Log(S2.1 GHz) vs. log(S843 MHz) for all ATCA XXL-S sources. Those not matched to a SUMSS source were assigned a SUMSS upper limit of S843 MHz < 6 mJy. The latter sources are clearly seen as the vertical line of data points with arrows at log(S843 MHz) ≈ 0.78 (arrows wereplotted at regular intervals below log(S2.1 GHz) ≈ 0.9 to avoid overcrowding the plot). The solid red line is the best fit line for all ATCA sources matched to a SUMSS source, corresponding to α = −0.81. At log(S2.1 GHz) ≈ 0.48 (S2.1 GHz ≈ 3 mJy), the spectral indices start to become highly censored because of the S843 MHz < 6 mJy limit. The dashed red line is the line corresponding to α = −0.66, the median spectral index of the ATCA sources with S2.1 GHz < 5 mJy (according to a survival analysis). The difference in slope between the two lines reflects the bias toward a steeper median spectral index for ATCA sources matched to SUMSS sources.
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