The link between SS433 and the ultra-luminous X-ray sources.

The nature of the ultra-luminous X-ray sources (ULXs) discovered in significant numbers in nearby galaxies by XMM and Chandra has been a subject of great debate. Interpreted purely as Eddington-limited accreting binaries leads to estimates of their compact object masses substantially greater than the ~10MΘ black holes typically found in galactic X-ray binaries. Consequently, some have suggested that ULXs are the long-sought intermediate-mass black holes with masses ≥100MΘ, a stepping-stone to the supermassive black holes found in galactic nuclei, such as our own. It has been noted that our Galaxy does not appear to harbour any ULXs, but this may be a result of observational difficulties due to obscuration in the galactic plane as well as the viewing angle of interacting binaries. The prototypical micro-quasar, SS433, one of the most bizarre objects in the Galaxy, is observed to be a weak X-ray source, yet the kinetic energy of its relativistic, precessing jets is vastly (at least a factor 1000) greater. In spite of its importance as the nearest example of directly observable relativistic phenomena, we know remarkably little about the nature of this binary system. There are ongoing arguments not only about the mass of the compact object, but even as to whether it is a black hole or a neutron star, an argument that recent high resolution optical spectroscopy has contributed to. Combined with the INTEGRAL discovery of a new class of highly obscured galactic high-mass X-ray binaries, one of which we have found to precess on a similar timescale to SS433, we suggest that these would indeed be seen by external observers as ULXs, once additional effects such as relativistic beaming are included.