NATURE OF THE EXTREME ULTRALUMINOUS X-RAY SOURCES

In this proof-of-concept study we demonstrate that in a binary system mass can be transferred toward an accreting compact object at an extremely high rate. If the transferred mass is efficiently converted to X-ray luminosity (with disregard of the classical Eddington limit) or if the X-rays are focused into a narrow beam, then binaries can form extreme ultraluminous X-ray (ULX) sources with an X-ray luminosity of L-X greater than or similar to 10(42) erg s(-1). For example, Lasota and King argued that the brightest known ULX (HLX-1) is a regular binary system with a rather low-mass compact object (a stellar-origin black hole (BH) or a neutron star (NS)). The predicted formation efficiencies and lifetimes of binaries with the very high mass transfer rates are large enough to explain all observed systems with extreme X-ray luminosities. These systems are not only limited to binaries with stellar-origin BH accretors. Notably, we have also identified such objects with NSs. Typically, a 10 M. BH is fed by a massive (similar to 10 M-circle dot) Hertzsprung gap donor with Roche lobe overflow (RLOF) rate of similar to 10(-3)M(circle dot) yr(-1) (approximate to 2600M over dot(Edd)). For NS systems the typical donors are evolved low-mass (similar to 2 M-circle dot) helium stars with RLOF rate of similar to 10(-2)M(circle dot) yr(-1). Our study does not prove that any particular extreme ULX is a regular binary system, but it demonstrates that any ULX, including the most luminous ones, may potentially be a short-lived phase in the life of a binary star.