Spectral index, , distribution between 327 MHz and 153 MHz (grey points). The difference in resolution is 54″ × 96″ (WENSS) and 25″ (GMRT) and multiple GMRT matches to a single WENSS source have been merged. The vertical line shows 5σ_avg, where σ_avg is the average rms noise in the GMRT mosaic. The diagonal dotted line indicates the incompleteness limit due to the sensitivity of WENSS. The horizontal dashed line shows the KM mean spectral index of −0.84 ± 0.02which takes the upper limits into account. The large black triangles show the median spectral index in 4 logarithmic bins. Error bars on individual points are not plotted for clarity, but a single bar in the top right indicates the maximum and minumum errors in the dataset.