We present the results of a study of the variability of X-ray sources in the central 30′ of the nearby Andromeda Galaxy ( M31 ) based on XMM-Newton Performance Verification observations .
Two observations of this field , with a total exposure time of about 50 ks , were performed in June and December of 2000 .
We found 116 sources brighter than a limiting luminosity of { 6 } { \times } 10 ^ { { 35 } } erg s ^ { -1 } ( 0.3–12 keV , d = 760 kpc ) .
For the \sim 60 brightest sources , we searched for periodic and non-periodic variability ; at least 15 % of these sources appear to be variable on a time scale of several months .
We discovered a new bright transient source \sim 2.9′ from the nucleus in the June observation ; this source faded significantly and was no longer detected in December .
The behaviour of the object is similar to a handful of Galactic LMXB transients , most of which are supposed to harbor black holes .
We detected pulsations with a period of \sim 865 s from a source with a supersoft spectrum .
The flux of this source decreased significantly between the two XMM observations .
The detected period is unusually short and points to a rapidly spinning magnetized white dwarf .
The high luminosity and transient nature of the source suggest its possible identification with classical or symbiotic nova , some of which were observed earlier as supersoft sources .