J/A+A/701/A138ivo://CDS.VizieR/J/A+A/701/A138doi:10.26093/cds/vizier/bc-nx/lbdiCDSAnastasopoulou K.Guarcello M.G.Drake J.J.Ritchie B.De Becker M.,Bayo A.Najarro F.Negueruela I.Sciortino S.Flaccomio E.,Castellanos R.Albacete-Colombo J.F.Andersen M.Damiani F.,Fraschetti F.Gennaro M.Gunderson S.J.Larkin C.J.K.Mackey J.,Moffat A.F.J.Pradhan P.Saracino S.Stevens I.R.Weigelt G.2026-08-03T06:56:57Z2025-09-09T10:59:58ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frx-ray-binary-starsspectroscopySupergiant 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 1.1Ms. 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~3.0keV), 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 Wd1.2025A&A...701A.138Ahttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/701/A138CatalogResearchIsServedByTAP VizieR generic servicerelated-toJ/A+A/682/A49 : EWOCS-I. X-ray sources in Westerlund 1 (Guarcello+, 2024)J/A+A/693/A120 : Westerlund 1 NIRCam/MIRI images (Guarcello+, 2025)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/701/A138https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/701/A138http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/701/A138https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/701/A138https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/701/A138http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/701/A138GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapX-raydefaultJ/A+A/701/A138/lcX-ray long-term light curve of the supergiant B[e] star Wd1-9 (Figure 2)recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintRateX-ray ratect/sphot.count;em.X-raydoublee_RateX-ray rate errorct/sstat.errordoubleTimeTime (MJD)dtime.epochdoubleJ/A+A/701/A138/listList of fits spectra (Figure 7)recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintNxNumber of pixels along X-axismeta.numberintNyNumber of pixels along Y-axismeta.numberintsizeSize of FITS fileKibytephys.size;meta.fitsintFileNameName of FITS file, in subdirectory fitsmeta.id;meta.fitscharTitleTitle of the FITS filemeta.title;meta.fitschar