CTQ 839: Candidate for the smallest projected separation binary quasar

We report the discovery of the new double quasar CTQ 839. This B = 18.3, ra dio-quiet quasar pair is separated by 2 ".1 in BRI and H filters, with magn itude differences of Delta m(B) = 2.5, Delta m(R) = Delta m(I) = 1.9, and D elta m(H) = 2.3. Spectral observations reveal both components to be z = 2.2 4 quasars, with relative redshifts that agree at the 100 km s(-1) level but exhibit pronounced differences in the equivalent widths of related emissio n features, as well as an enhancement of blue continuum flux in the brighte r component as compared with the fainter component longward of the Ly alpha emission feature. In general, similar redshift double quasars can be the r esult of a physical binary pair or of a single quasar multiply imaged by gr avitational lensing. Empirical point-spread function subtraction of R and H band images of CTQ 839 reveal no indication of a lensing galaxy and place a detection limit of R = 22.5 and H = 17.4 for a third component in the sys tem. For an Einstein-de Sitter cosmology and singular isothermal sphere mod el, the R band detection limit constrains the characteristics of any lensin g galaxy to z(1) greater than or similar to 1 with a corresponding luminosi ty of L greater than or similar to 5 L*, while an analysis based on the red shift probability distribution for the lensing galaxy argues against the ex istence of a z(l) greater than or similar to 1 lens at the 2 sigma level. A similar analysis for a Lambda-dominated cosmology, however, does not signi ficantly constrain the existence of any lensing galaxy. The broadband flux differences, spectral dissimilarities, and failure to detect a lensing gala xy make the lensing hypothesis for CTQ 839 unlikely. The similar redshifts of the two components would then argue for a physical quasar binary. At a p rojected separation of 8.3 h(-1) kpc (Omega(m) = 1), CTQ 839 would be the s mallest projected separation binary quasar currently known.