LBQS 0103-2753: A 0 ''.3 binary quasar

Imaging and spectroscopy with the Hubble Space Telescope show that LBQS 010 3-2753 (V = 17.8, z = 0.848) is a binary quasar with a separation of 0."3, or 2.3 kpc. This is by far the smallest separation binary quasar reported t o date. The two components have very different spectra, including the prese nce of strong broad absorption lines (BALs) in component A only. The emissi on-line redshifts, based on the broad high-ionization C IV lines, are z(A) = 0.834 and z(B) = 0.858; their difference is 3900 km s(-1) in velocity uni ts. The broad C IV lines, however, are probably not a good indicator of sys temic redshift, and LBQS 0103-2753A and B could have a much smaller systemi c redshift difference, like the other known binary quasars. If the systemic redshift difference is small, then LBQS 0103-2753 would most likely be a g alaxy merger that has led to a binary supermassive black hole. There is now one known 0."3 binary among roughly 500 QSOs that have been observed in a way that would reveal such a close binary. This suggests that QSO activity is substantially more likely for black hole binaries at spacings similar to 2 kpc than at similar to 15 to 60 kpc. Between 1987 and 1998, the observed Mg II BAL disappeared.