Spectral index based on 610 MHz and 1.4 GHz (NVSS) data (α) as a function of 610 MHz flux density, separately shown for the XMM-LSS (top panel), and outer XXL-N (bottom panel) areas. The dashed line in both panels indicates the constraint on α placed by the NVSS detection limit (2.5 mJy), and the vertical full line indicates the threshold beyond which the sample is not expected to be biased by the different detection limits. The horizontal line indicates the average spectral index for sources withflux densities above this threshold.