SDSS J153231.80+420342.7: A triple black hole candidate with a close binary black hole

We reported a triple black hole candidate with a close binary black hole (BBH) in the blue quasar SDSS J153231.80+420342.7 (=SDSS J1532) at a redshift of 0.209. It shows double-peaked profiles in all narrow emission lines, which can be a signature of a dual active galactic nucleus (AGN). If the double-peaked features are produced by a dual AGN, the estimated physical separation between the two cores is about 3 kpc. Alternative interpretations to the double-peaked profiles involving models of rotating disk-like narrow-line regions (NLRs) and AGN-driven outflows are also discussed for the double-peaked features. In addition, SDSS J1532 shows optical quasiperiodic oscillations (QPOs) of about 0.6 yr from the ZTF and CSS light curves, with time duration that is longer than 14 years, which may indicate a binary black hole with ∼1 mpc separation. Two alternative models, disk precession and jet precession, are also discussed. The current results cannot completely rule out alternative models for the characteristics of the spectrum and light curves. As a candidate for a triple black hole with two cores in kiloparsec scale and a close BBH in milliparsec scale in it may be going through a critical period in its evolution.