EWOCS-IV: 1Ms ACIS Chandra observation of the supergiant B[e] star Wd1-9

Supergiant B[e] (sgB[e]) stars are exceptionally rare objects, with only a
handful of confirmed examples in the Milky Way. The evolutionary pathways
leading to the sgB[e] phase remain largely debated, highlighting the need for
additional observations. The sgB[e] star Wd1-9, located in the massive cluster
Westerlund 1 (Wd1), is enshrouded in a dusty cocoon--likely the result of past
eruptive activity--leaving its true nature enigmatic. We present the most
detailed X-ray study of Wd1-9 to date, using X-rays that pierce through its
cocoon with the aim to uncover its nature and evolutionary state. We utilize 36
Chandra observations of Wd1 from the 'Extended Westerlund 1 and 2 Open Clusters
Survey' (EWOCS), plus eight archival datasets, totalling 1.1 Ms. This dataset
allows investigation of long-term variability and periodicity in Wd1-9, while
X-ray colours and spectra are analysed over time to uncover patterns that shed
light on its nature. Wd1-9 exhibits significant long-term X-ray variability,
within which we identify a strong 14-day periodic signal. We interpret this as
the orbital period, marking the first period determination for the system. The
X-ray spectrum of Wd1-9 is thermal and hard (kT approximately 3.0 keV),
resembling the spectra of bright Wolf-Rayet (WR) binaries in Wd1, while a
strong Fe emission line at 6.7 keV indicates hot plasma from a colliding-wind
X-ray binary. Wd1-9, with evidence of past mass loss, circumbinary material, a
hard X-ray spectrum, and a newly detected 14-day period, displays all the
hallmarks of a binary--likely a WR+OB--that recently underwent early Case B
mass transfer. Its sgB[e] classification is likely phenomenological reflecting
emission from the dense circumbinary material. This places Wd1-9 in a rarely
observed phase, possibly revealing a newly formed WN star, bridging the gap
between immediate precursors and later evolutionary stages in Wd1.