2001A&A...368..420P

The nearby galaxy M 33 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 M 33 X-8 is a black hole accreting from a binary companion. The 0.2-10 keV spectrum of M 33 X-8 can best be modeled by an absorbed power-law with a photon index, α, of 1.89±^0.40_0.79 and a disk-blackbody with a temperature, kT, of 1.10±0.05keV and a projected inner-disk radius of 55.4±^6.0_7.7km. This spectral shape is in good agreement with earlier ASCA results. The 2-10keV spectra of M 33 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±0.2keV and a projected inner-disk radius of 7±2km. The spectrum of M 33 X-9 is rather hard with α=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 and M 33 X-5 remained constant.