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Nature of the Extreme Ultraluminous X-ray Sources

Mar 30, 2015
8 pages
Published in:
  • Astrophys.J. 810 (2015) 1, 20
  • Published: Aug 25, 2015
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Abstract: (IOP)
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{L}_{{\rm{X}}}\gtrsim {10}^{42}\,\mathrm{erg}\,{{\rm{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⊙10\,{M}_{\odot } BH is fed by a massive (∼10 M⊙\sim 10\,{M}_{\odot }) Hertzsprung gap donor with Roche lobe overflow (RLOF) rate of ∼10−3 M⊙ yr−1\sim {10}^{-3}\,{M}_{\odot }\,{\mathrm{yr}}^{-1} (≈2600M˙Edd(\approx 2600{\dot{M}}_{\mathrm{Edd}}). For NS systems the typical donors are evolved low-mass (∼2 M⊙\sim 2\,{M}_{\odot }) helium stars with RLOF rate of ∼10−2 M⊙ yr−1\sim {10}^{-2}\,{M}_{\odot }\,{\mathrm{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.
Note:
  • ApJ - accepted (significant changes)
  • stars: black holes
  • stars: neutron
  • X-rays: binaries
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