Ultra-bright X-ray sources

For many years it was believed that the so-called Eddington brightness is an absolute limit for accretion of matter. However, brightness of ultra-bright X-ray sources seems to exceed that theoretical limit. Are such sources really very bright tails of some X-ray systems, accretion of which is somehow exceeding Eddinton's limit, or alse do they contain some medium mass black holes for which the limit is not necessarily broken at all? New results shed some light on that issue.

All interesed are invited.
Agnieszka Majczyna