Snapshot spectra showing the evolution of the spectral luminosity during the ESD in the non-dissipative scenario (left) and the dissipative scenario (right). Dashed lines have γ = 7/8, while solid lines have γ = 1.5, as also indicated in the figure. The spectra are plotted at even intervals in time from the onset of the ESD at t/t_b = 1 (dark colour) until t/t_b = 10 (bright colour). Note that the two spectra for γ = 7/8 and γ = 1.5 overlap at t/t_b = 1. The purple shading indicates the energy sensitivity of XRT. Parameter values used are given in Table 1.