Chandra X-ray observations of the young stellar cluster NGC 6193 in the Ara OB1 association

A 90-ks Chandra High Energy Transmission Grating observation of the young stellar cluster NGC 6193 in the southern Ara OB1 association detected 43 X-ray sources in a 2 × 2 arcmin
²
core region centred on the massive O stars HD 150135 (O6.5 V) 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 arcsec. The X-ray luminosity of HD 150136 is log L
X
= 33.39 (erg s
⁻¹
), 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-infrared (near-IR) counterparts, but existing JHK photometry provides little evidence for near-IR excesses. These core sources have typical mean photon energies 〈E〉 ≈ 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 at kT ≈ 0.3 keV, pointing to a wind-shock origin. However, the emission of HD 150136 is slightly hotter and four 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) spectroscopic binary HD 150136. A surprising result is that the X-ray emission of HD 150136 is slowly variable on a time-scale of <1 d. The origin of the variability is not yet known but the observed behaviour suggests that it is an occultation effect.