Xmm-newton Survey of the Central Region of M31. Spectral Properties and Variability of Bright X-ray Sources and Source Classification

We present the results of the systematic survey of X-ray sources in the central region of M31 using the data from XMM-Newton observations performed in the years 2000-2004. The spectral properties and variability of 123 bright X-ray sources with apparent luminosities between ∼ 1036 and 5×1038 ergs s−1 were studied in detail. The spectral properties of majority of sources in our sample have been found to be consistent with that of the low-mass X-ray binary systems. 20 sources in our sample coinside with globular clusters in M31, four sources are supernova remnant candidates and another six sources can be classified as background AGN candidates. The spectral distribution of M31 X-ray sources, based on the spectral fitting with a power law model is clearly bimodal with a main peak corresponding to a photon index Γ ∼ 1.75 and a shoulder at Γ ∼ 2.0−2.2 extending to the soft spectral region. The spectral distribution shows clear evolution with source luminosity, characterized by its narrowing and shift of its main peak to the harder spectral region for luminosities above ∼ (3−5)×1037 ergs s−1. The spectral properties of the high-luminosity sources in our sample closely resemble those of the bright LMXB in nearby galaxies of different morphological type. The distribution of absorbing columns towards M31 sources derived from spectral analysis has a peak at NH ∼ 1.2×1021 cm−2 extending up to 1.3×1022 cm−2, with an average value of (1.52±0.02)×1021 cm−2. More than 80% of sources observed in two or more observations show significant variability on the time scales of days to years. The fraction of variable sources in our survey is much higher than previously reported from Chandra survey of M31, and is remarkably close to the fraction of variable sources found in the M31 globular cluster X-ray source population. Timing analysis of several of the brightest sources reveals significant aperiodic variability in the 2× 10−5− 0.01 Hz frequency range, resembling the very-low frequency variability of the bright Galactic LMXB. About 50% of the sources in our sample are spectrally variable. The spectral evolution of a number of sources is correlated with the level of their X-ray flux, while some sources demonstrate complex patterns of evolution on the hardness-intensity diagram. Based on the similarity of the properties of M31 X-ray sources and their Galactic counterparts, we expect most of the X-ray sources in our sample to be accreting binary systems with neutron star and black hole primaries. A total of 44 X-ray sources can be identified as probable X-ray binaries. We show that X-ray hardness-luminosity (spectral photon index vs. luminosity) diagrams can be an effective tool for characterising X-ray binary populations in external galaxies and classification of individual sources as neutron star and black hole candidates. Combining the results of X-ray analysis (detailed X-ray spectra, hardness-luminosity diagrams and variability) with available data at other wavelengths, we classify 7% and 24% of sources in our sample as, respectively, probable black hole and neutron star candidates. Subject headings: galaxies: individual (M31) — X-rays: binaries — X-rays: stars