  A 90 ks Chandra HETG observation of the young stellar cluster NGC 6193 in the
southern Ara OB1 association detected 43 X-ray sources in a 2' x 2' core region
centered on the young O stars HD 150135 (O6.5V) and HD 150136 (O3+O6V). The
cluster is dominated by exceptionally bright X-ray emission from the two O
stars, which are separated by only 10 arcsecs. The X-ray luminosity of HD
150136 is log Lx = 33.39 (ergs/s), making it one of the most luminous O-star
X-ray sources known. All of the fainter X-ray sources in the core region have
near-IR counterparts, but JHK photometry provides little evidence for near-IR
excesses. These core sources have typical mean photon energies of 2 keV and
about one-third are variable. It is likely that some are young low-mass stars
in the cluster, but cluster membership remains to be determined. Grating
spectra show that the X-ray properties of HD 150135 and HD 150136 are similar,
but not identical. Both have moderately broadened unshifted emission lines and
their emission is dominated by cool plasma, pointing to a wind shock origin.
However, the emission of HD 150136 is slightly hotter and 4 times more luminous
than its optical twin HD 150135. We discuss the possibility that a radiative
colliding wind shock contributes to the prodigious X-ray output of the
short-period (2.66 d) binary HD 150136. A surprising result is that the X-ray
emission of HD 150136 is slowly variable on a timescale of less than one day.
The origin of the variability is not yet known, but the observed behavior
suggests that it is an occultation effect.
