The central region of M 31 observed with XMM-Newton - II. Variability of the individual sources

We present the results of a study of the variability of X-ray sources in th e central 30' of the nearby Andromeda Galaxy (M 31) based on XMM-Newton Per formance 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 x 10(35 ) erg s(-1) (0.3-12 keV, d = 760 kpc). For the similar to 60 brightest sour ces, 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 similar to2.9' from the nucleus in the June observation; this source faded significantly and was no longer de tected 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 similar to 865 s from a source wit h a supersoft spectrum. The flux of this source decreased significantly bet ween the two XMM observations. The detected period is unusually short and p oints to a rapidly spinning magnetized white dwarf. The high luminosity and transient nature of the source suggest its possible identification with cl assical or symbiotic nova, some of which were observed earlier as supersoft sources.