Abstract

We apply a reflection-based model to the best available XMM -Newton spectra of X-ray bright Ultraluminous X-ray (ULX) sources (NGC 1313 X-1, NGC 1313 X -2, M 81 X -6, Holmberg IX X -1, NGC 5408 X -1 and Holmberg II X -1). A spectral drop is apparent in the data of all the sources at energies 6-7 keV. The drop is interpreted here in terms of relativistically blurred ionized reflection from the accretion disk. A soft-excess is also detected from these sources (as usually found in the spectra of AGN ), with emission from O K and Fe L, in the case of NGC 5408 X -1 and Holmberg II X -1, which can be understood as features arising from reflection of the disk. Remarkably, ionized disk reflection and the associated powerlaw continuum provide a good description of the broad-band spectrum, including the soft-excess. With exception of NGC 1313 X -2 and M81 X -6, there is no requirement for thermal emission from the inner disk in the description of the spectra. The black holes of these systems must then be highly spinning, with a spin close to the maximum rate of a maximal spinning black hole. The results require the action of strong light bending in these sources.We discuss our results in terms of the masses of the black holes.