Jets and winds from the most powerful compact object in NGC 5408

Super-critical accretion onto stellar-mass compact objects is sometimes revealed by ultraluminous X-ray sources (ULXs). In other cases, X-rays are heavily obscured along our line of sight, but a super-Eddington accretion power can be indirectly revealed by multi-band studies. We have identified a candidate super-critical source in NGC,5408 (labelled X-2). We argue that it is powered by accretion; it is among the most powerful in the local Universe, despite its apparent X-ray faintness; and it has exceptional optical properties (both in the continuum and in the lines). We carried out a multi-band imaging and spectral study with the Australia Telescope Compact Array, the Very Large Array, the Space Telescope, the Very Large Telescope, and the X-ray Observatory. Hubble Chandra We found an unresolved source with variable radio emission (5 GHz luminosity of a few times 10^ 35 erg s -1 $); red optical continuum with absolute magnitudes M_ V,0 ≈ -9.1, M_ I,0 II łambda4686; point-like hard X-ray emission; and marginally resolved soft X-ray emission. We also found an ≈20-pc He^++ nebula near X-2 associated with candidate Wolf-Rayet stars. The optical continuum declined by 2 mag between 2010 and 2024 as did the intensity of the broad and narrow lines. We argue that the system is powered by the radiative and mechanical output of a super-Eddington stellar mass black hole and estimate an accretion power of ≳10^40 erg s$^ -1 . We propose that X-2 is a Compton-thick ULX seen at high inclination with a flaring jet and X-ray emission emerging from the polar funnel. We argue that the optical continuum and the broad emission lines are signatures of an optically thick outflow launched either from the accretion disk or from an luminous blue variable-like donor star, but more speculatively, we posit that it was caused by the long sought-for, elusive phase of common envelope ejection. We suggest that X-2 is undergoing a short-lived phase of hyper-Eddington accretion.