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

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica (IASF) del CNR, Sezione di Bologna, Via Gobetti 101, 40129 Bologna, Italy
² CEA Saclay, DSM/DAPNIA/SAp (CNRS FRE 2591), 91191 Gif-sur-Yvette Cedex, France
³ INTEGRAL Science Data Centre, Chemin d'Écogia 16, 1290 Versoix, Switzerland
⁴ The Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA
⁵ Observatory of Geneva, 51 chemin des Maillettes, 1290 Sauverny, Switzerland

Corresponding author: L. Foschini, foschini@bo.iasf.cnr.it

Received: 10 September 2003
Accepted: 3 December 2003

Abstract

We present observations with XMM-Newton of M 33 X-8, the ultraluminous X-ray source ( 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 >, but with proper parameters taking into account different effects, the best estimate becomes . 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.