Letter to the Editor

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

Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, GuangXi University, No. 100, Daxue Road, Nanning 530004, P. R. China

^⋆ Corresponding author: xgzhang@gxu.edu.cn

Received: 26 September 2025
Accepted: 12 November 2025

Abstract

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.