R. Barnard, M. Garcia, S. S. Murray
We examined 134 Chandra observations of the population of X-ray sources associated with globular clusters (GCs) in the central region of M31. These are expected to be X-ray binary systems (XBs), consisting of a neutron star or black hole accreting material from a close companion. We created long term lightcurves for these sources, correcting for interstellar absorption and instrumental effects. We tested for variability by examining the goodness of fit for the best fit constant intensity. We found significant variability in 27 out of 33 GCs and GC candidates; the other 6 sources had 0.3--10 keV luminosities fainter than ~2E+36 erg/s, limiting our ability to detect similar variability. We identify 3 new black hole candidates (BHCs), bringing the total number of M31 GC BHCs to 9, with 8 covered in this survey. The variable GC XBs either exhibited substantially higher amplitudes than expected from ensemble studies of AGN, or exhibited fluxes expected for <1 AGN per square degree. The two AGN in our sample did vary, but less often, and with smaller amplitudes, than the GC XBs. These encouraging results suggest that examining the long term lightcurves of other X-ray sources in the field may provide an important distinction between X-ray binaries and background galaxies, as the X-ray emission spectra from these two classes of X-ray sources are similar. We also identify a possible transient HMXB associated with a H{\sc ii} region that was previously identified as a GC; its outbursts are consistent with a ~120 day period.
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http://arxiv.org/abs/1203.2583
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