HST Images and KPNO Spectroscopy of the Binary Black Hole Candidate SDSS
  J153636.22+044127.0
  We present HST WFPC2/PC images and KPNO 4-m longslit spectroscopy of the QSO
SDSS J153636.22+044127.0, which we advanced as a candidate binary supermassive
black hole. The images reveal a close companion coincident with the radio
source identified by Wrobel & Laor (2009). It appears to be consistent with a
M_g ~ -21.4 elliptical galaxy, if it is at the QSO redshift. The spectroscopy,
however, shows no spatial offset of the red or blue Balmer line subcomponents.
The companion is thus not the source of either the red or blue broad line
systems; SDSS J153636.22+044127.0 cannot be explained as a chance superposition
of objects, or as an ejected black hole. Over the Delta T=0.75 yr difference
between the rest frame epochs of the present and SDSS spectroscopy, we find no
velocity shift to within 40 km/s, nor any amplitude change in either broad line
system. The lack of a shift can be admitted under the binary hypothesis if the
implied radial velocity is a larger component of the full orbital velocity than
was assumed in our earlier work. A strong test of the binary hypothesis
requires yet longer temporal baselines. The lack of amplitude variations is
unusual for the alternative explanation of this object as a "double-peaked"
emitter; we further argue that SDSS J153636.22+044127.0 has unique spectral
features that have no obvious analogue with other members of this class.
