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We present a new X-ray study of NGC188, one of the oldest open clusters knownin our Galaxy (7 Gyr). Our observation with the Chandra X-ray Observatory isaimed at uncovering the population of close interacting binaries in NGC188. Wedetect 84 sources down to a luminosity of Lx ~ 4e29 erg/s (0.3-7 keV), of which73 are within the half-mass radius rh. Of the 60 sources inside rh with morethan 5 counts, we estimate that ~38 are background sources. We detected 55 newsources, and confirmed 29 sources previously detected by ROSAT and/orXMM-Newton. A total of 13 sources detected are cluster members, and 7 of theseare new detections: four active binaries, two blue straggler stars (BSSs), and,surprisingly, an apparently single cluster member on the main sequence(CX33/WOCS5639). One of the BSSs detected (CX84/WOCS5379) is intriguing as itsX-ray luminosity cannot be explained by its currently understood configurationas a BSS/white-dwarf binary in an eccentric orbit of ~120 days. Its X-raydetection, combined with reports of short-period optical variability, suggeststhe presence of a close binary, which would make this BSS system a hierarchicalmultiple. We also classify one source as a new cataclysmic-variable candidate;it is identified with a known short-period optical variable, whose membershipto NGC188 is unknown. We have compared the X-ray emissivity of NGC188 withthose of other old Galactic open clusters. Our findings confirm the earlierresult that old open clusters have higher X-ray emissivities than other oldstellar populations.

A Chandra X-ray census of the interacting binaries in old open clusters - NGC188