Numerical solution of the merger-nova emission in different energy bands. Left panel: optical and NIR upper limits of FXT 14 and the expected merger-nova emission (from u- to K-bands) assuming their best fitting parameters (see Table 2) and z = 2.23 (solid lines). As a middle and right panels: optical and NIR upper limits of FXT 22, and the merger-nova emission considering the derived magnetar parameters (see Table 2), two redshifts of z = 1.0 (solid lines) and 0.1 (dashed lines), and assuming lanthanide-free (middle panel; κ = 0.1 cm² g⁻¹ and M_ej = 10⁻⁴ M_⊙) and rich (right panel; κ = 30 cm² g⁻¹ and M_ej = 10⁻³ M_⊙) scenarios. The upper limits of FXT 14 were taken from Bauer et al. (2017), while for FXT 22 from Andreoni et al. (2021a,b,c), Xin et al. (2021), Rossi et al. (2021), and Eappachen et al. (2023). The time is measured from the X-ray trigger detection of the FXTs.