A Chandra survey of Milky Way globular clusters-IV. Periodic X-ray sources

We present a systematic search for periodic X-ray sources in ten Galactic globular clusters (GCs) using deep archival Chandra observations. By applying the Gregory–Loredo algorithm, we detect 28 periodic signals from 27 independent X-ray sources across six GCs, including 21 newly discovered X-ray periodic sources. The remaining four GCs show no periodic X-ray sources, primarily due to lower data sensitivity. Based on X-ray timing and spectral analyses, complemented by available optical and ultraviolet data, 21 of these periodic sources are identified as cataclysmic variables (CVs). Combined with 11 previously identified periodic CVs in 47 Tuc, this work constitutes the most comprehensive sample to date of GC CVs with probable orbital periods. The observed scarcity of old, short-period CVs in GCs compared to the Galactic inner bulge and solar neighbourhood is likely due to both selection effects favoring younger, dynamically formed systems and the suppression of primordial CV formation by stellar dynamical interactions typical of GC environments. Furthermore, a significant fraction of GC CVs—most with orbital periods below or within the CV period gap—are identified as probable magnetic CVs, while luminous intermediate polars appear underrepresented compared to the solar neighbourhood.