ASKAP Data Products for Project AS034 (ASKAP Early Science Broadband Survey): derived images

These data are a combined mosaic of two overlapping ASKAP tiles covering the GAMA-23 field.

The following is copied from the validation notes of the source SBIDs and also apply to these data.

These data represent a test observation with the full ASKAP array. While it may be useful for science purposes the primary goal of the release is to seek feedback on data quality from the science community. Although the image approaches expected noise levels in most areas, a common feature of early ASKAP data has been some low-level artefacts (at ~1%) very close to bright sources (a few hundred mJy and above). These artefacts generally appear as radial stripes and rapidly fade away from the source. The origin of these artefacts is yet to be determined, but they do not seem to emulate any known instrumental effect taken into account in commonly used wideband/widefield calibration and imaging software packages (such as ASKAPsoft, casa etc.). The spectral indices given in the catalogues were derived from Taylor-term imaging, which is a technique not yet widely used in radio astronomy. It has been identified that the distribution of the spectral indices in these catalogues deviates significantly from expectations for reasons that are not yet understood. Fitting spectra to sources in the continuum cubes is recommended to attain more reliable spectral indices. While the commissioning team investigates the cause of these issues in detail, we do not wish to hold the release of otherwise good quality data. Hence these test observations are being released, but with the validation flag set to “uncertain” — meaning user discretion is necessary in the interpretation for science purposes. Users should perform their own analysis of the flux scale, source positions and spectral indices given. The ASKAP operations team would be glad to receive feedback on any issues discovered.