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

Abstract

In this manuscript, variations in optical spectral index $\alpha_{5100}$ 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 $\alpha_{5100}$ can be expected. First, if a BLAGN harbours a central sub-pc BBH, the expected unique variability in $\alpha_{5100}$ should lead the BLAGN to be an outlier in the space of $\alpha_{5100}$ versus continuum luminosity $L_{5100}$ determined from normal BLAGN. Second, BLAGN harbouring central sub-pc BBHs could lead to periodic variations in $\alpha_{5100}$. 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 $\alpha_{5100}$ 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 $\alpha_{5100}$ 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.