Abstract

The spectra from ultraluminous X-ray sources have been described by several different models which give very different results for the inferred black hole mass, ranging from intermediate (>100 solar masses) to standard mass (~ 5 – 20 solar masses) black holes. Here we revisit the question of the nature of ULX through a detailed investigation of their spectral shape using the highest quality X-ray data currently available in the XMM -Newton public archives. We apply phenomenological models to characterise the spectra of these objects and more physically motivated models in order to explore the physical processes underlying these characteristics. Results show that the spectra of these sources are fundamentally different to that of Galactic X-ray binaries, whilst the application of physical models indicates a more extreme version of the highest known luminosity state, the very high state. We therefore speculate that in observing ULXs we are observing stellar-mass black holes residing in a new ‘ultraluminous’ state.