SDSS J1056+5516: A Triple AGN or an SMBH Recoil Candidate?
  We report the discovery of a kiloparsec-scale triple supermassive black hole
system at $z=0.256$: SDSS J1056+5516, discovered by our systematic search for
binary quasars. The system contains three strong emission-line nuclei, which
are offset by $<250~{\rm km~s^{-1}}$ and by 15-18 kpc in projected separation,
suggesting that the nuclei belong to the same physical structure. The system
includes a tidal arm feature spanning $\sim100$ kpc in projected distance at
the systems' redshift, inhabiting an ongoing or recent galaxy merger. Based on
our results, such a structure can only satisfy one of the three scenarios; a
triple supermasive black hole (SMBH) interacting system, a triple AGN, or a
recoiling SMBH. Each of these scenarios is unique for our understanding of the
hierarchical growth of galaxies, AGN triggering, and gravitational waves.
