Recent highlights from NuSTAR Observations of extreme ULXs

The origin of the intense luminosities displayed by ultraluminous X-ray asources (ULXs) may relate to either super-Eddington accretion or the presence of black holes more massive than the standard stellar remnants observed in our own Galaxy, e.g. intermediate mass black holes with masses of 100's or 1000's of solar masses. Although recent broadband X-ray spectroscopy of bright ULXs with the Nuclear Spectroscopic Telescope Array (NuSTAR) is suggestive of the former, a definitive answer has not yet been reached owing to continued difficulty constraining ULX masses. Here we report on highlights from our continued NuSTAR observations of extreme (Lx > 1e40 erg/s) ULXs, extending our broadband X-ray analysis into the time domain with multi-epoch observations which allow us to examine the evolution of these enigmatic sources, and thus shed further light on their accretion processes.