2016ApJ...827...56Z

We report a candidate centi-parsec supermassive black hole binary (SMBHB) in the radio-quiet quasar SDSS J0159+0105 at z=0.217. With a modified Lomb-Scargle code (GLSdeDRW) and auto-correlation analysis, we detect two significant (at P > 99%) periodic signals at ∼741 day and ∼1500 day from the 8.1 yr Catalina V-band light curve of this quasar. The period ratio, which is close to 1:2, is typical of a black hole binary system with a mass ratio of 0.05 < q < 0.8 according to recent numerical simulations. SDSS J0159+0105 has two SDSS spectroscopic observations separated by ∼10 yr. There is a significant change in the broad Hβ profile between the two epochs, which can be explained by a single broad-line region (BLR) around the binary system illuminated by the aforementioned mini-disks, or a stream of gas flowing from the circumbinary disk to one of the SMBHs. From the single BLR assumption and the orbital period t_orb ∼ 1500 day, we estimate the total virial masses of M_SMBHB ∼ 1.3 x 10^8 M_☉, the average distances of BLR of ∼0.04 pc (∼50 lt-day, with ±0.3 dex uncertainty), and an SMBHB separation of d=(0.01 pc){M}_{8,tot}^{1/3} (T_rest/3.3 yr)^2/3 ∼ 0.013 pc (15 lt-day). Based on analytical work, the postulated circumbinary disk has an inner radius of 2d=0.026 pc (30 lt-day). SDSS J0159+0105 also displays unusual spectral energy distribution. The unique properties of SDSS J0159+0105 are consistent with it being a centi-parsec SMBHB.