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

¹ Dipartimento di Fisica “Ettore Pancini”, Università di Napoli Federico II, Via Cintia 80126, Naples, Italy
² Ruđer Bošković Institute, Bijenička Cesta 54, 10000 Zagreb, Croatia
³ INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Naples, Italy
⁴ Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski Trg 16, 11000 Belgrade, Serbia
⁵ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁶ PIFI Research Fellow, Key Laboratory for Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, 19B Yuquan Road, 100049 Beijing, China
⁷ Faculty of Science, University of Kragujevac, Radoja Domanovića 12, 34000 Kragujevac, Serbia
⁸ Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia
⁹ Department of Physics & Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC V8P 5C2, Canada
¹⁰ Department of Astronomy and the DiRAC Institute, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA
¹¹ MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Ave., Cambridge, MA 02139, USA
¹² Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA

^⋆ Corresponding author; marta.fatovic@unina.it

Received: 23 December 2024
Accepted: 27 January 2025

Abstract

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.