Modelling of broadband spectra of GRB 131231A in the time intervals 60 s-1day and 52 000–56 000 s. Thick black curves represent the theoretical spectra of synchrotron plus SSC corresponding to slow cooling in the external forward shock scenario. Dotted line and dashed line represent the synchrotron and SSC component, respectively. The adopted parameters are ϵ_e = 0.16, ϵ_B = 0.00001, p = 2.13, and n = 1 cm⁻³.