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

¹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
² Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astro-nomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ Lockheed Martin Solar and Astrophysics Laboratory, 3251 Hanover Street, Palo Alto, CA 94304, USA
⁴ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁵ Space Sciences, Technologies and Astrophysics Research (STAR) Institute, University of Liège, Quartier Agora, 19c, Allée du 6 Aôut, B5c, 4000 Sart Tilman, Belgium
⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁷ Departamento de Astrofísica, Centro de Astrobiología, (CSIC-INTA), Ctra. Torrejón a Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
⁸ Departamento de Física Aplicada, Facultad de Ciencias, Universi-dad de Alicante, Carretera de San Vicente s/n, 03690, San Vicente del Raspeig, Spain
⁹ Universidad de Rio Negro, Sede Atlántica - CONICET, Viedma CP8500, Río Negro, Argentina
¹⁰ Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD, 21218, USA
¹¹ The William H. Miller III Department of Physics & Astronomy, Bloomberg Center for Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
¹² Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA
¹³ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
¹⁴ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
¹⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁶ Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dublin D15 XR2R, Ireland
¹⁷ Universite de Montreal, Montreal, Quebec, H2V 0B3, Canada
¹⁸ Embry Riddle Aeronautical University, Department of Physics & Astronomy, 3700 Willow Creek Road, Prescott, AZ 86301, USA
¹⁹ Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astrofisico di Arcetri, Largo E.Fermi 5, 50125 Firenze, Italy
²⁰ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
²¹ School of Physics & Astronomy, University of Birmingham, Birmingham B15 2TT, UK
²² Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany

^★ Corresponding author: konstantina.anastasopoulou@cfa.harvard.edu

Received: 25 April 2025
Accepted: 23 July 2025

Abstract

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.