Time Variability of the X-Ray Sources in M33

We have analyzed the variability of the X-ray sources in M33 observed by the Einstein Observatory telescope. Our analysis has been based on statistical techniques sensitive to variability over several time scales and has shown that two of the 15 known sources are variable above the 99.73% confidence level. The light curve of one of these sources, M33 X-7, exhibits a variability pattern of high and low states, suggesting an eclipsing binary X-ray source. The result would be the first identification of a close accreting binary system with an X-ray source in an external galaxy other than the Magellanic Clouds. The data suggest a binary period P = 1.7857 days and an eclipse duration of ~0.4 days. The nuclear source M33 X-8 varies only in the softest part of the spectrum (kT <= 1.2 keV). The observations suggest a rapid variability (t ~ 3000 s) and show a rapid flare with a rise time shorter than 3 days together with longer time scale variability.