  The physical nature of ultraluminous x-ray sources is uncertain. Stellar mass
black holes with beamed radiation and intermediate mass black holes with
isotropic radiation are two plausible explanations. We discovered radio
emission from an ultraluminous x-ray source in the dwarf irregular galaxy NGC
5408. The x-ray, radio and optical fluxes as well as the x-ray spectral shape
are consistent with beamed relativistic jet emission from an accreting stellar
black hole. If confirmed, this would suggest that the ultraluminous x-ray
sources may be stellar-mass rather than intermediate mass black holes. However,
interpretation of the source as a jet-producing intermediate-mass black hole
cannot be ruled out at this time.
