Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-12-04
Astron.Astrophys.416:529,2004
Astronomy and Astrophysics
Astrophysics
9 pages, 5 figures. Accepted for publication on A&A Main Journal. A bug in the introduction has been corrected (a citation)
Scientific paper
10.1051/0004-6361:20034303
We present observations with XMM-Newton of M33 X-8, the ultraluminous X-ray source (L_{0.5-10 keV} ~ 2x10^39 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_sun, but with proper parameters taking into account different effects, the best estimate becomes 12 M_sun. Our analysis favours the hypothesis that M33 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.
Cocco G. Di
Courvoisier Thierry J. -L.
Dadina Mauro
Foschini Luca
Fuchs Ya"el
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