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Spectroscopic Indication for a Centi-parsec Supermassive Black Hole Binary in the Galactic Center of NGC 5548

As a natural consequence of cosmological hierarchical structure formation, sub-parsec supermassive black hole binaries (SMBHBs) should be common in galaxies but thus far have eluded spectroscopic identification. Based on four decades of optical spectroscopic monitoring, we report that the nucleus of NGC 5548, a nearby Seyfert galaxy long suspected to have experienced a major merger about one billion years ago, exhibits long-term variability with a period of 14 years in the optical continuum and broad Hbeta emission line. Remarkably, the double-peaked profile of Hbeta shows systematic velocity changes with a similar period. These pieces of observations plausibly indicate that a SMBHB resides in the center of NGC 5548. The complex, secular variations in the line profiles can be explained by orbital motion of a binary with equal mass and a semi-major axis of 22 light-days (corresponding to 18 milli-parsec). At a distance of 75 Mpc, NGC 5548 is one of the nearest sub-parsec SMBHB candidates that offers an ideal laboratory for gravitational wave detection.