Binary black hole systems with subparsec scales are predicted in merging scenarios for galaxy evolution and also factor prominently in predictions for the gravitational wave background. But do such binary systems exist? Twitter has been atwitter over the past few months following the astro-ph announcement in January that such a system may have been found by Boroson & Lauer .
The quasar SDSS J1536+0441, at a redshift of ~ 0.4, shows two broad-line emission systems . One interpretation is that the emission lines are caused by a binary system of two black holes, with masses of about 10ˆ7 and 10ˆ9 solar masses separated by ~ 0.1 pc (0.02 mas) with an orbital period of ~ 100 years. The subparsec scale is significant, as it implies that this black hole system has solved its so-called final parsec problem, in contrast to the 7 pc binary in a radio galaxy . Boroson & Lauer use the quasar's SDSS localization region to rule out a superposition of two unrelated quasars.
At meetings in March and April, we  announced the surprising VLA discovery of a 1 arcsec double source within the SDSS localization region (see Figure), suggesting a second interpretation for the emission lines: a 5 kpc binary quasar hosting VLA-A and VLA-B. Word rapidly circulated at those meetings about a Palomar telegram reporting a third broad-line emission component , suggesting a third interpretation as a so-called double-peaked emitter (DPE). A DPE is thought to originate from a geometrically thin Keplerian gaseous disk (also proposed for SDSS J1536+0441 by ). Importantly, no velocity drifts were detected between the epochs separating the SDSS and Palomar spectra , seemingly inconsistent with the interpretation as a subparsec binary system .
Then, in late May, an ESO/VLT telegram announced that each radio source had a 2 micron counterpart with a 2micron-to-radio ratio that was characteristic of a quasar . This endorsed our  interpretation of SDSS J1536+0441 as a 5 kpc binary quasar.
Meanwhile, Boroson & Lauer regrouped as Lauer & Boroson and presented, at the end of May, HST WFPC2/PC images and KPNO 4-m longslit spectra of SDSS J1536+0441 . Their images showed that the companion coincident with VLA-B was an elliptical galaxy. Significantly, the spectra, obtained with an east-west slit, showed no spatial offsets among the broad-line emission systems. Thus the companion elliptical galaxy could not host any of the broad-line gas. For this reason, SDSS J1536+0441 itself could not be explained as a superposition of separate broad-line objects hosting VLA-A and VLA-B. Alas, the interpretation of a 5 kpc binary quasar [3,6] bit the dust!
As reported in June , a high quality Keck spectrum of SDSS J1536+0441 excludes even better both spatial shifts in the broad-line light entering the slit and drifts of the broad-line peaks over time. These traits reinforce the difficulties with both the 5 kpc binary [3,6] and 0.1 pc binary  interpretations. A DPE interpretation for SDSS J1536+0441 remains viable but is not free of complications: DPEs always produce two nice broad peaks, while here the blue peak is narrow and strong, not the way a DPE produced by a Keplerian disk should appear. This has led to SDSS J1536+0441 being billed as an unusual DPE  and the theorists are enjoying the challenge of trying to understand its properties, including its radio-quiet nature . The companion elliptical galaxy is also somewhat unusual, being a low-power radio galaxy .
Twitter has calmed down on the Boroson & Lauer binary black hole system, but we will surely hear more about this puzzling system. The Holy Grail of a binary black hole system with a subparsec scale is probably still out there. We'll be watching astro-ph for an announcement of its discovery!
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