2006ApJ...639L..35S

Recent optical and infrared studies have revealed that the heavily reddened starburst cluster Westerlund 1 (Wd 1) contains at least 22 Wolf-Rayet (W-R) stars, constituting the richest W-R population of any Galactic cluster. We present results of a sensitive Chandra X-ray observation of Wd 1 that detected 12 of the 22 known W-R stars and the mysterious emission-line star W9. The fraction of detected WN stars is nearly identical to that of WC stars. The WN stars WR-A and WR-B, as well as W9, are exceptionally luminous in X-rays and have similar hard, heavily absorbed X-ray spectra with strong Si XIII and S XV emission lines. The luminous high-temperature X-ray emission of these three stars is characteristic of colliding-wind binary systems, but their binary status remains to be determined. Spectral fits of the X-ray-bright sources WR-A and W9 with isothermal plane-parallel shock models require high absorption column densities, log N_H=22.56 (/cm2), and yield characteristic shock temperatures kT_s~3 keV (T_s~35 MK).