Comparison between arc cross sections. Left panel: contours of constant arc cross section in terms of the PNFW parameters. Solid lines correspond to and dashed lines correspond to . The contours of constant arc cross section from left to right are 1 × 10^-6,1 × 10^-5, 1 × 10^-4, 10^-3,0.01,0.02,0.04,0.08,0.16 and 0.24. Right panel: contours of (Eq. (24)). In both plots, the dash-dotted line shows . Calculations were made for R_th = 10.