M31 Globular Cluster X-ray Sources: XMM-Newton and Chandra results

Abstract

(Abridged) We present the results of M31 globular cluster (GC) X-ray source survey, based on the data of XMM-Newton and Chandra observations covering ~6100 sq.arcmin of M31. We detected 43 X-ray sources coincident with globular cluster candidates from optical surveys. The estimated isotropic X-ray luminosities of GC sources lie between ~10e35 and ~10e39 erg/s in the 0.3 - 10 keV energy band. The spectral properties and variability of M31 GC X-ray sources are consistent with that derived for the LMXBs in the bulges of M31 and Milky Way. We found that ~80% of the M31 GC sources with multiple flux measurements available show significant variability on a time scales from days to years. The X-ray luminosity function of GC sources is found to be significantly different from that of the point sources in the bulge and disk of M31 and that of the Galactic GC X-ray sources. GC sources make dominant contribution to the bright source counts in the areas of M31 covered by the survey: ~40% of the total number of sources with luminosities above 10e37 ergs/s reside in GCs with fraction of GC sources rising to 67-90% for the luminosities above 10e38 ergs/s. The contribution of the GC sources to the total number of bright sources found in M31 is much higher than in the Milky Way galaxy, but surprisingly close to that of the early-type galaxies. The brightest M31 GC sources tend to reside at large galactocentric distances outside the central bulge. We found that globular clusters hosting bright X-ray sources are optically brighter and more metal rich than the rest of M31 globular clusters. The brightest sources with luminosities above ~10e38 ergs/s show tendency to reside in more metal poor clusters.