Speaker
Description
There is a mysterious point-like source in the starburst dwarf galaxy NGC 5408. It is the strongest compact radio source in that galaxy (5-GHz flux density of $\approx$2 mJy, corresponding to a luminosity of $\approx$3 $\times 10^{35}$ erg s$^{-1}$). In the optical, it is too bright and too red ($V \approx 19.5$, $R \approx 18.1$, corresponding to $M_V \approx -9$, $M_R \approx -10.4$) to be consistent with any individual star or HMXB or young star cluster. Its optical spectrum is completely dominated by two sets of strong emission lines: broad and narrow. Very broad Balmer and HeI lines imply a dense, powerful and highly variable outflow with speeds of up to $\pm$3,000 km s$^{-1}$. Photo-ionized, narrow higher-ionization lines (eg He II 4686) suggest an intrinsic X-ray luminosity of several times $10^{40}$ erg s$^{-1}$ (mostly blocked from our view). The amount of power required to explain X-ray and radio emission suggests the presence of a fast-accreting black hole, but the optical spectrum is very different from any previously known ULX. We speculate that it is a rare type of microquasar going through a short-lived evolutionary phase of its massive donor star. The optically thick outflow may come from a super-critical accretion disk or from common envelope ejection, with the large outflow photosphere responsible for the optical continuum (blackbody radius $\approx 1000 R_\odot$). The radio emission may trace a compact (very young?) shock-ionized bubble.
| Contribution | Oral talk |
|---|---|
| Affiliation | INAF-Turin Observatory |
| roberto.soria@inaf.it |
