NATURE OF THE EXTREME ULTRALUMINOUS X-RAY SOURCES

Abstract

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 LX ≳ 1042 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 (∼10 M⊙) Hertzsprung gap donor with Roche lobe overflow (RLOF) rate of ∼10-3 M⊙ yr-1 (≈2600M⊙Edd). For NS systems the typical donors are evolved low-mass (∼2 M⊙) helium stars with RLOF rate of ∼10-2 M⊙ 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.