Spectral index between 1.4 GHz and 153 MHz for 417 sources, plotted as a function of the 153 MHz flux (black dots). The detection limit is almost fully determined by the 153 MHz survey due to the sensitivity of the 1.4 GHz observations. The median spectral index is  − 0.76 (dashed line). 16 sources have a spectral index below  − 1.3 (dotted line). The spectral indices are binned to emphasize the spectral flattening towards lower flux densities (white squares; see Table 6).