2018ApJ...856...77X

The X-ray emissivity (i.e., luminosity per unit stellar mass) of globular clusters (GCs) is an important indicator of their dynamical evolution history. Based on deep archival Chandra observations, we report a stacking analysis of 44 GCs with 0.5-8 keV luminosities L_X <= 10³⁵ erg s^–1 in the M31 bulge, which are supposed to be dominated by cataclysmic variables (CVs) and coronally active binaries (ABs). We obtain a significant detection at the 5σ level in 0.5-8 keV band. The average X-ray luminosity per GC and the average X-ray emissivity are determined to be 5.3 ± 1.6 x 10³³ erg s^–1 and 13.2 ± 4.3 x 10²⁷ erg s^–1 M_☉^–1, respectively. Both of these values are consistent with those of Milky Way GCs. Moreover, the measured emissivity of M31 GCs is also consistent with that of the Milky Way field stars. Massive GCs have X-ray luminosities that are marginally higher than those of less massive ones. Massive GCs also show a lower emissivity (5.0±2.5×10²⁷ergs^–1M_☉^–1) than less massive ones (26.5±14.3×10²⁷ergs^–1M_☉^–1), which is consistent with the scenario that the (progenitors of) CVs and ABs were more efficiently destroyed via stellar encounters in the more massive GCs. No dependence of the X-ray emissivity on GC color or on the projected galactocentric distance of GCs is found.