M31 Globular Cluster X‐Ray Sources:XMM‐NewtonandChandraResults

Abstract

We present the results of an M31 globular cluster (GC) X-ray source survey, based on the data of XMM-Newton and Chandra observations covering ~6100 arcmin2 of M31. We detected 43 X-ray sources coincident with GC candidates from various optical surveys. The inferred isotropic X-ray luminosities of GC sources lie between ~1035 and ~1039 ergs s-1 in the 0.3-10 keV energy band. The spectral properties of the 31 brightest sources in our sample were found to be similar to those of the low-mass X-ray binaries (LMXBs) located in the bulge and the GCs of the Milky Way. The spectral distribution of the M31 GC X-ray sources is consistent with that derived for the bulge of M31 and other nearby galaxies of different morphological type. Several sources demonstrate a correlation between the level of X-ray flux and the hardness of their energy spectrum reminiscent of the Galactic Z and atoll sources. We found that ~80% of the M31 GC sources with multiple flux measurements available show significant variability on timescales from days to years. The X-ray source RX J0043.2+4127, coincident with GC Bo 163, has been found to show recurrent transient outbursts with peak luminosities of ~1038 ergs s-1. Several sources in our sample show significant variability on a timescale of individual observations, ranging from aperiodic fluctuations to regular dipping. 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. The luminosity distribution of M31 GC sources has ~10 times higher peak luminosity and a much higher fraction of bright sources than the Milky Way GC distribution. Six persistent sources in our sample (or ~14% of the total number) have luminosities exceeding 1038 ergs s-1 during all observations, and three other sources occasionally exceed that luminosity level. Our observations indicate that GC sources make the 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 1037 ergs s-1 reside in GCs, with the fraction of GC sources rising to 67%-90% for luminosities above 1038 ergs s-1. The contribution of the GC sources to the total number of bright sources found in M31 is much higher than in the Milky Way but surprisingly close to that of early-type galaxies. We found that the brightest M31 GC sources tend to reside at large galactocentric distances outside the central bulge. We found that GCs hosting bright X-ray sources are optically brighter and more metal-rich than the rest of the M31 GCs, in agreement with previous studies. The brightest sources, with luminosities above ~1038 ergs s-1, show a tendency to reside in more metal-poor clusters. The remarkable similarities between the properties of the M31 GC X-ray sources and those of Galactic neutron star LMXBs lead us to expect most of the persistent M31 GC X-ray sources to be LMXB systems with neutron star primaries. However, the current X-ray spectral and timing data cannot rule out the possibility of finding active accreting black holes in our GC source sample.