SIMBAD references

We report on Chandra observations of the central region of M31. By combining eight Chandra ACIS-I observations taken between 1999 and 2001, we have identified 204 X-ray sources within the central ∼17'x17' region of M31, with a detection limit of ∼2x10³⁵ ergs.s^–1. Of these 204 sources, 22 are identified with globular clusters, two with supernova remnants, nine with planetary nebula, and nine with supersoft sources. By comparing individual images, about 50% of the sources are variable on timescales of months. We also found 13 transients, with light curves showing a variety of shapes. We also extracted the energy spectra of the 20 brightest sources; they can be well fitted by a single power law with a mean photon index of 1.8. The spectral shapes of 12 sources are 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 1.7x10³⁷ ergs.s^–1). However, when the X-ray sources in different regions are considered separately, different luminosity functions are obtained. This indicates that the star formation history might be different in different regions.