positions. Including an external shear changes the time delay proportional to the shear strength, and varying the radial profile of the potential changes the time delay approximately linearly. These analytic results can be used to obtain simple estimates of the time delay and the Hubble constant in observed gravitational lenses. The naive estimates for four of five time delay lenses show surprising agreement with each other and with local measurements of H_0; the complicated Q 0957+561 system is the only outlier. The agreement suggests that it is reasonable to use simple isothermal lens models to infer H_0, although it is still important to check this conclusion by examining detailed models and by measuring more lensing time delays.

Cosmological Parameters from Multiple‐Arc Gravitational Lensing Systems. I. Smooth Lensing Potentials

We describe a new approach for the determination of cosmological parameters using gravitational lensing systems with multiple arcs, exploiting the fact that a given cluster can produce multiple arcs from sources over a broad range in redshift. The coupling between the critical radius of a single arc and the projected mass density of the lensing cluster can be avoided by considering the relative positions of two or more arcs. Cosmological sensitivity appears through the angular size-redshift relation. We consider simulated data constructed using a more general form for the potential, realistic sources, and an assumed cosmology and present a method for simultaneously inverting the lens and extracting the cosmological parameters. The input data required are the image and measured redshifts for the arcs. The technique relies upon the conservation of surface brightness in gravitationally lensed systems. We find that for a simple lens model our approach can recover the cosmological parameters assumed in the construction of the simulated images.

Low Surface Brightness Radio Structure in the Field of Gravitational Lens 0957+561

We have produced deep radio maps of the double quasar 0957+561 from multiple-epoch VLA observations. To achieve high sensitivity to extended structure we have rereduced the best available 1.6 GHz observations and have combined 5 GHz data from multiple array configurations. Regions of faint emission approximately 15'' north and south of the radio source G are probably lobes associated with the lensing galaxy. An arc 5'' to the east of G may be a stretched image of emission in the background quasar's environment. At 14 southwest of G we detect a source that we interpret as an image of emission from the quasar's western lobe, which could provide a constraint on the slope of the gravitational potential in the central region of the lens. We explore the consequences of these new constraints with simple lens models of the system.

1608+656: A Quadruple Lens System Found In The CLASS Gravitational Lens Survey

The first phase of a large gravitational lens survey using the Very Large Array at a wavelength of 3.6 cm has been completed, yielding images for 3258 radio sources. The Cosmic Lens All-Sky Survey, or CLASS, is designed to locate gravitational lens systems consisting of multiply-imaged compact components with separations > 0.″2. From this first phase has come the discovery of 1608+656, a quadruply-imaged object with maximum separation of 2.″1. Images from the Palomar 5-m and Keck 10-m telescopes show the lensed images and the lensing galaxy. An optical spectrum obtained with the Palomar 5-m Telescope indicates a redshift of z = 0.63 for the lensing galaxy, and a newly-obtained Palomar spectrum indicates a redshift of z = 1.39 for the lensed source, which appears to be a galaxy. A simple single-galaxy lens model derived from the radio image reproduces the observed configuration and relative fluxes of the images, as well as the position, shape, and orientation of the lensing galaxy. Because a simple mass model is able to fit the observations, we argue that this lens system is promising for determining H0. CLASS has also yielded the new double image lens system 1600+434. The second phase of the survey is scheduled for August and September 1995 on the VLA, and should yield images for an additional 5000+ targets, bringing the CLASS total to over 8000.

