Fraction of 40 MeV CR secondary electron energy passed to the inverse-Compton X-ray heating channel (red solid line) compared to the direct Coulomb heating channel (black solid line) as a function of density. The left hand y-axis corresponds to the relative fraction, and this indicates a strong preference for the IX heating channel in low density media, or the DC channel in higher density media (if ionised). Timescales of respective processes are shown in grey to illustrate the underlying physics: the solid line gives the total energy loss timescale for CR electrons. This is calculated by combining the contributions from inverse-Compton (dashed horizontal line), Coulomb (dotted line) and free–free losses (dot-dashed line). The respective timescale y-axis is shown to the right of the plot.