2009ApJ...703..930L

We present Hubble Space Telescope WFPC2/PC images and KPNO 4 m long-slit 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. 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 between the red or blue Balmer lines. The companion is thus not the source of either the red or blue broad-line systems; SDSS J153636.22+044127.0 is highly unlikely to be a chance superposition of objects or an ejected black hole. Over the ΔT = 0.75 yr difference between the rest-frame epoch 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.