Macrolensing, microlensing, and BL Lac objects

It has been suggested that BL Lac objects may result from the gravitational lensing of quasars by foreground galaxies. A simple lens model is used to demonstrate that the observed properties of BL Lac objects are not consistent with this hypothesis. The absence of multiply imaged BL Lac objects rules out pure macrolensing by the overall potential of a foreground galaxy. This objection can be overcome by supposing that the production of a BL Lac object also requires microlensing by individual compact objects in the foreground galaxy

A gravitational lens candidate discovered with the Hubble Space Telescope

Evidence is reported for gravitational lensing of the high-redshift (z = 3.8) quasar 1208 + 101, observed as part of the Snapshot survey with the HST Planetary Camera. An HST V image taken on gyroscopes resolves the quasar into three point-source components, with the two fainter images having separations of 0.1 and 0.5 arcsec from the central bright component. A radio observation of the quasar with the VLA at 2 cm shows that, like most quasars of this redhsift, 1208 + 101 is radio quiet. Based on positional information alone, the probability that the observed optical components are chance superpositions of Galactic stars is small, but not negligible. Analysis of a combined ground-based spectrum of all three components, using the relative brightnesses of the HST image, supports the lensing hypothesis. If all the components are lensed images of the quasar, the observed configuration cannot be reproduced by simple lens models.

Gravitational Lens Optics

Several instances of multiple imaging of cosmologically distant sources by intervening galaxies and galaxy clusters have been discovered over the past decade. These "gravitational lenses" have distinctive optical properties. Pointlike sources such as quasars generally produce two or four images when lensed, whereas extended sources such as galaxies produce spectacular arcs and rings. The salient features of most of the observations can be reproduced with the use of simple elliptical lens models that approximate the lenses made by ellipsoidal mass distributions such as are common in the universe. In addition to illustrating simple optics in operation on a cosmological scale, multiple images and arcs provide useful probes of the lensing galaxies and clusters. Also, gravitational lenses can make magnified images of cosmologically distant sources and may eventually furnish important cosmographic data such as the Hubble constant.

A gravitational lens explanation for the arclike structure in CL 0500 - 24

view Abstract Citations (20) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Gravitational Lens Explanation for the Arclike Structure in CL 0500-24 Wambsganss, Joachim ; Giraud, Edmond ; Schneider, Peter ; Weiss, Achim Abstract We present two simple gravitational lens models for the arclike structure in the rich cluster Cl 0500-24. These models are constructed in accordance with the observations, which suggest that beside the cluster, two galaxies are important for the light deflection. Our models account naturally for the shape as well as for the orientation of the observed elongated feature. The lens parameter can be determined to within about 20%. We conclude that gravitational lensing of a background galaxy is a possible explanation for this arclike structure. Publication: The Astrophysical Journal Pub Date: February 1989 DOI: 10.1086/185381 Bibcode: 1989ApJ...337L..73W Keywords: Galactic Clusters; Galactic Structure; Gravitational Lenses; Interacting Galaxies; Galactic Evolution; Mass Distribution; Mass To Light Ratios; Red Shift; Astrophysics; GALAXIES: CLUSTERING; GRAVITATIONAL LENSES full text sources ADS | data products NED (6) SIMBAD (2)

Acoustic refraction by a spark discharge in air

It is observed that, when the acoustic impulse from an electric spark is reflected normally from a rigid plate and detected after passage through the remains of the spark region, a precursor component appears in the received waveform. To explain this phenomenon in terms of refraction, a two-spark experiment is used to examine the acoustic properties of the post-discharge region near a spark gap. The measured times of arrival of the precursor signal are fitted to a simple lens model consisting of two parameters: lens radius and acoustic index of refraction. The temporal evolution of these lens parameters is also measured and interpreted in terms of a diffusion model. The diffusion coefficient characterizing the process is found to be much larger than that for thermal diffusion in air at room temperature. This is consistent with the much higher temperatures corresponding to the low acoustic index of refraction of the measured lens models.

2237 + 0305 - A new and unusual gravitational lens

A new gravitational lens system has been discovered which consists of a quasar at z = 1.695 nearly centered on a 15-mag spiral galaxy, 2237 + 0305, at z = 0.0394. At 2 arcsec resolution, only a single optical image of the quasar is visible; its centroid is located within approximately 0.3 arcsec of the center of the galaxy. 'Snapshot' observations at the VLA yielded no detectable radio radiation, placing an upper limit of about 0.5 mJy on the combined flux density of the galaxy and quasar at a wavelength of 6 cm. A simple gravitational lens model that accounts for these observations is discussed.