Left panel: Thin slice of the simulated light cone. The coloured points correspond to MBHBs with P_obs ≤ 5 yr, while the black points represent all other galaxies hosted in the lightcone. We note that the lightcone slice shows some spatial periodicity of structures, which results from the box replication needed to build a wide and deep lightcone with the MSII merger trees. Right panels: Light curves in the LSST bands (u,  g,  r,  i,  z,  y) associated with a MBHB at redshift ∼1.22 with mass ∼7.3 × 10⁷ M_⊙, mass ratio ∼0.77 and eccentricity ∼0.24. The error bars correspond to the associated photometric uncertainty. The modelling and construction of the MBHB emission is illustrated in detail in Sections 3–4.