Timing the bright X-ray population of the core of M31 with XMM-Newton

Abstract

All 63 discrete X-ray sources brighter than L_{\rm X} = 5x10^{36} erg/s in any of four XMM-Newton observations of the core of M31 were surveyed for time variability over time-scales of seconds to thousands of seconds, and for intensity variations between observations. This population is likely to be dominated by low mass X-ray binaries (LMXBs). Analysis of the shapes of power density spectra (PDS) of these sources allows us to determine whether they are accreting at a high or low rate; in the observed frequency range a broken power law (Type A) PDS indicates a low accretion rate while a simple power law (Type B) indicates a high accretion rate. We obtained the 0.3--10 keV luminosities of the sources by modelling their spectral energy distributions. The luminosity function for Type A PDS appears to consist of two populations, which we tentatively classify as neutron star and black hole LMXBs. We find that 46 sources are likely X-ray binaries, 13 with possible black hole primaries. Remarkably, 5 out of the 13 black hole candidates appear persistently bright; there are no persistently bright black hole LMXBs known in our Galaxy. The lightcurves of 55 X-ray sources had a probability of variability >99% in at least one observation. Also, 57 of the sources show a luminosity variation > ~5$\sigma$ between observations; 7 of these are classed as transients, since they are absent in at least one observation, and vary in luminosity by a factor of > ~10. Scaling this sample with the known Galactic LMXBs, we find that the M31 sample has ~50% of the dippers, Z-sources and transients.