EWOCS-IV: 1Ms ACIS Chandra observation of the supergiant B[e] star Wd1-9
arXiv:2507.17816 · doi:10.1051/0004-6361/202555305
Abstract
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.
arXiv commentaccepted for publication in A&A, 17 pages, 14 figures, 5 tables
References in corpus (44)
- The Gaia mission
- Gaia Data Release 3: Summary of the content and survey properties
- On the Absorption of X-rays in the Interstellar Medium
- Collisional Plasma Models with APEC/APED: Emission Line Diagnostics of Hydrogen-like and Helium-like Ions
- The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
- HI4PI: A full-sky HI survey based on EBHIS and GASS
- Mass loss from hot massive stars
- Bayesian Estimation of Hardness Ratios: Modeling and Computations
- The Galactic WN stars: Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation
- The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars
- Nonlinear Instabilities in Shock-Bounded Slabs
- On the massive stellar population of the Super Star Cluster Westerlund 1
- Introduction to astroML: Machine Learning for Astrophysics
- Periodograms for Multiband Astronomical Time Series
- A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy
- High-Mass X-ray Binaries in the Milky Way: A closer look with INTEGRAL
- Constraining the mass transfer in massive binaries through progenitor evolution models of Wolf-Rayet+O binaries
- Predictions for mass-loss rates and terminal wind velocities of massive O-type stars
- High resolution radio observations of the colliding-wind binary WR140
- The Galactic WN stars revisited. Impact of Gaia distances on fundamental stellar parameters
- Masses and Luminosities of O and B - type stars and red super giants
- High-Resolution Spectroscopy of the Yellow Hypergiant Rho Cassiopeiae from 1993 Through the Outburst of 2000-2001
- A VLT/FLAMES survey for massive binaries in Westerlund 1. IV. Wd1-5 - binary product and a pre-supernova companion for the magnetar CXOU J1647-45?
- Kinematical structure of the circumstellar environments of galactic B[e]-type stars
- A serendipitous survey for variability amongst the massive stellar population of Westerlund 1
- Variability of Young Massive Stars in the Galactic Super Star Cluster Westerlund 1
- Radio emission from the massive stars in the Galactic Super Star Cluster Westerlund 1
- The circumstellar environment and evolutionary state of the supergiant B[e] star Wd1-9
- A Rich Population of X-ray Emitting Wolf-Rayet Stars in the Galactic Starburst Cluster Westerlund 1
- Suppression of X-rays from radiative shocks by their thin-shell instability
- Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage
- One of the most massive stars in the Galaxy may have formed in isolation
- A Systematic Search for Corotating Interaction Regions in Apparently Single Galactic Wolf-Rayet Stars. II. A Global View of the Wind Variability
- CIR Modulation of the X-ray Flux from the O7.5 III(n)((f)) Star xi Persei?
- Discovery of extended radio emission in the young cluster Wd1
- An ALMA 3mm continuum census of Westerlund 1
- A VLT/FLAMES survey for massive binaries in Westerlund 1: VII. Cluster census
- Suzaku monitoring of the Wolf-Rayet binary WR140 around periastron passage: An approach for quantifying the wind parameters
- Westerlund 1 under the light of Gaia EDR3: Distance, isolation, extent, and a hidden population
- VLT/MUSE discovers a jet from the evolved B[e] star MWC 137
- A VLT/FLAMES survey for massive binaries in Westerlund 1: VIII. Binary Systems and Orbital Parameters
- ALMA observations of the supergiant B[e] star Wd1-9
- A Radio Census of the Massive Stellar Cluster Westerlund 1
- Investigation of the nature of the wind interaction in HD93205 based on multi-epoch X-ray observations