Suzaku observations of ultraluminous X-ray sources in nearby galaxies

Recent results indicate that the majority of ULXs are stellar remnant black holes accreting material at or above the Eddington rate, rather than sub-Eddington accretion onto intermediate mass black holes. However, precisely how these ULXs accrete material at a super-Eddington rate remains an open question. In this work, we present the results of an analysis of 21 high quality spectra (> 4000 counts) extracted from a sample of 15 ULXs detected in nearby galaxies (D < 4.2 Mpc), using Suzaku data. We confirm that these high quality datasets require two component models to provide an adequate description of the spectra. A disc plus Comptonised corona model suggests an anti-correlation between the corona's temperature and its optical depth, as found in previous ULX studies. Alternatively, fitting by a blackbody plus Comptonised corona model allows a simple calculation of an outflowing wind radius (assuming this provides the soft component) and we find that the wind becomes larger as the accretion rate increases, consistent with theoretical predictions of Super-Eddington accretion. Finally, we demonstrate that the mass of the black hole powering these ULXs can be constrained to 3-16 M_{⊙} by modelling the hard spectral component using a Kerr black hole model.