Radio Counterparts of Ultraluminous X-Ray Sources in Nearby Galaxies

Abstract Radio surveys of ultraluminous X-ray sources (ULXs) allow us to find supercritically accreting compact objects (SS 433/W50-like systems) or stripped nuclear black holes in nearby galaxies. We identified 21 such objects by crossmatching a ULX catalog with the Rapid ASKAP Continuum Survey and Very Large Array Sky Survey. They may have a diverse population. (i) Three have a double-lobed radio structure with a compact core found in two of them and could be 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. Those with a steep spectrum are arguably candidates for SS 433/W50-like objects, with radio emission due to optically thin synchrotron radiation in a surrounding jet/wind powered nebula. Remarkable cases include NGC 925 ULX 1 and NGC 6946 ULX 1, which are associated with an optical nebula. Those with a flat or inverted spectrum could be accreting black holes with a compact jet, while the black hole mass is estimated to be several 10 6 –10 8 M ⊙ based on the fundamental plane. Redshift measurements are needed to firmly determine the association with their apparent host galaxy.