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 , \alpha , of 1.89 \pm ^ { 0.40 } _ { 0.79 } and a disk-blackbody with a temperature , kT , of 1.10 \pm 0.05 keV and a projected inner-disk radius of 55.4 \pm ^ { 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 \pm 0.2 keV and a projected inner-disk radius of 7 \pm 2 km .
The spectrum of M33 X-9 is rather hard with \alpha = 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 .