Magnetar model from Cano et al. (2016) fit to the X-ray (top) and r-band (bottom) luminosity LCs. Times are shown in the rest-frame. Top: sum of the SPL (dot-dashed line) and magnetar-powered AG (dotted line) phases is shown as the solid black line. Visual inspection shows that the model fails to reproduce the decay seen in the observations at t − t₀> 3 × 10⁵ s. The best-fitting model gives B₀ = (1.0 ± 0.1) × 10¹⁵ G, and P₀ = 8.1 ± 0.3 ms. Bottom: sum of the SPL (dot-dashed line), magnetar-powered AG (dotted line) and magnetar-powered SN (dashed line) phases is shown as the solid red line. The solid blue line includes an additional multiplicative factor (Ψ) to get the magnetar-powered SN to match the observations. The best-fitting model gives B₀ = (2.4 ± 0.3) × 10¹⁵ G, and P₀ = 35.2 ± 3.0 ms, which are clearly discrepant with those determined from the X-ray. In addition, we found a diffusion time scale of t_diff = 9.43 ± 0.10 days, and Ψ = 4.2 ± 0.1. The conclusion is that the magnetar model cannot self-consistently explain the X-ray and optical observations of GRB 161219B/SN 2016jca. Even if SN 2016jca was powered in part by a magnetar central engine, an additional source of energy is needed to reproduce the observations, most likely a reservoir of radioactive nickel. The required nickel mass to get the model to match observations is M_Ni ~ 0.4M_⊙.