Annu. Rev. Astron. Astrophys. 1992. 30: 311-358
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2.1 Multiple Quasars

The most famous, and also still the best studied gravitational lens, is the ``double quasar,'' Q0957+561 (Walsh et al. 1979). Two optical and radio images of a z = 1.41 quasar, separated by ~ 6", are formed on opposite sides of a brightest cluster galaxy with redshift z = 0.36 (Figure 1). Detailed lens modeling can account for the observed images (e.g. Falco et al. 1991a) and a 1.48 yr time lag has been measured in the variation of the two images (Lehar et al. 1992a, Press et al. 1992a, b). There is one other secure double quasar - Q0142-100 (Surdej et al. 1987), and five candidates - Q1120+019 (Meylan & Djorgovski 1989), Q1208+101 (Maoz et al. 1992, Magain et al. 1992), Q1429-008 (Hewett et al. 1989), Q1635+267 (Djorgovski & Spinrad 1984, Turner et al. 1988), and Q2345+007 (Weedman et al. 1982, Tyson et al. 1986), where the evidence for multiple imaging is not conclusive and where any lensing galaxies, if present, must be highly subluminous. The radio galaxy 3C 324 may also be doubly imaged (Le Fevre et al. 1987). The source Q2016+112 (Lawrence et al. 1984, Heflin et al. 1991) is compellingly argued on the basis of spectroscopic and morphological data to be triply-imaged by two observed galaxies, but the image geometry is not well understood (however, see Narasimha et al. 1987). There are four convincing examples of quadruple imaging: 0414+053 [which, although a radio source, may not be a quasar (Hewitt et al. 1989; Elston and Lawrence, personal communication)], Q1115+080 [the ``triple quasar'' (Weymann et al. 1980, Young et al. 1981a)], H1413+117 [the ``clover leaf'' (Magain et al. 1988)], and Q2237+031 [the ``Einstein cross'' (Huchra et al. 1985); see Figure 2]. Despite the absence of a detected lensing galaxy in 0414+053 and Q1413+117, all four examples are considered secure because the image arrangement is a pattern quite characteristic of a gravitational lens (see Figure 6).

Q0957+561
Figure 1. The ``double quasar'' Q0957+561. (Upper left) Radio VLA map showing the two quasar images A, B separated by 6.1" and the lensing galaxy G1. (Lower left) Optical image of the same field. (Right) High resolution VLBI maps of the radio jets with size 50 milliarcsec by 100 milliarcsec. (Images supplied courtesy J. Hewitt, R. Schild, and E. Falco.)

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