Examples of 400 MHz, uGMRT images where the A399-A401 bridge visibilities were injected, as a function of the spectral index. In particular, we show the different contributions of the bridge when its spectral index steepens, from α = 0 to α = 2.0. In the first two top panels, the bridge is clearly detectable, while approaching the lower limit for the spectral index (left-bottom panel) the emission is less visible, until there is no significant change with respect the image without any injected visibilities. Contour levels are drawn from 2σ_rms (in green) and increase up to 5σ_rms (black). A negative contour level at −3σ_rms is shown in red.