Fit to the relic surface brightness profile with a broken power-law gas-density profile. The best-fitting edge position is at 79′. At the edge the density changes by a factor of ≈2.18, which in the gas with adiabatic index 5/3 corresponds to the shock Mach number M ≈ 1.9. For comparison, the blue dashed curve shows the same model for M = 3.5 that is needed to explain the slope of the relic integrated radio spectrum (under the standard set of assumptions). This model is clearly excluded by the data.