Abstract :
[en] We present the most comprehensive and deepest X-ray study to date of the
properties of the richest Wolf-Rayet (WR) population observed in a single
stellar cluster, Westerlund 1 (Wd1). This work is based on 36 Chandra
observations obtained from the "Extended Westerlund 1 and 2 Open Clusters
Survey" (EWOCS) project, plus 8 archival Chandra observations. The overall
exposure depth (~1.1 Ms) and baseline of the EWOCS observations extending over
more than one year enable us to perform a detailed photometric, colour, and
spectral analysis, as well as to search for short- and long-term periodicity.
In X-rays, we detect 20 out of the 24 known Wolf-Rayet stars in Wd1 down to an
observed luminosity of ~7$\times10^{29}$erg s$^{-1}$ (assuming a distance of
4.23 kpc to Wd1), with 8 WR stars being detected in X-rays for the first time.
Nine stars show clear evidence of variability over the year-long baseline, with
clear signs of periodicity. The X-ray colours and spectral analysis reveal that
the vast majority of the WR stars are hard X-ray sources (kT$\geq$2.0keV). The
Fe XXV emission line at ~6.7 keV, which commonly originates from the wind-wind
collision zone in binary systems, is detected for the first time in the spectra
of 17 WR stars in Wd1. In addition the ~6.4 keV fluorescent line is observed in
the spectra of three stars, indicating that dense cold material coexists with
the hot gas in these systems. Overall, our X-ray results alone suggest a very
high binary fraction ($\geq$80%) for the WR star population in Wd1. When
combining our results with properties of the WR population from other
wavelengths, we estimate a binary fraction of $\geq$92%, which could even reach
unity. This suggests that either all the most massive stars are found in binary
systems within Wd1, or that binarity is essential for the formation of such a
rich population of WR stars.
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