Fast time optical variability in Be/X-ray binaries. Pulsation and rotation

P. Reig and J. Fabregat

Submitted on 14 September 2022


Classical Be stars, regardless of spectral subtype, display multiperiodic light modulations in the frequency range 0.1-12 d1, when observed with high-cadence and long duration. This behaviour is attributed to non-radial pulsations and/or rotation of the Be star. 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. We analyzed 21 sources with TESS. High-cadence photometry with two ground-based telescopes was also performed for 15 sources. Standard Fourier analysis and least-squares fitting methods were employed in the frequency analysis. 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 d1. 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 type of sources displays frequency spectra characterized by a mixed pattern of stochastic variability and high-frequency peaks. 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.


Subjects: Astrophysics - Solar and Stellar Astrophysics; Astrophysics - High Energy Astrophysical Phenomena