  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 to 2.0 arcseconds. 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 pm 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 and the minimum energy
required to power the nebula is ~1 x 10^49 erg. These values are two orders of
magnitude larger than in any Galactic nebula powered by an accreting compact
object.
