Plot of the instantaneous non-thermal electron density as a function of ambient density for three different low-energy cut-offs. The non-thermal electron density is calculated and plotted only where the thin-target assumption is valid. The shaded regions mark the ranges of the estimated volume from the source size estimations. The grey lines at 100%, 10%, and 1% mark where the ambient density equals the respective percentages of the non-thermal density (i.e. 100% is the extreme cases when the ambient density equals the non-thermal electron density.) The black star marks the value of the instantaneous non-thermal electron density if we assume the ambient density is that of the thermal source.