University of Leicester
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ULXs: Neutron stars versus black holes

journal contribution
posted on 2016-12-02, 12:30 authored by Andrew King, J-P. Lasota
We consider ultraluminous X–ray systems (ULXs) where the accretor is a neutron star rather than a black hole. We show that the recently discovered example (M82 X–2) fits naturally into the simple picture of ULXs as beamed X–ray sources fed at super-Eddington rates, provided that its magnetic field is weaker (≃1011G) than a new-born X-ray pulsar, as expected if there has been mass gain. Continuing accretion is likely to weaken the field to the point that pulsing stops, and make the system indistinguishable from a ULX containing a black hole. Accordingly we suggest that a significant fraction of all ULXs may actually contain neutron star accretors rather than black holes, reflecting the neutron-star fraction among their X-ray binary progenitors. We emphasize that neutron-star ULXs are likely to have higher apparent luminosities than black hole ULXs for a given mass transfer rate, as their tighter beaming outweighs their lower Eddington luminosities. This further increases the likely proportion of neutron-star accretors among all ULXs. Cygnus X–2 is probably a typical descendant of neutron-star ULXs, which may therefore ultimately end as millisecond pulsar binaries with massive white dwarf companions.


JPL acknowledges support from the French Space Agency CNES and Polish NCN grants UMO-2013/08/A/ST9/00795 and DEC- 2012/04/A/ST9/00083. ARK thanks the Institut d’Astrophysique, Paris, for hospitality during a visit where this Letter was performed. Theoretical astrophysics research at the University of Leicester is supported by an STFC Consolidated Grant.



Monthly Notices of the Royal Astronomical Society, (May 01, 2016) 458 (1): L10-L13.

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/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


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Monthly Notices of the Royal Astronomical Society


Oxford University Press (OUP), Royal Astronomical Society





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