Variability of optical spectral index to support a central sub-parsec binary black hole system in quasar SDSS J001224-102226.51

TL;DR

The paper addresses detecting central sub-parsec BBHs in BLAGN by analyzing the optical continuum spectral index $α_{5100}$ and its relation to continuum luminosity $L_{5100}$. It proposes a two-component continuum model with orbital-phase dependent obscurations to explain outliers in the $α_{5100}$–$L_{5100}$ plane, illustrated with SDSS J0012-1022, and shows that periodic obscurations can produce time-dependent variability in $α_{5100}$. It pairs spectroscopic evidence from multi-epoch SDSS data with optical QPOs observed in ZTF light curves, finding a robust ~827-day periodicity in both the gr-band variability and the g−r color, with red-noise unlikely to account for the signal. Together, these results introduce a novel, spectroscopy-informed pathway to identify sub-pc BBHs in BLAGN and connect continuum variability with photometric QPOs to bolster the BBH interpretation.

Abstract

In this manuscript, variations in optical spectral index $α_{5100}$ are applied for detecting central sub-parsec binary black hole systems (sub-pc BBHs) in broad line active galactic nuclei (BLAGN), due to apparent effects of obscurations on central two BH accreting systems. For sub-pc BBHs in BLAGN, two main characteristics on $α_{5100}$ can be expected. First, if a BLAGN harbours a central sub-pc BBH, the expected unique variability in $α_{5100}$ should lead the BLAGN to be an outlier in the space of $α_{5100}$ versus continuum luminosity $L_{5100}$ determined from normal BLAGN. Second, BLAGN harbouring central sub-pc BBHs could lead to periodic variations in $α_{5100}$. Here, after checking the two-epoch SDSS spectra of quasar SDSS J0012-1022 reported as a candidate of sub-pc BBH by large velocity offset between narrow and broad Balmer emission lines, unique variability of $α_{5100}$ can be explained by effects of obscurations related to an assumed central sub-pc BBH. In the near future, to detect and report periodic variations of $α_{5100}$ for sub-pc BBHs in BLAGN should be our main objective. The results provide a new method by applications of properties of optical continuum emissions for detecting sub-pc BBHs in BLAGN.

Figures (4)

• Figure 1: Top panel shows the contour (filled by greenish colors) for the correlation between $L_{5100}$ and $\alpha_{5100}$ of the selected 3274 SDSS quasars. Circles plus error bars in blue and in red show the results of SDSS J0012-1022 with MJD=52141 and 56628. Solid and dashed lines in red show the best fitting results and corresponding 5$\sigma$ confidence bands. Contour with reddish levels represents the results through the 20000 artificial $f_{\lambda}$. The contour levels in each contour represent 0.1, 0.3, 0.5, 0.7 and 0.9 of the 2D volume contained. Bottom panel shows four examples on periodic variations in $\alpha_{5100}$ related to sub-pc BBHs, with corresponding model parameters listed in the same color. The six parameters in each square bracket are $\log(L_1)$, $\log(L_2)$, $\alpha_1$, $\alpha_2$, $E_{10}$ and $E_{20}$.
• Figure 2: SDSS spectra of SDSS J0012-1022 with MJD = 52141 (in blue) and 56628 (in red). Dashed lines in blue and in red show the power law function determined optical continuum emissions underneath broad H$\beta$ through the two regions filled by dark green lines. Dashed purple line represents the first component related to the obscured power law component shown as dashed blue line with E(B-V)=0.1, and dashed dark green line shows the second component (formula listed in dark green characters) from the other one BH accreting system. The sum of the two components is one solution related to a sub-pc BBH to the continuum emissions in the spectrum with MJD=56628.
• Figure 3: Left panel shows the ZTF $gr$-band (in blue and in red) light curves of SDSS J0012-1022. Solid and dot-dashed lines in blue and in red represent the best fitting results and the corresponding 1RMS scatters for the $gr$-band light curves, horizontal dashed lines in blue and in red show the determined base line components in the $gr$-band light curves. Middle panel shows the determined LS powers of the $gr$-band (in blue and in red) light curves and the g-r color (in purple), with horizontal dashed red line representing the 5$\sigma$ significance level (false alarm probability 3e-7) and with vertical dashed red line marking periodicity=827 days. Top left corner in middle panel shows the bootstrap method determined distributions (in blue, in red and in purple) of the 2000 re-determined periodicities through the ZTF $gr$-band light curves, and through the g-r color. Right panel shows the g-r color, with solid and dot-dashed lines in purple as the best fitting results and the corresponding 1RMS scatters.
• Figure 4: Contour with reddish levels represents the same results shown in top panel of Fig. \ref{['sed']} after considering periodic obscurations in assumed sub-pc BBHs. Pluses in blue represent the results from the 3000 newly created artificial $f_\lambda$ after considering $E_1(B-V,\phi)~=~E_2(B-V,\phi)~=~E_0$ with $E_0$ as a random value between 0 and 0.5. The solid red line shows the dependence determined from SDSS quasars as shown in top panel of Fig. \ref{['sed']}.