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

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.

Figures (6)

• Figure 1: The rest-frame optical spectrum of SDSS J1532 (top-left panel), and best fitting results of SDSS J1532 with rest-frame spectrum from 3850Å to 4450Å (top-right panel), from 4750Å to 5050Å (bottom-left panel), and from 6400Å to 6800Å (bottom-right panel). In the top of the panels, the solid dark green lines show the line spectra; the dashed blue and red lines represent the blue-shifted and red-shifted components of narrow H$\alpha$, H$\beta$, H$\gamma$, H$\delta$, He i, [Ne iii], [O iii], [N ii] and [S ii] doublets; the solid blue lines represent the extended components of [O iii] emission lines; the solid green lines represent broad H$\alpha$, H$\beta$ and H$\gamma$ emission lines with corresponding parameters of central wavelength (Å), line width $\sigma$ (Å) and line flux ($10^{-17}{\rm erg/s/cm^{2}}$) listed in the first two lines of the panels; the dashed cyan lines show the continuum emissions $\alpha\times(\frac{\lambda}{5100\textsc{\AA}})^{\beta}$ with parameters listed in the third line. In the bottom of the panels, the solid blue lines represent the residuals computed by subtracting the best-fitting results from the line spectra and then dividing by the uncertainties of the SDSS spectra; the solid and dashed green lines show 0 and $\pm$1, respectively.
• Figure 2: The observed optical light curves of SDSS J1532 directly downloaded from the survey archives of the CSS (v-band) and ZTF (g-band, r-band, i-band). The solid and dashed lines show the best-fitting results and their corresponding 1RMS scatter bands, respectively. The dotted lines represent the results of polynomial function.
• Figure 3: The results of LSP method in ZTF r-band (solid red line) and g-band (solid blue line) light curves. The vertical red line and blue line mark the position of the corresponding peaks of the power, the dashed and solid orange lines show 3$\sigma$ and 5$\sigma$ significance level determined by false alarm probability, respectively.
• Figure 4: The results of WWZ method in ZTF r-band (left panel) and g-band (right panel) light curves of SDSS J1532. In each panel, the horizontal red line represents the position of the corresponding periodicity.
• Figure 5: The epoch-folding method results of ZTF 5-day binned r-band light curve with a 226 day periodicity (left panel), ZTF 5-day binned g-band light curve with a 225 day periodicity (middle panel) after subtracting the contributions of the six-degree polynomial function, and CSS 1-day binned light curve with a 223 day periodicity (right panel). In each panel, the solid and dashed purple lines show the best-fitting descriptions by sinusoidal function and their corresponding 1RMS scatters, respectively.