Issue |
A&A
Volume 688, August 2024
|
|
---|---|---|
Article Number | A181 | |
Number of page(s) | 14 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361/202449737 | |
Published online | 20 August 2024 |
X-raying the ζ Tau binary system
1
Groupe d’Astrophysique des Hautes Energies, STAR, Université de Liège, Quartier Agora (B5c, Institut d’Astrophysique et de Géophysique),
Allée du 6 Août 19c,
4000
Sart Tilman, Liège,
Belgium
e-mail: ynaze@uliege.be
2
Université de Strasbourg, CNRS,
Observatoire Astronomique de Strasbourg, 11 rue de l’Université, UMR 7550,
67000
Strasbourg,
France
3
NSF OIR Lab,
950 N Cherry Ave,
Tucson,
AZ
85721,
USA
4
Hamburger Sternwarte, University of Hamburg,
Gojenbergsweg 112,
21029
Hamburg,
Germany
Received:
26
February
2024
Accepted:
19
June
2024
Context. The Be star ζ Tau was recently reported to be a γ Cas analog; that is, it displays an atypical (bright and hard) X-ray emission. The origin of these X-rays remains debated.
Aims. The first X-ray observations indicated a very large absorption of the hot plasma component (NH ~ 1023 cm−2). This is most probably related to the edge-on configuration of the ζ Tau disk. If the X-ray emission arises close to the companion, an orbital modulation of the absorption could be detected as the disk comes in and out of the line of sight.
Methods. New XMM-Newton data were obtained to characterize the high-energy properties of ζ Tau in more detail. They are complemented by previous Chandra and SRG/eROSITA observations as well as by optical spectroscopy and TESS photometry.
Results. The high-quality XMM-Newton data reveal the presence of a faint soft X-ray emission, which appears in line with that recorded for non-γ Cas Be stars. In addition, ζ Tau exhibits significant short-term variability at all energies, with larger amplitudes at lower frequencies (“red noise”), as is found in X-ray data of other γ Cas stars. Transient variability (softness dip, low-frequency signal) may also be detected at some epochs. In addition, between X-ray exposures, large variations in the spectra are detected in the 1.5–4 keV energy band. They are due to large changes in absorption toward the hottest (9 keV) plasma. These changes are not correlated with either the orbital phase or the depth of the shell absorption of the Hα line. These observed properties are examined in the light of proposed γ Cas models.
Key words: stars: emission-line, Be / stars: individual: ζ Tau / X-rays: stars
© The Authors 2024
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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