Cumulative distribution function of R(α), normalised to unit area over the interval 0 ≤ α ≤ 4. The horizontal green line marks the 95% probability that the spectral index of the bridge in A399-A401 takes on a value smaller than α ∼ 2.2 (vertical blue line) if the bridge were detected in our uGMRT observations. The non-detection sets a lower limit for the spectral index at α_l > 2.2 with a 95% confidence level.