XMM-Newton observations of the ultraluminous nuclear X-ray source in M 33

Abstract

We present observations with XMM-Newton of M 33 X-8, the ultraluminous X-ray source (L0.5-10kev ≈ 2×1039 erg/sclosest to the centre of the galaxy. The best-fit model is similar to the typical model of Galactic black holes in very high state. Comparison with previous observations indicates that the source is still in a very high state after about 20 years of observations. No state transition has been observed even during the present set of XMM-Newton observations. We estimate the lower limit of the mass of the black hole >6 M⊙, but with proper parameters taking into account different effects, the best estimate becomes 12 M⊙. Our analysis favours the hypothesis that M 33 X-8 is a stellar mass black hole candidate, in agreement with the findings of other authors. In addition, we propose a different model where the high luminosity of the source is likely to be due to orientation effects of the accretion disc and anisotropies in the Comptonized emission.