BeppoSAX spectroscopy of the luminous X-ray sources in M 33

The nearby galaxy M 33 was observed by the imaging X-ray instruments on-boa rd BeppoSAX. Two observations at different phases of the 105.9 day intensit y cycle of the luminous central source X-8 failed to reveal the expected mo dulation, suggesting that it is probably transitory. Similar behavior has b een observed from several X-ray binary sources. This strengthens somewhat t he idea that M 33 X-8 is a black hole accreting from a binary companion. Th e 0.2-10 keV spectrum of M 33 X-8 can best be modeled by an absorbed power- law with a photon index, alpha, of 1.89 +/- (0.79)(0.40) and a disk-blackbo dy with a temperature, kT, of 1.10 +/- 0.05 keV and a projected inner-disk radius of 55.4 +/- (7.7)(6.0) km. The spectral shape is in good agreement w ith earlier ASCA results. The 2-10 keV spectra of M 33 X-4, X-5, X-7, X-9 a nd X-10 are ah consistent with power-law or bremsstrahlung models, whilst t hat of X-6 appears to be significantly more complex and may be reasonably w ell modeled with a disk-blackbody with kT = 1.7 +/- 0.2 keV and a projected inner-disk radius of 7 +/- 2 km. The spectrum of M 33 X-9 is rather hard w ith alpha = 1.3. Compared to earlier Einstein and ROSAT observations, M 33 X-7 and X-9 varied in intensity, M 33 X-4, X-6, and X-10 may have varied an d hi 33 X-5 remained constant.