Probing Nature of Ultraluminous X-ray Sources using Broad-band X-ray Data

The origin of the extreme luminosities displayed by ultraluminous X-ray sources (ULXs) may relate to either super-Eddington accretion or the presence of black holes more massive than standard stellar remnants, but is as yet undetermined, despite frequent observational efforts with soft X-ray missions. The Nuclear Spectroscopic Telescope Array (NuSTAR), launched in June 2012, will for the first time allow the nature of the hard X-ray emission from ULXs to be determined, opening up a totally new observational window into these enigmatic sources. In combination with a decicated XMM legacy program and Suzaku observations providing simultaneous soft X-ray coverage, these high energy observations will achieve unprecedented broadband X-ray spectra for a small sample of bright ULXs, resulting in ~1.7 Ms of new X-ray data 400 ks XMM, ~500 ks Suzaku, ~800 ks NuSTAR) in the first year of operation, allowing us to further probe the nature of these sources. Here, we discuss plans and predictions for the NuSTAR ULX program, and also present some preliminary results.