Illustration of projected and convolved volumetric emissivity profiles fitted to the segment profiles. Top row: postshock volumetric emissivity profile with radius R (dotted lines) in Eq. (3) (left panel), projected emissivity profile using Eq. (2) (middle panel), and convolved with the PSF (right panel). Bottom row: same as top but for the projected “cap” model in Eq. (4). The same postshock volumetric emissivity profile is used (left panel). The projected emissivity profile is modified to fall to a fitted background value b (dash-dot lines) below r_f which is a fraction r_c of the shell radius R, r < r_f = r_cR (middle panel, dashed lines). This was then convolved with the PSF (right panel).