X-ray spectral variability as a probe of the compact objects powering ULXs

The nature of the accretor in most ultraluminous X-ray sources (ULXs) has remained unknown with the exception of a few ULXs unambiguously identified as pulsating neutron star accreting at super-Eddington rates. In this talk, I will present a novel method to estimate the nature of the accreting compact objects by accurately tracking the Luminosity-Temperature (L-T) diagram. We used data collected with the XMM-Newton satellite for several ULXs and modelled the spectra with a double thermal component. In some ULXs such as those in the NGC 5204 and NGC 1313 galaxies, there are significant deviations from the expectation of thin disc models. This occurs particularly at high luminosities. Such deviations are likely due to the accretion rate exceeding the supercritical rate with a consequent launching of powerful winds and deformation of the accretion disc. This interpretation allows us to forecast the mass of the compact object and, therefore, its nature which in some cases, e.g. Holmberg II X-1, is likely a black hole and in others, e.g. NGC 1313 X-2, is a neutron star.