2 VARIABLE X-RAY SOURCES IN M31

We have analyzed the entire set of Einstein observations of M31, consisting of five imaging proportional counter and nine high resolution imager observations, for variability of the X-ray sources. The time scales explored range up to ∼ 105 s, the maximum time span of individual observations. We detect variability above the adopted 99.73% significance level in two sources and derive constraints on their possible periodicity. The observed X-ray luminosities, the pattern of variability, and the optical identifications suggest the possibility that one of them is a low-mass X-ray binary and the other one a compact massive X-ray binary. We also discuss the analysis of the nonvariable sources and derive upper limits on the detectable variability of the various sources. The number of variable sources detected is in agreement with the expectations based on the features of intense Galactic X-ray sources and on the characteristics of the observations.