Spectral index distribution against the LOFAR integrated flux density for compact sources. The median value α = −0.78 (horizontal line) is calculated for the sources with integrated flux density above 40 mJy at 150 MHz. The surrounding dashed curves show ±σ_α,S error due to flux density uncertainties. The inclined solid lines show spectral index lower limit calculated for the weakest Apertif source and for 1 mJy level corresponding to the completeness of 95%. An overall scatter, ±σ_α, of the spectral index is shown with dash-dotted lines.