Fast time optical variability in Be/X-ray binaries. Pulsation and rotation
Abstract
Context. Classical Be stars, regardless of spectral subtype, display multi-periodic light modulations in the frequency range 0.1-12 c d−1 when observed with a high cadence and a long duration. This behavior is attributed to non-radial pulsations and/or the rotation of the Be star. A similar study on the optical counterparts to Be/X-ray binaries is yet to be carried out.
Aims: The main goal of this work is to investigate the fast photometric variability of the optical counterparts to Be/X-ray binaries and compare the general patterns of such variability with the Galactic population of classical Be stars.
Methods: The main core of our analysis is based on space-based observations performed by TESS. We analyzed 21 sources with TESS. High-cadence photometry with two ground-based telescopes was also performed for 15 sources. The TESS light curves were created from the full-frame images using the Lightkurve package. The ground-based light curves were obtained through differential photometry between the target and a number of non-variable stars in the same field of view. Standard Fourier analysis and least-squares fitting methods were employed in the frequency analysis.
Results: All sources exhibit intra-night light variations with intensity variations of 0.01-0.06 mag in the ground-based observations and up to 5% in flux in TESS observations. This variability manifests itself as multi-periodic signals in the frequency range 0.2-12 c d−1. We find that the patterns of variability of the Be stars in Be/X-ray binaries agree with that of classical early-type Be stars in terms of the general shape of the periodograms. Based on the general shape and number of peaks in the periodograms, Be/X-ray binaries can be classified into different types. The most common case is the presence of groups of closely spaced frequencies (67%), followed by sources that exhibit isolated signals (18%). The remaining source type displays frequency spectra characterized by a mixed pattern of stochastic variability and high-frequency peaks.
Conclusions: This study reveals that short-term optical photometric variability is a very common, if not ubiquitous, feature intrinsic to the Be optical companions in Be/X-ray binaries. This variability is mainly attributed to pulsations that originate in the stellar interior.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- November 2022
- DOI:
- 10.1051/0004-6361/202243664
- arXiv:
- arXiv:2209.06440
- Bibcode:
- 2022A&A...667A..18R
- Keywords:
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- stars: emission-line;
- Be;
- pulsars: general;
- X-rays: binaries;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- A&