The CLASS Gravitational Lens Survey : Survey Selection and Early Results for 3271 Radio Sources
Session 23 -- Gravitational Lensing
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

## [23.01] The CLASS Gravitational Lens Survey : Survey Selection and Early Results for 3271 Radio Sources

C.D. Fassnacht, S.T. Myers, T.J. Pearson, A.C. Readhead (Caltech), I.W.A. Browne, P.N. Wilkinson (NRAL Jodrell Bank), N. Jackson, G. Miley (Leiden University), G. de Bruyn, R. Schilizzi (NFRA Dwingeloo)

The Cosmic Lens All-Sky Survey (CLASS) is a large-scale search for gravitational lenses using the VLA. Operating at an observing wavelength of 3.6 cm in its most widely-spaced configuration, a resolution of around 0.2 arc-seconds was obtained. The sources were observed as $30^{\rm s}$ snapshots'' at an average rate of one target source per minute reaching a rms noise level of 0.4 mJy in the final images. The survey sources were selected from the 4.85 GHz Green Bank Survey (87GB) to have 6~cm flux densities 50 mJy or greater, two-point spectral indices of $\alpha \geq -0.6$ versus the 365~MHz Texas Survey (ie. `flat' spectrum), $\vert b \vert > 10^\circ$, and to be not previously observed. Phase-1 observations of 3271 sources were made in Feb -- Apr 1994. Of these sources, 709 were selected from the 325~MHz Westerbork Northern Sky Survey (WENSS) mini-survey regions. All 87GB sources meeting the criteria for declinations $\delta \geq 45^\circ 20^\prime$ and were observed in this session, as well as other selected regions. For the bulk of the survey, reduction was carried out with the Caltech difference mapping software (difmap) using an automatic mapping procedure --- this allowed the mapping of sources to be completed at a rate of one field image every 2 minutes on a SPARC-10.

Previous lens surveys have shown that $\sim 0.2\%$ of flat-spectrum radio sources exhibit lensing, and thus we expect to find 6 lenses in our initial sample, and a total of 20 when the entire sample of $\sim 10,000$ sources is completed. By selecting targets based upon a flat radio spectrum, the CLASS survey aims to find a large number of new gravitational lens systems with multiply-imaged compact variable components suitable for measuring time-delays and thus leading to a determination of $H_0$. From this survey, two confirmed lens systems have so far been found, as well as more than 100 multiple component systems that are being followed up as candidates.