X-ray study of Ultraluminous X-ray sources with XMM-Newton and NuSTAR

We propose simultaneous observations of four Ultraluminous X-ray sources with XMM-Newton and the hard X-ray observatory Nuclear Spectroscopic Telescope Array (NuSTAR), to be launched in February 2012. ULXs are of prime interest, as they may be associated with a poorly explained regime of super-Eddington accretion onto a stellar mass black hole (BH), or even contain sub-Eddington intermediate mass BHs. The combined observations spanning from 0.3 to more than 50 keV will remove the degeneracy of the emission model parameters derived from multi-component fitting of the spectra below 10keV. This broadband study will enable detailed comparisons of the ULX timing and spectral properties with those of better studied Galactic BH X-ray binaries, delivering breakthrough information on ULX BHs.