Close supermassive binary black holes/Boroson and Lauer reply

If this verifies that the Hβ profile of J1536+0441 is the result of normal disk emission, it has significant implications for the evolution of SMBBs, as J1536+0441 is the only candidate so far for a sub-parsec SMBB out of ~17,500 AGNs with z < 0.70 in the SDSS2,13. Because SMBB formation must be common, the absence of clear evidence for close SMBBs in AGNs needs to be explained. Competing financial interests: declared none. doi:10.1038/nature08665 Boroson and Lauer reply Replying to: C. M. Gaskell Nature 463, doi:10.1038/nature08665 (2010) Gaskell1 makes the point that the profile of the Balmer lines in the candidate binary supermassive black hole2 AGN J1536+0441 bears some similarity to those that are thought to arise from disk emission in other objects.