Outflows in Ultraluminous X-ray sources: unveiling properties of super-Eddington accretion


Advisors : Jean-François OLIVE, Olivier GODET

Start : Octobre 2018

Group : GAHEC

Ultraluminous X-ray sources (ULXs) are off-nuclear extragalactic sources emitting X-ray luminosity Lx > 3×10^39 erg/s. These sources are most likely powered by accretion of matter coming from a companion around a compact object. The observed luminosities are larger than the Eddington limit for a black hole (BH) with a mass around 20 solar masses. The Eddington limit corresponds to the maximum luminosity for an astrophysical source that is powered by isotropic accretion of matter. For luminosities larger than this limit, radiation pressure is assumed to overcome gravity leading to the appearance of strong outflows with complex structures. The characteristics of these accretion flows and associated outflows/winds as well as the nature of the central compact object are still highly debated.

In some cases, the compact object was demonstrated to be a neutron star implying that these particular sources exceed the Eddington limit by several orders of magnitude. The propreties of such accretion flows, in particular the existence of strong outflows ejected by the central source in that extreme accretion regime are still poorly understood.

A better understanding of the ULX nature could shed important lights on how supermassive BHs (SMBHs) with masses ranging from 10^6 to 10^10 solar masses are formed and evolve. These supermassive BHs are mostly located in the nucleus of the most massive galaxies. Indeed, IMBHs could be the fundamental bricks to form SMBHs. Moreover, multiple episodes of super-Eddington accretion onto these IMBHs along with BH mergers could play an essential role to grow supermassive BHs.