NATURE OF THE EXTREME ULTRALUMINOUS X-RAY SOURCES

• Published in: The Astrophysical journal 2015-09, Vol.810 (1), p.1-8
• Main Authors: Wiktorowicz, Grzegorz, Sobolewska, Ma gorzata, Sa dowski, Aleksander, Belczynski, Krzysztof
• Format: Article
• Language: English
• Subjects: Approximation; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BINARY STARS; Binary systems; Binary systems (materials); BLACK HOLES; HELIUM; LUMINOSITY; MASS TRANSFER; NEUTRON STARS; NEUTRONS; ROCHE EQUIPOTENTIALS; stars: black holes; stars: neutron; X RADIATION; X-RAY SOURCES; X-rays; X-rays: binaries
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.1088/0004-637X/810/1/20

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 . 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 BH is fed by a massive ( ) Hertzsprung gap donor with Roche lobe overflow (RLOF) rate of ). For NS systems the typical donors are evolved low-mass ( ) helium stars with RLOF rate of . 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.