ULXs: Neutron stars versus black holes
Abstract
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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- May 2016
- DOI:
- 10.1093/mnrasl/slw011
- arXiv:
- arXiv:1601.03738
- Bibcode:
- 2016MNRAS.458L..10K
- Keywords:
-
- accretion;
- accretion discs;
- black hole physics;
- binaries: close;
- pulsars: general;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 4 pages, accepted for publication as a Letter in Monthly Notices of the Royal Astronomical Society