We study three transient X-ray sources , that were bright in the central region of M31 galaxy in the year 2000 .
Observations with Chandra and XMM-Newton allowed us for the first time in the history of X-ray astronomy , to build light curves of transient sources in M31 suitable for studying their variability on a time scale of months and , in some periods , weeks .
The three sources demonstrate distinctly different types of X-ray variability and spectral evolution .
XMMU J004234.1+411808 is most likely a black hole candidate based on the similarity of its X-ray light curve and spectra to typical transient low-mass X-ray binaries observed in our Galaxy .
The outburst of CXO J004242.0+411608 lasted longer than a year , which makes the source an unusual X-ray transient .
The supersoft transient XMMU J004319.4+411759 is probably a classical nova-like system containing a magnetized , rapidly-spinning white dwarf .
We estimate a total rate of X-ray transient outbursts in the central bulge of M31 to be of the order \sim 10 per year .
The rate of the hard X-ray transients ( \sim 5 year ^ { -1 } ) in the central part of the Andromeda Galaxy appears to be comparable to that of the central part of our own Galaxy .