Spectral energy distribution of the gamma-ray emission from HB 21 modeled as a flat circle of 1.125 radius. Red error bar is statistical uncertainty. An additional systematic uncertainty of 10% (E < 560 MeV) and of 5% (E > 560 MeV) is represented by the black error bar. The solid thin black curve is the log-parabola (Eq. (3)) used to model the overall spectrum. Curves with extreme values of α and β, within statistical uncertainty are also shown (solid thick black curves). Dotted, dashed, and dash-dotted curves are the best spectral descriptions for segments NW, S, and NE respectively (see Table 2).