Accretion geometry and variability of ULX pulsars

The recent discovery that extreme (L ≥ 10^{40} erg/s) ULXs can be powered by neutron stars raises the question how these objects can sustain such inferred luminosities and challenges models for accretion dominated by magnetic fields. All three known ULX pulsars (M82 X-2, NGC 5907 ULX1, and NGC 7793 P13) have pulse periods of around one second and also show orbital and super-orbital periods. By performing time- and phase-resolved spectroscopy over these periods we can constrain the accretion flow, begin to understand the role of beaming, and investigate the level to which the disk accretion is super Eddington. I will present an analysis of NuSTAR and XMM-Newton data of NGC 5907 ULX1 at four different super-orbital phases. While the spectral evolution is compatible with a precessing accretion disk over the super-orbital phase, this is difficult to reconcile with a regime where accretion happens in a slim disk regime. I will also discuss preliminary comparisons between the pulsed spectra of NGC 7793 P13 and M82 X-2 which show distinct differences. This analysis points to potential differences in their accretion geometry, even though their inferred luminosities are similar.