Variability of the X-ray sources in M 33.

Variability analysis of the X-ray sources in M33 observed with the Einstein telescope is presented. Two of the fifteen sources of M33 have been found to be variable beyond 99.77 percent significance level: M33 X-7 and M33 X-8. M33 X-7 showed a very strong variability on many time scales ranging from months to less than hours. This result signifies the first observation of a close accreting binary system with an X-ray source in an external galaxy other than the Magellanic Clouds. The nuclear source M33 X-8 is variable on a six-month time scale in the High Resolution Imager (HRI), the Imaging Proportional Counter (IPC), and the nonimaging Monitor Proportional Counter (MPC). The spectral characteristics of the X-ray emission of this source and the similarity with other low luminosity nuclei favor the possibility of galactic nuclear activity.