An ultraluminous X-ray microquasar in NGC 5408?

We studied the radio source associated with the ultraluminous X-ray source in NGC 5408 (L(X) approximate to 10(40) erg s(-1)). The radio spectrum is steep (index approximate to -1), consistent with optically thin synchrotron emission, not with flat-spectrum core emission. Its flux density (approximate to 0.28 mJy at 4.8 GHz, at a distance of 4.8 Mpc) was the same in the March 2000 and December 2004 observations, suggesting steady emission rather than a transient outburst. However, it is orders of magnitude higher than expected from steady jets in stellar-mass microquasar. Based on its radio flux and spectral index, we suggest that the radio source is either an unusually bright supernova remnant, or, more likely, a radio lobe powered by a jet from the black hole (BH). Moreover, there is speculative evidence that the source is marginally resolved with a radius similar to 30 pc. A faint H II region of similar size appears to coincide with the radio and X-ray sources, but its ionization mechanism remains unclear. Using a self-similar solution for the expansion of a jet-powered electron-positron plasma bubble, in the minimum-energy approximation, we show that the observed flux and (speculative) size are consistent with an average jet power approximate to 7 x 10(38) erg s(-1)similar to 0.1L(X) similar to 0.1L(Edd), an age approximate to 10(5) yr, a current velocity of expansion approximate to 80 km s(-1). We briefly discuss the importance of this source as a key to understand the balance between luminosity and jet power in accreting BHs.