An equal mass ratio supermassive binary black holes in Q J0158−4325 with periodic microlensing signature?

ABSTRACT This study aims to test whether a supermassive binary black hole (SMBBH) system with a triple-disc accretion structure can explain the observed $\sim$173-d periodic microlensing variations and spectral energy distribution (SED) of the gravitationally lensed quasar Q J0158−4325. We construct a triple-disc model for the SMBBH system, incorporating realistic accretion disc structures, orbital motion, and microlensing effects. The model is used to simulate optical and X-ray microlensing light curves and SEDs, which are compared with long-term optical monitoring, X-ray observations, and ultraviolet (UV)–optical spectra from the Hubble Space Telescope and XSHOOTER. Bayesian analysis and Markov chain Monte Carlo fitting are applied to constrain model parameters. The model successfully reproduces the periodic microlensing variations. Combined light curve and SED fitting favour a high mass ratio ($q\gt 0.5$) SMBBH system with total mass ${\sim} 10^{9.5} \, {\rm M}_\odot$, and nearly equal-mass binaries ($q\sim 1$) provide the best agreement with both the optical/UV spectrum and the microlensing signal. This model predicts larger X-ray microlensing amplitudes than in the optical, but the available X-ray observations lack the precision needed to place strong constraints. We emphasize the need for future high-cadence monitoring to resolve remaining uncertainties. This study demonstrates the effectiveness of combining multiwavelength microlensing signatures with spectral modelling to provide robust constraints on SMBBH systems, with the developed framework applicable to other lensed quasars for identifying and characterizing candidate SMBBHs.