S. Nair

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 (CLASS) is designed to locate gravitational lens systems consisting of multiply imaged compact components with separations greater than 02. We report here the first discovery of a gravitational lens from the survey: 1608+656, a quadruply imaged object with a maximum separation of 21. 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=0.6304 for the lensing galaxy. No conclusive redshift for the lensed object has been determined, although a single strong emission line is found at 9240 A in the Keck low-resolution imaging spectrograph spectrum. The two most likely identifications for this line are Hβ (z=0.90) and Mg II (z=2.30). The preliminary 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.

PKS 1830-211 as a gravitationally lensed system

The unusually strong, flat-spectrum, 1″ separation double radio source, PKS 1830-211, which has been found to exhibit a connecting radio ringlike feature, is believed to be a gravitationally lensed system. It is attempted here to reconcile the available multifrequency radio observations with the lensing hypothesis. The source can be modeled as a core-knot-jet structure; this suggests an explanation for the differing radio spectra of the two dominant features which are believed to be images of the compact part of the source. Simulations of the extended structure at 8 GHz and 15 GHz are presented. Our model of the system indicates that a faint feature that has shown up in previous observations is probably a combined third/fifth image of some portions of the source

X-Ray Absorption Toward the Einstein Ring Source PKS 1830-211

PKS 1830-211 is an unusually radio-loud gravitationally lensed quasar. In the radio spectrum, the system appears as two compact, dominant features surrounded by relatively extended radio emission that forms an Einstein ring. As the line of sight to it passes close to our Galactic center, PKS 1830-211 has not been detected in wave bands other than the radio and X-ray so far. Here we present X-ray data of PKS 1830-211 observed with ROSAT Position Sensitive Proportional Counter. The X-ray spectrum shows that absorption in excess of the Galactic contribution is highly likely, which at the redshift of the lensing galaxy (zl = 0.886) corresponds to NH=3.5−0.5+0.6×1022 atoms cm-2. The effective optical extinction is large, AV(observed) 5.8. When corrected for this additional extinction, the two-point optical to X-ray slope αox of PKS 1830-211 lies just within the observed range of quasars. It is argued here that both compact images must be covered by the X-ray absorber(s) that we infer to be the lensing galaxy (galaxies). The dust-to-gas ratio along the line of sight within the lensing galaxy is likely to be somewhat larger than for our Galaxy.

Modelling the 10‐image lensed system B1933+503

A gravitational lens model is presented for the newly discovered 10-image system B1933+503. The underlying object, revealed by modelling, is a triple radio source on the scale of a couple of hundred mas that is well-aligned along the line of sight with a foreground and somewhat flattened lensing galaxy, the orientation and location of which match those of an observed galaxy, known to be at a redshift of 0.755. Uncertainties in the modelling are obtained by a Monte Carlo exercise. Observational tests of the lens model an proposed, and the time delays between various pairs of images are determined, as the core of the source is known to be significantly variable. Future observations of the lens hold the key to using B1933+503 to constrain Hubbles constant. Despite the absence of a source redshift, the utility of the system as a probe of the structure of the lens galaxy is unparalleled as it provides a surfeit of easily identifiable constraints for modelling the system.

Multi-epoch VLBA 15 and 43 GHz observations of the gravitational lens PKS 1830-211: Extending VLBI towards the micro-arcsecond scale

Abstract We present multi-epoch, VLBA maps of PKS 1830-211 at 15 and 43 GHz. The 43 GHz observations reveal complicated submilliarcsecond radio structure in the cores of both lensed images. This structure appears to change dramatically between the two epochs (separated by 44 days). One explanation for these unusually rapid structural changes is that the lens magnifies each image, such that the nominal submilliarcsecond scale of these maps is enhanced by a factor of between 5 and 10—effectively boosting our resolution towards the micro-arcsecond scale.

The JVAS/Class Gravitational Lens Surveys

The JVAS/CLASS surveys are designed to find gravitational lens systems in which the flat spectrum cores of radio sources are multiply imaged. The prime motivation for the searches is to find systems from which the Hubble constant can be determined by measuring time delays and to produce statistically clean samples of lenses which can be used for cosmological investigations. Up to now some 8000 flat spectrum sources have been mapped with the VLA with a resolution of 200 mas. When complete we hope to have surveyed 10,000 sources. High resolution MERLIN and VLBA follow-up of the candidates is still in progress. To date 11 lens systems have been discovered. Time delays have been measured for 2 of these systems. HST and other observations almost always show a lensing galaxy so there is no evidence for “dark lenses”. The maximum image separations peak between 1 and 2 arcsec.

Properties of Lensing Galaxies in the Class Survey

Followup of lenses discovered in both the JVAS (Jodrell Bank-VLA Astrometric Survey) and CLASS (Cosmic Lens All-Sky Survey) is underway. Here we report on new Hubble Space Telescope (HST) and MERLIN observations of a sample of CLASS gravitational lenses. All the lensing galaxies are detected, and all of the lensed images are detected in all but one of the systems. The lensing galaxies appear to be surprisingly elongated and not especially centrally concentrated. We argue that, in at least four of the five systems, we are dealing with disk lenses rather than massive elliptical systems.