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

Zhang XueGuang

First listed 2025-11-12 | Last updated 2025-11-12

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.

Short digest

Uses optical spectral index α_5100 variability as a continuum-based flag for sub-parsec BBHs and applies it to SDSS J0012-1022 (z=0.228). Between two SDSS epochs (MJD 52141 → 56628; Δt=4487 d) the quasar brightened yet reddened around 5100 Å, making it an outlier to the α_5100–L_5100 trend from 3274 SDSS quasars; a two-component continuum with modest extinction (e.g., E(B–V)=0.1) from a second accreting BH reproduces the later spectrum. Simulations with periodic obscuration naturally populate this outlier locus and predict periodic α_5100 variability, while ZTF g/r data show a candidate ~827-day signal plus color changes consistent with orbital modulation. The work motivates monitoring α_5100 for secured periodicity as a new, continuum-based BBH diagnostic.

Key figures to inspect

• Figure 1: Check where SDSS J0012-1022 lands in the α_5100–L_5100 plane at MJD 52141 vs 56628 and how the 20,000 binary-driven artificial SEDs shift the locus—this shows the source’s redder-when-brighter outlier status relative to normal quasars.
• Figure 2: Inspect the two-epoch spectra and the continuum windows (4180–4250 Å, 5550–5750 Å); the MJD 56628 spectrum is reproduced by summing an extincted power law (E(B–V)=0.1) and a second BH continuum—key for the obscuration-based BBH interpretation.
• Figure 3: Look at ZTF g and r light curves, the Lomb–Scargle periodogram peak near 827 days, bootstrap period distributions, and the g−r evolution—evidence for quasi-periodic, color-dependent modulation expected from orbital obscuration.
• Figure 4: Compare the normal-quasar α_5100–L_5100 relation to the distribution produced when periodic obscuration is included; note how the artificial sample occupies the same region as the observed outlier point for SDSS J0012-1022.