The NuSTAR ULX program

We present the results of the first large program of broadband ULX observations with Chandra, Suzaku, XMM-Newton and NuSTAR, yielding high-quality spectra and timing measurements from 0.3-30 keV in 7 ULXs, providing powerful information for understanding the accretion modes and nature of the central BHs. In particular, we find that all ULXs in our sample have a clear cutoff above 10 keV. This cutoff is less pronounced than expected by Comptonization from a cold, thick corona. We confirm the presence of a soft excess at low energies in the brightest ULXs, with temperatures below ∼ 0.5 keV. We make an estimates on the masses of several ULXs based on spectral variability and model fitting.