Images of the spectral index and spectral index limits for the sources where part of the emission is USS. The contours represent the emission 5σ at 1.4 GHz (starting at 0.15 mJy beam⁻¹ and increasing with factors of 2). The spectral index inside those contours are detections, while the ones outside are lower limitsto the spectral index (due to the non-detection of Apertif; see the text for details). The beam size is 12′′ × 16′′ and is indicated in the bottom-left corner. The edges around the sources with a systematically flatter spectral index are the results of bias in the way the limits were derived. Further details are provided in Sect. 3. The white crosses represent the optical identifications.