2006MNRAS.368.1527S

We studied the radio source associated with the ultraluminous X-ray source in NGC5408 (L_X~ 10⁴⁰erg/s). The radio spectrum is steep (index ~ -1), consistent with optically thin synchrotron emission, not with flat-spectrum core emission. Its flux density (~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 ∼30 pc. A faint Hii 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 ~ 7x10³⁸erg/s ∼ 0.1L_X∼ 0.1L_Edd, an age ~10⁵ yr, a current velocity of expansion ~80 km/s. We briefly discuss the importance of this source as a key to understand the balance between luminosity and jet power in accreting BHs.