We present a new image of a 26.5 square degree region in the Bootes constellation obtained at 1.4GHz using the Aperture Tile in Focus (Apertif) system on the Westerbork Synthesis Radio Telescope. We use a newly developed processing pipeline which includes direction-dependent self-calibration which provides a significant improvement of the quality of the images compared to those released as part of the Apertif first data release. For the Bootes region, we mosaic 187 Apertif images and extract a source catalog. The mosaic image has an angular resolution of 27x11.5 arcsec and a median background noise of 40uJy/beam. The catalog has 8994 sources and is complete down to the 0.3mJy level. We combine the Apertif image with LOFAR images of the Bootes field at 54 and 150MHz to study spectral properties of the sources. We find a spectral flattening towards low flux density sources. Using the spectral index limits from Apertif non-detections we derive that up to 9% of the sources have ultra-steep spectra with a slope steeper than -1.2. Steepening of the spectral index with increasing redshift is also seen in the data showing a different dependency for the low-frequency spectral index and the high frequency one. This can be explained by a population of sources having concave radio spectra with a turnover frequency around the LOFAR band. Additionally, we discuss cases of individual extended sources with an interesting resolved spectral structure. With the improved pipeline, we aim to continue processing data from the Apertif wide-area surveys and release the improved 1.4GHz images of several famous fields.

Cone search capability for table J/A+A/676/A37/table1 (Apertif source catalog)

Cone search capability for table J/A+A/676/A37/table3 (Spectral indices of the sources)