Distribution of the τ probability density as a function of the estimator (defined in the text) derived from the 100 × 143 cross-spectra. The top panel uses simulations containing only white noise, while the bottom panel uses simulations with the systematic effects residuals added. As an example of a cut through this two-dimensional plot, in the top panel an observed value of (horizontal black line) gives a probability distribution (red curve) that peaks at τ = 0.060. In the bottom panel, the same peak value of the probability distribution is obtained with an observed .