Searching for unresolved massive black hole pairs through active galactic nucleus photometric variability

¹ Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
² INAF – Osservatorio Astronomico di Brera, Via Brera 20, I-20121 Milano, Italy
³ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, I-20126 Milano, Italy
⁴ Como Lake centre for AstroPhysics (CLAP), DiSAT, Università dell’Insubria, Via Valleggio 11, 22100 Como, Italy
⁵ Department of Physics and Astronomy, Washington State University, Pullman, WA 99163, USA
⁶ Institute of Astrophysics, FORTH, GR-71110 Heraklion, Greece

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 25 August 2025
Accepted: 20 March 2026

Abstract

Since the discovery of active galactic nuclei (AGN), their light curves have proved to be intrinsically variable. In the optical/UV band, this variability is consistent with correlated or red noise and is particularly well described by the damped random walk (DRW) model. In this work, we evaluate the feasibility of a new method for identifying spatially unresolved couples of active galactic nuclei through a fully Bayesian time-domain analysis of the observed light curves. More specifically, we check whether observed light curves are better described by a single DRW, which we interpret as emitted by a single massive black hole, or a pair of independent damped random walks, generated by a pair of massive black holes. We test the method on mock light curves associated with single massive black holes and pairs generated with different cadences and lengths of observational campaigns. We constrained the occurrence of false positives, that is, the percentage of single MBH light curves that show substantial evidence in favour of the unresolved MBH pair scenario, finding a fraction of 0.2% and 0.59% in the even and uneven sampling scenarios. We discuss how well the method recovers the model parameters, showing that about 51% and 7% of the simulated light curves have all the recovered parameters within 20% of their true values in our best scenario of evenly sampled light curves for the single massive black hole and massive black hole pair scenarios, respectively. We finally study the region of the parameter space in which the detection of a massive black hole pair is possible, finding that such objects can be correctly identified if the timescales of the process describing the noise are very different, with a ratio smaller than ∼0.2, and the variability amplitudes are similar, with their ratio bigger than ∼0.2. When limiting to such a region of the parameter space, the fraction of pairs with all the recovered parameters within 20% of the injected values increases up to about 14% and 8% for evenly and unevenly sampled light curves, respectively.