Pulsing ULXs: tip of the iceberg?
Abstract
We consider the three currently known pulsing ultraluminous X-ray sources (PULXs). We show that in one of them the observed spin-up rate requires super-Eddington accretion rates at the magnetospheric radius, even if magnetar-strength fields are assumed. In the two other systems, a normal-strength neutron star field implies super-Eddington accretion at the magnetosphere. Adopting super-Eddington mass transfer as the defining characteristic of ULX systems, we find the parameters required for self-consistent simultaneous fits of the luminosities and spin-up rates of the three pulsed systems. These imply near equality between their magnetospheric radii RM and the spherization radii Rsph where radiation pressure becomes important and drives mass-loss from the accretion disc. We interpret this near equality as a necessary condition for the systems to appear as pulsed, since if it is violated the pulse fraction is small. We show that as a consequence all PULXs must have spin-up rates \dot{ν }≳ 10^{-10} s^{-2}, an order of magnitude higher than in any other pulsing neutron-star binaries. The fairly tight conditions required for ULXs to show pulsing support our earlier suggestion that many unpulsed ULX systems must actually contain neutron stars rather than black holes.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- June 2017
- DOI:
- 10.1093/mnrasl/slx020
- arXiv:
- arXiv:1702.00808
- Bibcode:
- 2017MNRAS.468L..59K
- Keywords:
-
- accretion;
- accretion discs;
- binaries: close;
- stars: black holes;
- stars: neutron;
- pulsars: general;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 5 pages, MNRAS Letters, in press