A radio nebula surrounding the ultraluminous X-ray source in NGC 5408

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 - 2.0. This corresponds to a physical size of 35-46 pc, and rules out interpretation of the radio emission as beamed emission from a relativistic jet. In addition, the radio spectral index of the counterpart is well determined from three frequencies and found to be alpha = -0.8 +/- 0.2. The radio emission is likely to be optically thin synchrotron emission from a nebula surrounding the X-ray source. The radio luminosity of the counterpart is 3.8 x 10(34) erg s(-1) and the minimum energy required to power the nebula is similar to 1 x 10(49) erg. These values are 2 orders of magnitude larger than in any Galactic nebula powered by an accreting compact object.