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

TL;DR

This study reanalyzed the variability of the SBHB candidate SDSS J2320+0024, finding no statistically significant evidence of periodic modulation and highlighting the importance of rigorous statistical methods in such analyses.

Contribution

The paper provides a comprehensive Bayesian reanalysis of the light curves, challenging previous claims of periodicity and emphasizing the need for robust statistical validation in SBHB candidate studies.

Findings

No significant periodicity detected in g, r, i bands.

Broad posterior peaks do not favor periodic models over red noise.

Previous periodicity claims are not statistically supported.

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 (g, r, 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.