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Astron. Astrophys. 334, L33-L36 (1998)

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2. The JVAS/CLASS radio lens searches

The objective of the JVAS/CLASS surveys is to observe all northern-hemisphere sources with flux densities [FORMULA] 30 mJy at 5 GHz, and with a radio spectral indices flatter than -0.5 ([FORMULA]). The brighter section of the survey, the Jodrell Bank-VLA Astrometric Survey (JVAS) consists of the brightest [FORMULA] 2400 sources ([FORMULA]  mJy) and is described by Patnaik et al. (1992a) and Wilkinson et al. (1998). The Cosmic Lens All Sky Survey (CLASS), which is a collaboration between groups at Jodrell Bank, Caltech, Dwingeloo and Leiden, is an extension of this work to weaker sources. The surveys used to define the JVAS/CLASS sample, and determine the radio spectral indices, are the NRAO 5-GHz surveys at the high-frequency end (Gregory & Condon 1991; Gregory et al. 1996) and either the Westerbork Northern Sky Survey (WENSS) at 325 MHz (de Bruyn et al., in preparation) or the Texas 365 MHz catalogue (Douglas et al. 1996) at the low frequency end. So far, approximately 9000 sources have already been observed of which about 8000 have been detected with 8.4 GHz flux densities [FORMULA] 20 mJy.

Every effort is being made to ensure that the finished survey is complete in the sense that all gravitational lenses with image separations [FORMULA], and with image flux ratios [FORMULA] 10 will be detected. This task is made easier by the fact that flat-spectrum radio sources have intrinsically simple structures (mostly unresolved on the arcsecond scale). Initial observations are made in snapshot mode with the VLA in A-configuration at 8.4GHz. These give 0.2 arcsec-resolution images with dynamic range [FORMULA] 100 on the brightest component. Any object which consists of more than one component with angular separation 0:003 [FORMULA]  arcsec between the components is followed up by higher-resolution radio observations using some combination of the VLA, MERLIN and the VLBA. In this phase of the search for lens systems, candidates are rejected if the images have very different radio spectra, if the images have very different percentage polarizations, or if the high resolution maps reveal extended structure inconsistent with lensing (most often the putative lensed images are shown to have very different surface brightnesses). More details of the lens search procedure are given by King et al (1998a).

The radio follow-up observations of the candidates from all the sources so far observed with the VLA are virtually complete. So far we have twelve confirmed lens systems which we list in Table 1. There are no systems that have passed all the radio tests which have been subsequently rejected by optical observations. Hence we take as a confirmed system as one which has passed the radio tests; here we discuss the subset of these which have so far been observed with HST. There remain five recently-discovered good candidates (not included in Table 1) which still require optical follow-up observations. These five candidates have separations [FORMULA]  arcsec.


Table 1. Radio-loud lensed systems from JVAS/CLASS, in order of decreasing image separation. In column 2 we give the reference to the discovery paper: column 3 gives the number of images of each source component. [FORMULA] (column 4) and [FORMULA] (column 5) are the source and lens redshift. Redshifts for three systems are taken from Fassnacht & Cohen (1998) and the lens galaxy redshift for B1422+231 is from Kundic et al. (1997). The image separation (column 6) is given in arcsec. In columns 7-9, the lens magnitudes (Johnson-Cousins V, I, CIT H; typical errors [FORMULA] 0.2 magnitudes from measurements of different frames) and mass-to-light ratios are given. Magnitudes and inferred lens masses given are within the Einstein radius, and are from our own data, except for B1422+231 (Impey et al. 1996), B0218+357 (fits by Hjorth 1997: the magnitudes are total magnitudes rather than within the Einstein radius, making the inferred [FORMULA] ratio slightly too small), B1030+074 (Xanthopoulos et al. 1998). [FORMULA] ratios for 6 objects, plus other non-JVAS/CLASS objetcs, are given by Keeton, Kochanek & Falco (1997). The mass-to-light ratios (bold type) have no K- or evolution corrections (see the text for a discussion of these) and are in units of [FORMULA]. In column 10 we give the lens mass in units of 1010 [FORMULA]. MG0414+054 is a rediscovery of a lens system from the MIT-Greenbank Survey. B2114+022 has 4 compact components, of which at least two are gravitational images; it is not clear if all four components are images of the same object; two galaxies are associated with the lens (Augusto et al. 1998, in preparation), as is also the case in B1127+385. References: 1. Augusto et al. 1998; 2. Hewitt et al. 1992; 3. Myers et al. 1995; 4. Fassnacht et al. 1998; 5. Xanthopoulos et al. 1998; 6. Jackson et al. 1995; 7. Patnaik et al. 1993; 8. Jackson et al. 1998; 9. Sykes et al. 1998; 10. King et al. 1997, King et al. 1998; 11. Koopmans et al. 1998; 12. Patnaik et al. 1992.

In Fig. 1 we show the histogram of the maximum image separations given in Table 1 for the twelve confirmed lens systems. Our search technique is designed to pick up systems with image separations in the range 0.3 arcsec to 6 arcsec, and we believe it to be complete in this range. In particular, we have found no lens systems with separations [FORMULA]  arcsec amongst 8000 radio sources mapped. However, three systems have separations between 2 arcsec and 3 arcsec and hence lie in the separation range discussed by Hawkins.

[FIGURE] Fig. 1. Histogram of the maximum image separation (in arcsec) for the 12 confirmed JVAS/CLASS lens systems.

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Online publication: May 15, 1998