Pulsing ULXs: tip of the iceberg?

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 spinup 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 ˙ν ˙≳10−10s−,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.