Fast optical variability in supergiant X-ray binaries

Context. Recent studies of massive stars using high-precision space photometry have revealed that they commonly exhibit stochastic low-frequency variability. Aims. The main goal of this work is to investigate the fast photometric variability of the optical counterparts to supergiant X-ray binaries and to compare the general patterns of this variability with that observed in the Galactic population of other early-type stars. Methods. We selected a sample of 14 high-mass X-ray binaries with supergiant companions observed by the Transiting Exoplanet Survey Satellite (TESS). We also studied 4 Be/X-ray binaries with persistent X-ray emission for comparison. The TESS light curves were created from the full-frame images using the Lightkurve package. The light curves were background subtracted and corrected for scattered light and instrumental effects. Standard Fourier analysis was used to obtain the periodograms. We used a phenomenological model to fit the amplitude spectra and derive the best-fit parameters. Results. All sources exhibit fast aperiodic light variations. The shape of the periodogram is well described by a red noise component at intermediate frequencies (∼1 − 10 d −1 ). At lower frequencies, the noise level flattens, while at higher frequencies the periodogram is dominated by white noise. We find that the patterns of variability of the massive companions in supergiant X-ray binaries agree with those of single early-type evolved stars in terms of the general shape of the periodograms. However, they exhibit higher amplitude at low frequencies and lower characteristic frequencies than those of Be/X-ray binaries. Unlike Be/X-ray binaries, supergiant X-ray binaries exhibit a total lack of coherent signals at high frequencies. Most sources have been analyzed over multiple TESS sectors, spanning a duration of 4 years. We do not find any significant variation over time in the low-frequency variability characteristics. Conclusions. This study reveals that stochastic low-frequency variability is a very common, if not ubiquitous, feature intrinsic to supergiant optical companions in X-ray binaries. The phenomenology of this variability is similar to that of single early-type supergiant stars.