RADIO EMISSION ASSOCIATED WITH THE ULTRALUMINOUS X-RAY SOURCE IN HOLMBERG Ⅱ

摘要

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 ～60 x 40 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.