The NuSTAR ULX program

¹ Université de Toulouse; UPS-OMP; IRAP , Toulouse, France
² CNRS; Institut de Recherche en Astrophysique et Planétologie, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France
³ Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
⁴ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Lyngby, Denmark
⁵ Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
⁶ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁷ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125
⁸ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
⁹ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁰ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
¹¹ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
¹² INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
¹³ Harvard-smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA

^a e-mail: matteo.bachetti@irap.omp.eu

Published online: 8 January 2014

Abstract

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.