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3rd of August 2021
ASKAP LMC and SUMSS comparison
by Pennock et al.
The Sydney University Molonglo Sky Survey (SUMSS) was carried out between 1997 and 2003 at 843 MHz with a beam size of 45 arcseconds × 45 arcseconds to create a catalogue that covers the whole sky South of a declination of = −30 degrees. The positions in the catalogue are accurate to within 1-–2 arcseconds for sources brighter than 20 mJy/beam and are always better than 10 arcsec. The SUMSS catalogue is therefore a good comparison for the new 888 MHz ASKAP catalogue of the Large Magellanic Cloud by Pennock et al.

The new ASKAP 888 MHz source lists ( Gold and Silver, 53,547 sources) were cross-matched with the SUMSS 843 MHz catalogue (3,829 sources), with a search radius of 10 arcseconds, yielding 3,211 matches. The flux densities are compared in the figure above; the dashed blue line indicates the one-to-one relation and the red line indicates the line of best fit. The sources that show higher than expected flux density from ASKAP compared to SUMSS were found to be near bright point sources with nearby artefacts which, when coinciding with a fainter source, increases their total flux density. The sources that show lower than expected flux density from ASKAP compared to SUMSS were found to be extended sources -- these were seen as one source in SUMSS but multiple components in ASKAP, hence the lower flux density.

The fit is not perfect, in part due to the small but appreciable effect that the small frequency difference from 843 to 888 MHz causes on average a lower flux density at 888 MHz due to the general tendency for most celestial radio sources to be brighter at lower frequencies. Also, sources that have brightened or faded in the ~20 years between the surveys will have different flux densities in this comparison. Overall, however, the correlation between surveys is good.

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