  We present a new X-ray study of NGC188, one of the oldest open clusters known
in our Galaxy (7 Gyr). Our observation with the Chandra X-ray Observatory is
aimed at uncovering the population of close interacting binaries in NGC188. We
detect 84 sources down to a luminosity of Lx ~ 4e29 erg/s (0.3-7 keV), of which
73 are within the half-mass radius rh. Of the 60 sources inside rh with more
than 5 counts, we estimate that ~38 are background sources. We detected 55 new
sources, and confirmed 29 sources previously detected by ROSAT and/or
XMM-Newton. A total of 13 sources detected are cluster members, and 7 of these
are 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 its
X-ray luminosity cannot be explained by its currently understood configuration
as a BSS/white-dwarf binary in an eccentric orbit of ~120 days. Its X-ray
detection, combined with reports of short-period optical variability, suggests
the presence of a close binary, which would make this BSS system a hierarchical
multiple. We also classify one source as a new cataclysmic-variable candidate;
it is identified with a known short-period optical variable, whose membership
to NGC188 is unknown. We have compared the X-ray emissivity of NGC188 with
those of other old Galactic open clusters. Our findings confirm the earlier
result that old open clusters have higher X-ray emissivities than other old
stellar populations.
