2 XMM-Newton observations

The bulge of M 31 was observed with XMM-Newton  on June 25, 2000 and again on Dec. 28, 2000 (see Table 1). The observations were centered on the core of M 31 ( $\alpha=00^{\rm h}42^{\rm m}43\hbox{$.\!\!^{\rm s}$ }0,~\delta =+41\hbox{$^\circ$ }15\hbox{$^\prime$ }46.0\hbox{$^{\prime\prime}$ }$ J2000), with a field of view of 30$^\prime$ in diameter for the three European Photon Imaging Camera (EPIC) instruments. The two EPIC MOS instruments (Turner et al. 2001) and the EPIC PN (Strüder et al. 2001) operated in full-window mode with the medium optical blocking filter. The Optical/UV Monitor Telescope (OM; Mason et al. 2001) filter wheel was set to the blocked position during the June observation. During the December observation, exposures were obtained with the B and UVW1 filter of the OM; results of these observations will be presented elsewhere.

A background flare occurred during the final 5 ks of the EPIC exposures from the June observation. Data obtained during this background flare were excluded from the variability studies.

We used the XMM-Newton  Science Analysis System (versions 4.1 and 5.0.1) to reduce the EPIC data to calibrated event lists, produce images, and extract light curves. A combination of SAS programs and external software was applied to further analyze the data. The data from MOS1, MOS2 and PN were extensively compared to confirm the consistency of our results. The variability of individual sources within the longest (June) observation was studied by Fourier analysis. The value of the pulsation period detected in one of the supersoft sources (see below) was refined by the epoch-folding method.