Empirical probability density function (EPDF) of the pixel values for the linear composition map of the first sky region after the application of the filter. The EPDF is different from zero in the range  [− 11,   132] . Here, only the range  [− 10,   30]  is shown because outside this interval the EPDF is close to zero. The vertical red and green lines show the detection threshold based on the 98th percentile and the 5σ₁₅ level, respectively. Here, σ₁₅ is the standard deviation of the pixels with values less or equal to 15. The cyan line provides the Gaussian probability density function with zero mean and standard deviation given by σ₁₅.