Spectral index diagrams of all sources (see text for details). The left panel shows sources detected in catalogs C4, C5, and C6. The size of the markers is proportional to the source S/N in C5. The typical errors are shown in the bottom right corner. The right panel shows the spectral index limits of the sources that are missing in either C4 or C6 estimated from the noise RMS of images M4 and M6 at the location of a source. The contours show the Gaussian-kernel density estimate. The dashed lines indicates the zero spectral index, and the dotted lines show median values of and calculated for the common sources.