Variability in the supermassive black hole binary candidate SDSS J2320+0024: No evidence of periodic modulation

¹ INAF – Osservatorio Astronomico di Brera, via Brera 20, I-20121 Milano, Italy
² INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, I-20126 Milano, Italy
³ Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
⁴ Como Lake centre for AstroPhysics (CLAP), DiSAT, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy

^⋆ Corresponding author: fabio.rigamonti@inaf.it

Received: 16 May 2025
Accepted: 27 August 2025

Abstract

Supermassive black hole binaries (SBHBs) are a natural outcome of galaxy mergers, and they are expected to be among the loudest gravitational-wave sources at low frequencies. The source SDSS J2320+0024 was recently proposed as a promising SBHB candidate due to a possible periodicity in its light curve and variability in the MgII emission line. In this work, we reanalysed the optical (gr, and i bands) light curves of J2320+0024 within the framework of Bayesian model selection. When periodicity was searched for together with red noise, analysis of the g-band light curve reveals a peak in the posterior of the period at ∼290 days. The posterior profile is too broad to yield a preference for periodic models over models that include only red noise. Furthermore, the same peak is not present in the analysis of the r-band and i-band light curve. A periodic model without red noise identified a different (∼1100 days) periodicity, but this model is statistically significantly disfavoured relative to the other models tested. In summary, we find no significant evidence in favour of a true periodic signal over red-noise variability. Our analysis questions the robustness of the previously proposed periodicity and emphasises the importance of rigorous statistical treatment. While our findings challenge the binary interpretation for J2320+0024, they do not rule it out. A statistically robust joint analysis of the photometric light curves and evolving broad-line profiles would shed further light on the true nature of this object.