Two Years of the X-ray Sky in M31

We have been monitoring M31 with HRC and ACIS onboard Chandra regularly in the past two years. By combining eight Chandra ACIS-I observations taken between 1999 to 2001, we have identified 204 X-ray sources within the central ∼ 17 ′ ×17 ′ region of M31, with a detection limit of ∼ 1.6 × 10 35 erg s −1. Of these 204 sources, 21 are identified with globular clusters, 2 with supernova remnants (one of them is spatially resolved with Chandra), and 8 with planetary nebula. By comparing individual images, about 50% of the sources are variable in time scales of months. We also found 14 transients. Combining all the available transients found in literatures, there are 25 tran-sients in M31 and M32 detected by Chandra and XMM-Newton; we present some of the long-term lightcurves by using the HRC, ACIS and XMM-Newton data. The spectral shape of 12 sources is shown to be variable, suggesting that they went through state changes. The luminosity function of all the point sources is consistent with previous observations (a broken power-law with a luminosity break at 1.7 × 10 37 erg s −1). However, when the X-ray sources in different regions are considered separately, different lumi-nosity functions are obtained. This indicates that the star formation history might be different in different regions.