The central X-ray source in M 33.

Abstract

The Local Group galaxy M33 was observed with the European X-ray observatory Exosat. Two of the previously reported 11 X-ray sources in M33 were detected; analysis confirmed the dominance of the source M33 X-8, which coincides with the nucleus of M33. With an 0.1-6 keV X-ray luminosity of about 1.5 x 10 to the 39th erg/s, it is the brightest steady X-ray source in the Local Group. The X-ray spectrum between 0.1 and 6 keV can be described either by a thermal bremsstrahlung model or by a power law with an exponential cutoff. In both cases, the best-fitting parameters are in agreement with a soft spectrum. The X-ray luminosity of M33 X-8 during the Exosat observation is identical with the one at the time of the Einstein observations. Results are discussed in the context of various models concerning the nature of the central X-ray source in M33.