Radio surveys of ultraluminous X-ray sources (ULXs) allow the identification of supercritically accreting compact objects or stripped nuclear black holes in nearby galaxies. Zhang et al. identified 21 such objects by cross-matching a ULX catalog with the Rapid ASKAP Continuum Survey and Very Large Array Sky Survey. Selected targets were then followed up with ATCA observations. The ULXs appear to have a diverse population. (i) Three have a double-lobed radio structure with a compact core found in two of them and could be background quasars. (ii) Five associated with an extended radio structure reside in star-forming regions, and their radio emission is likely due to star-forming activities. Two of them show X-ray variability suggesting that they are ULXs embedded in star-forming regions. (iii) Thirteen are associated with an unresolved radio source, with a steep spectrum seen in eight and a flat or inverted spectrum seen in two. Remarkable cases include NGC 925 ULX 1 and NGC 6946 ULX 1, which are associated with an optical nebula.

The image above shows radio contours around the position of ESO 352-41 ULX 1. There is a spatially unresolved radio source at the ULX position, detected in RACS — placing the source in the third category above. However, no radio source is detected in VLASS above 4σ with an rms of 0.14 mJy at 3 GHz. The source was detected with ATCA at both 5.5 and 9 GHz. No optical counterpart can be seen in the underlying DES image. The red circle indicates the ULX position with a 3σ error radius. Radio contours are shown for RACS (blue), VLASS (magenta), and ATCA (green). The corresponding beam sizes are shown at the bottom-left corner. The scale bar has a length of 10 arcseconds.