Near-infrared counterparts to Chandra X-ray sources toward the Galactic
  Center. II. Discovery of Wolf-Rayet stars and O supergiants
  We present new identifications of infrared counterparts to the population of
hard X-ray sources near the Galactic center detected by the Chandra X-ray
Observatory. We have confirmed 16 new massive stellar counterparts to the X-ray
population, including nitrogen-type (WN) and carbon-type (WC) Wolf-Rayet stars,
and O supergiants. For the majority of these sources, the X-ray photometry is
consistent with thermal emission from plasma having temperatures in the range
of kT=1-8 keV or non-thermal emission having power-law indices in the range of
-1<gamma<3, and X-ray luminosities in the range of Lx~1e32-1e34 erg/s. Several
sources have exhibited X-ray variability of several factors between separate
observations. The X-ray properties are not a ubiquitous feature of single
massive stars but are typical of massive binaries, in which the high-energy
emission is generated by the collision of supersonic winds, or by accretion
onto a compact companion. However, the possibility of intrinsic hard X-ray
generation from single stars cannot be completely ruled out. The spectral
energy distributions of these sources exhibit significant infrared excess,
attributable to free-free emission from ionized stellar winds, supplemented by
hot dust emission in the case of the WC stars. With the exception of one object
located near the outer regions of the Quintuplet cluster, most of the new stars
appear isolated or in loose associations. Seven hydrogen-rich WN and O stars
are concentrated near the Sagittarius B HII region, while other similar stars
and more highly evolved hydrogen-poor WN and WC stars lie scattered within ~50
pc, in projection, of Sagitarrius A West. We discuss various mechanisms capable
of generating the observed X-rays and the implications these stars have for
massive star formation in the Galaxy's Central Molecular Zone.
