Accretion rates in HFS. Upper panel: comparison between observed total accretion rates (Ṁ_HFS) and the HFS mass (M_HFS) reported in the literature for HFS (see also Table A.1). Lower panel: comparison between observed accretion rates per-unit-length (ṁ_HFS) and those predicted following Eq. (19) (ṁ_pred). A red dotted line shows the expected 1:1 correlation between these values. A grey dashed line indicates a fit to the data (in log-log space) in both panels (see legend).