X-ray reflection in a sample of X-ray bright ultraluminous X-ray sources

We apply a reflection-based model to the best available XMM-Newton spectra of X-ray bright ultraluminous X-ray 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 disc. A soft excess is also detected from these sources [as usually found in the spectra of active galactic nuclei (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 disc. Remarkably, ionized disc reflection and the associated power-law continuum provide a good description of the broad-band spectrum, including the soft excess. There is no requirement for thermal emission from the inner disc 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 suggest that they could be strongly accreting black holes in which most of the energy is extracted from the flow magnetically and released above the disc thereby avoiding the conventional Eddington limit.

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