Rotation diagram for C₃ lines observed with SOFIA/GREAT, upGREAT and with HIFI/Herschel towards SgrB2(M). The excitation temperature, T_ex = 44.4 K, was derived from a linear fit to the state-specific column densities N_l (J_l = 2, 4,...12) of the main isotopologue, CCC (blue solid line and blue shaded ± 1σ error range). A straight line with the temperature slope of the main isotopologue was fitted to the data J_l = 2, 4 of the isotopologues C¹³CC (red) and ¹³CCC (green). Filled symbols represent measured values, empty symbols are obtained from the least-squares fit. Note, the centro-symmetric species, CCC and C¹³CC, only have even-numbered J_l levels. The numbers to the left are the log (N₀) values for J_l = 0 obtained from the fit.