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

We present observations with XMM-Newton of M 33 X-8, the ultraluminous X-ray source (L_0.5-10 keV ≈ 2× 10³⁹ erg/s) closest 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.

Based on public observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA).