Multiwavelength periodic microlensing signatures of macrolensed supermassive binary black holes

Abstract

The microlensing of lensed quasars presents a promising avenue for understanding the structure of accretion disks around supermassive binary black holes (SMBBHs). We investigated the microlensing signatures in multiband (optical, UV, and X-ray) light curves of active SMBBH systems, focusing on how these signatures depend on the mass ratio, separation, and accretion rate. We analyzed the periodic fluctuations in microlensing light curves induced by the orbital motion of SMBBHs. We examined the relation between the mass ratio and the period of variations in light curves across optical, UV, and X-ray bands. We find that the periodic fluctuations in the light curves depend on the mass ratio of the black holes: for nearly equal masses, variations occur at half the orbital period, whereas for low mass ratios, the period corresponds to the orbital period influenced by the secondary mini-disk. Furthermore, all optical, UV, and X-ray light curves exhibit the same period and phase, but the amplitude of variation is greater in the UV and X-ray bands than in the optical bands. These light curves provide insights into the motion and radiation regions of the disks through wavelength-dependent periodic variations, although they yield limited constraints on the system's black hole mass or Eddington ratio, which can instead be derived from the spectral energy distribution (SED). Integrating microlensing data with SED observations is crucial for accurately constraining the parameters of SMBBH systems.