BeppoSAX spectroscopy of the luminous X-ray sources in M33

The nearby galaxy M33 was observed by the imaging X-ray instruments on-board BeppoSAX. Two observations at different phases of the 105.9 day intensity cycle of the luminous central source X-8 failed to reveal the expected modulation, suggesting that it is probably transitory. Similar behavior has been observed from several X-ray binary sources. This strengthens somewhat the idea that M33 X-8 is a black hole accreting from a binary companion. The 0.2-10 keV spectrum of M33 X-8 can best be modeled by an absorbed power-law with a photon index of 1.89 +0.40 -0.79 and a disk-blackbody with a temperature, kT, of 1.10 +/0 0.05 keV and a projected inner-disk radius of 55.4 +6.0 -7.7 km. This spectral shape is in good agreement with earlier ASCA results. The 2-10 keV spectra of M33 X-4, X-5, X-7, X-9 and X-10 are all consistent with power-law or bremsstrahlung models, whilst that of X-6 appears to be significantly more complex and may be reasonably well 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 M33 X-9 is rather hard with a photon index of 1.3. Compared to earlier Einstein and ROSAT observations, M33 X-7 and X-9 varied in intensity, M33 X-4, X-6, and X-10 may have varied and M33 X-5 remained constant.