J/A+A/701/A138 X-ray long term lightcurve of Wd1-9 (Anastasopoulou+, 2025)
EWOCS-IV: 1Ms ACIS Chandra observation of the supergiant B[e] star Wd1-9.
Anastasopoulou 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.
<Astron. Astrophys. 701, A138 (2025)>
=2025A&A...701A.138A 2025A&A...701A.138A (SIMBAD/NED BibCode)
ADC_Keywords: Binaries, X-ray ; Spectroscopy
Keywords: binaries: general - stars: emission-line, Be - stars: massive -
Galaxy: open clusters and associations: individual: Westerlund 1 -
X-rays: stars
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 1.1Ms. 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.
Description:
X-ray long-term light curve of the supergiant B[e] star Wd1-9
in the young supermassive star cluster Westerlund 1 (top panel of
Figure 2).
Rate and rate errors are given in counts per second. Time is given in
modified Julian date.
All relevant files of the combined X-ray spectrum of Wd1-9 (Figure 7)
are given in FITS format.
Objects:
-----------------------------------------------------------
RA (2000) DE Designation(s)
-----------------------------------------------------------
16 47 04.14 -45 50 31.3 Wd1-9 = Cl* Westerlund 1 W 9
-----------------------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
lc.dat 27 72 X-ray long-term light curve of the
supergiant B[e] star Wd1-9 (Figure 2)
list.dat 79 5 List of fits spectra (Figure 7)
fits/* . 5 Individual fits spectra
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See also:
J/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)
Byte-by-byte Description of file: lc.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 F7.5 ct/s Rate X-ray rate
9- 15 F7.5 ct/s e_Rate X-ray rate error
17- 27 F11.5 d Time Time (MJD)
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 I7 --- Nx Number of pixels along X-axis
9- 12 I4 --- Ny Number of pixels along Y-axis
14- 17 I4 Kibyte size Size of FITS file
19- 45 A27 --- FileName Name of FITS file, in subdirectory fits
47- 79 A33 --- Title Title of the FITS file
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Acknowledgements:
Konstantina Anastasopoulou, konstantina.anastasopoulou(at)inaf.it
References:
Guarcello et al., Paper I 2024A&A...682A..49G 2024A&A...682A..49G, Cat. J/A+A/682/A49
Guarcello et al., Paper II 2024A&A...690A..25A 2024A&A...690A..25A
Guarcello et al., Paper III 2025A&A...693A.120G 2025A&A...693A.120G, Cat. J/A+A/693/A120
(End) Patricia Vannier [CDS] 04-Aug-2025