Periodic X-ray sources in globular clusters: diagnosing dynamical formation and evolution of cataclysmic variables

It has long been recognized that dynamical interaction plays an important role in the formation and evolution of close binaries in globular clusters (GCs). The orbital period distribution of cataclysmic variables (CVs), has arguably been the most powerful observational tool for studying CV formation and evolution. We present a systematic study of periodic sources in 11 Galactic GCs, utilizing deep archival Chandra observations and recently available eROSITA observations (for 47 Tucanae only). By applying the Gregory-Loredo algorithm, which employs Bayes’s theorem to the phase-folded light curve and is well-suited for irregularly sampled X-ray data, dozens of periodic X-ray sources are newly discovered. We identify the orbital periods of CVs based on their X-ray temporal properties, as well as optical and ultraviolet information, enlarging the periodic CV sample of GCs by nearly 3 times. Despite of the selection effect that CVs of closer orbits tend to be fainter, for the first time, we discover eight short period CVs (less than 2 hour) that greatly improved the previous sample, since no such systems have been discovered before in GCs. In particular, the orbital period distribution of 47 Tuc CVs is likely an outlier, with a substantial fraction of CVs within the period gap and a steep radial surface profile. They are best understood as a group of CVs having recently formed via dynamical interactions in the dense cluster core. Overall, CVs in all GCs together exhibit a positive correlation between orbital period and X-ray luminosity, in accord with the theoretical anticipation. Compared to the periodic CV sample in galactic bulge that derived by similar method and comparable detection sensitivity, an apparent overabundance of long period systems (larger than 3 hour) emerges in the GC CVs. Our study provides direct evidence for dynamically-formed CV group in GCs.