The Quasar SDSS J153636.22+044127.0: A Double-Peaked Emitter in a
  Candidate Binary Black-Hole System
  Double-peaked emission lines are believed to be originated from accretion
disks around supermassive black holes (SMBHs), and about 3% of z<0.33 AGNs are
found to be double-peaked emitters. The quasar SDSS J153636.22+044127.0 has
recently been identified with peculiar broad-line emission systems exhibiting
multiple redshifts. We decompose the H$\alpha$ and H$\beta$ profiles into a
circular Keplerian disk line component and other Gaussian components. We
propose that the system is both a double-peaked emitter and a binary SMBH
system, where the extra-flux in the blue peaks of the broad lines comes from
the region around the secondary black hole. We suggest that such black hole
binary systems might also exist in many known double-peaked emitters, where the
tidal torques from the secondary black hole clear the outer region of the disk
around the primary black hole, similar to the gap in a protostellar disk due to
the process of planetary migration, and might also stimulate the formation of a
vertical extended source in the inner region around the primary which
illuminates the disk. However, most secondary SMBHs in such systems might be
too small to maintain a detectable broad line region (BLR), so that the disk
line from the primary dominates.
