Time evolution of Mg ii in SDSS J2320+0024: Implications for a subparsec binary supermassive black hole system

Context. We present results from our spectroscopic follow-up of SDSS J2320+0024, a candidate binary supermassive black hole (SMBH) with a suspected subparsec separation, identified by a 278-day periodicity observed in its multiband optical light curves. Such systems serve as a crucial link between binaries with long periods (tens of years), which are influenced by tidal forces with minimal gravitational wave damping, and ultra-short-period binaries (≤order of days), which are dominated by gravitational wave-driven inspiral. Aims. We investigated the dramatic variability in the complex Mg ii emission line profile with the aim of testing the alignments of the observed photometric light curves and the spectroscopic signatures in the context of the binary SMBH system. Methods. We extracted the pure broad Mg ii line from newly obtained Gemini and Magellan spectra and measured the emission line parameters to determine the fundamental dynamical parameters of the SMBH’s binary system. We adopted the PoSKI subparsec binary SMBH model, which includes a broad-line region around a less massive component and a circumbinary broad-line region, to interpret the observed variability in the spectral profile. Results. We find that the Mg ii broad-line profile has a distinctive complex shape, with asymmetry and two peaks, which has varied across recent and archival observations. The temporal variability of the Mg ii line profile may be associated with emission from the binary SMBH system, whose components have masses M₁ = 2 × 10⁷ M and M₂ = 2 × 10⁸ M and eccentricity e = 0.1. We discuss other plausible physical interpretations. With a total estimated mass of ∼10⁹ M and a sub-annual orbital period, this system may be a rare example of a high-mass compact SMBH binary candidate and, thus, should be part of further investigations of the evolution of binary systems. This study highlights the synergies between spectroscopic follow-up and future massive time-domain photometric surveys, such as the Vera C. Rubin Observatory Legacy Survey of Space and Time.