Theoretical results of the trapped-zone emission. Left panel: evolution of Lorentz factor (Γ). Middle panel: logarithmic optical depth (τ). The black horizontal line represents the optical depth limit at τ = 1. Right panel: X-ray luminosity (L_X) of the magnetar model (Eq. (8), first term). The solid black line shows the free zone spin-down luminosity (ηL_sd). The color code shows the different ejecta masses (M_ej = 2 × 10⁻⁴ − 5 × 10⁻³ M_⊙), while the solid and dashed lines represent opacities of 1.0 and 10.0 cm² g⁻¹, respectively, expected for the blue and red kilonova (Tanaka et al. 2017; Metzger 2019; Tanaka et al. 2020). The initial conditional assumed are: the initial velocity and internal ejecta energy of the ejecta are β_i = 0.1, and , respectively. The magnetar parameters are: B_p = 5 × 10¹⁵ G, P_i = 10⁻³ s, I = 10⁴⁵ g cm², ϵ = 10⁻⁵, and ξ = 0.3.