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

TL;DR

This paper presents SDSS J153231.80+420342.7 as a candidate for a triple black hole system, featuring a close binary black hole and dual AGN signatures, with evidence from spectral lines and optical QPOs.

Contribution

It reports the identification of a triple black hole candidate with a close binary in a quasar, analyzing spectral and light curve data to support this claim.

Findings

Double-peaked emission lines suggest dual AGN at ~3 kpc separation.

Optical QPOs indicate a possible binary black hole at ~1 mpc separation.

Alternative models like disk or jet precession are also considered.

Abstract

We report 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 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 rotating disk-like narrow line region (NLR) and AGN-driven outflow models are also discussed for the double-peaked features. Besides, SDSS J1532 shows optical quasi-periodic oscillations (QPO) of about 0.6 yr from the ZTF and CSS light curves, with time duration longer than 14 years, which may indicate a binary black hole with about 1 mpc separation. Two alternative explanations, the disk precession and the jet precession models, are also discussed. The current results cannot completely rule out alternative models for the characteristics of spectrum and light curves. As a candidate for triple black hole with two cores in kpc scale and a close BBH in milli-pc scale in SDSS J1532, it may be going through a critical period in its evolution.