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

Abstract

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 multiband 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; it has exceptional optical properties (both in the continuum and in the lines). We carried out a multiband imaging and spectral study with the ATCA, the VLA, HST, the VLT and Chandra. We found an unresolved source with: variable radio emission (5-GHz luminosity of a few x 10^{35} erg/s); red optical continuum with absolute mag M_{V,0} = -9.1, M_{I,0} = -10.6; exceptionally broad low-ionization emission lines (FWHM ~ 1700 km/s) with P-Cygni profiles; narrow, photoionized emission lines including strong He II 4686; point-like hard X-rays; marginally resolved soft X-rays. 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 broad and narrow lines. We argue that the system is powered by the radiative and mechanical output of a super-Eddington stellar mass BH, with an accretion power >~10^{40} erg/s. 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 LBV-like donor star, or, more speculatively, caused by the elusive phase of common envelope ejection. We suggest that we are looking at a short-lived phase of hyper-Eddington accretion.