  The long term evolution of ULX with their spectral and luminosity variations
in time give important clues on the nature of ULX and on the accretion process
that powers them. We report here the results of a Swift-XRT 6-year monitoring
campaign of the closest example of a persistent ULX, M33 X-8, that extends to
16 years the monitoring of this source in the soft X-rays. The luminosity of
this source is a few 10^39 erg/s, marking the faint end of the ULX luminosity
function. We analysed the set of 15 observations collected during the Swift
monitoring. We searched for differences in the spectral parameters at different
observing epochs, adopting several models commonly used to fit the X-ray
spectra of ULX. The source exhibits flux variations of the order of 30%. No
significant spectral variations are observed along the monitoring. The average
0.5-10 keV spectrum can be well described by a thermal model, either in the
form of a slim disk, or as a combination of a Comptonized corona and a standard
accretion disk.

High Energy Astrophysical Phenomena