Close supermassive binary black holes
  It has been proposed that when the peaks of the broad emission lines in
active galactic nuclei (AGNs) are significantly blueshifted or redshifted from
the systemic velocity of the host galaxy, this could be a consequence of
orbital motion of a supermassive blackhole binary (SMB) (Gaskell 1983). The AGN
J1536+0441 (=SDSS J153636.22+044127.0) has recently been proposed as an example
of this phenomenon (Boroson & Lauer 2009). It is proposed here instead that
1536+044 is an example of line emission from a disc. If this is correct, the
lack of clear optical spectral evidence for close SMBs is significant and
argues either that the merging of close SMBs is much faster than has generally
been hitherto thought, or if the approach is slow, that when the separation of
the binary is comparable to the size of the torus and broad-line region, the
feeding of the black holes is disrupted.
