12 years of X-ray variability in M31 globular clusters, including 8 black hole candidates, as seen by Chandra

TL;DR

This study analyzed 12 years of Chandra X-ray observations of M31 globular clusters, revealing significant variability in many sources and identifying 8 new black hole candidates, advancing understanding of X-ray binary populations.

Contribution

It introduces the use of structure functions to analyze X-ray binary variability and reports the discovery of 8 new black hole candidates in M31 globular clusters.

Findings

28 out of 34 GCs showed significant variability

X-ray binaries exhibit more variability than AGN in structure functions

Identified 8 new black hole candidates in M31 GCs

Abstract

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 background, interstellar absorption and instrumental effects. We tested for variability by examining the goodness of fit for the best fit constant intensity. We also created structure functions (SFs) for every object in our sample, the first time this technique has been applied to XBs. We found significant variability in 28 out of 34 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. The SFs of XBs with 0.3--10 keV luminosities ~2--50 E+36 generally showed considerably more variability than the published ensemble SF of AGN. Our brightest XBs were mostly consistent with the AGN SF; however, their 2--10 keV fluxes could be matched by <1 AGN per square degree. 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. Additionally, we identify 3 new black hole candidates (BHCs) using additional XMM-Newton data, bringing the total number of M31 GC BHCs to 9, with 8 covered in this survey.