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

In this work, we use variations in optical spectral index, α5100, to detect central sub-parsec binary black hole systems (sub-pc BBHs) in broad line active galactic nuclei (BLAGNs) through apparent effects of obscurations on two central BH accreting systems. For sub-pc BBHs in BLAGNs, two main characteristics of α5100 can be expected. First, if a BLAGN harbours a central sub-pc BBH, the expected unique variability in α5100 should lead to the BLAGN being an outlier in the space of α5100 versus continuum luminosity, L5100, determined from normal BLAGNs. Second, BLAGNs harbouring central sub-pc BBHs can lead to periodic variations in α5100. Here, after checking the two-epoch Sloan Digital Sky Survey (SDSS) spectra of quasar SDSS J0012-1022 reported as a candidate of sub-pc BBH by a large velocity offset between narrow and broad Balmer emission lines, the unique variability of α5100 can be explained by the effects of obscurations related to an assumed central sub-pc BBH. The results of this work provide a new method of detecting sub-pc BBHs in BLAGNs through the application of properties of optical continuum emissions. In the near future, detecting and reporting periodic variations of α5100 for sub-pc BBHs in BLAGNs should be main objective of our research group.