The NuSTAR ULX program

• Main Authors: Bachetti Matteo, Barret Didier, Boggs Steven E., Christensen Finn E., Craig William W., Fabian Andrew C., Forster Karl, Fürst Felix, Grefenstette Brian W., Hailey Charles J., Harrison Fiona A., Hornschemeier Ann E., Madsen Kristin K., Miller Jon M., Parker Michael, Ptak Andrew, Rana Vikram R., Risaliti Guido, Stern Daniel, Walton Dominic J., Webb Natalie A., Zhang William W.
• Format: Article
• Language: English
• Published: EDP Sciences 2014-01-01
• Series: EPJ Web of Conferences
• Online Access: http://dx.doi.org/10.1051/epjconf/20136406010
• ISSN: 2100-014X

We present the results of the first large program of broadband ULX observations with NuSTAR, XMM–Newton and Suzaku, yielding high-quality spectra and timing measurements from 0.3–30 keV in 6 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.