2005ApJ...623L.109M

We report the detection of radio emission coincident with the ultraluminous X-ray source (ULX) in Holmberg II. The radio emission is diffuse and resolved, covering an area ∼60x40 pc in extent and well matched to the recently discovered He II nebula surrounding the X-ray source. Comparison of the radio and optical properties of this extended radio emission argue against its association with either an H II region or a supernova remnant. This is additional evidence that this ULX is not powered by a stellar mass object whose emission is relativistically beamed toward the observer, and thus is either a super-Eddington source or an intermediate-mass black hole as suggested by optical observations. Implications of this result to future and existing radio studies of ULXs are discussed.