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

Context . Supergiant B[e] (sgB[e]) stars are exceptionally rare objects, with only a select number 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. Aims . We present the most detailed X-ray study of Wd1-9 to date, using X-rays that pierce through its cocoon with the aim of uncovering its nature and evolutionary state. Methods . We utilised 36 Chandra observations of Wd1 from the ‘Extended Westerlund 1 and 2 Open Clusters Survey’ (EWOCS), plus eight archival datasets, totalling 1.1 Ms. We used this dataset to investigate long-term variability and periodicity in Wd1-9, and analysed X-ray colours and spectra over time to uncover patterns that shed light on its nature. Results . 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 ∼ 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. Conclusions . 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.