Fast optical variability in Supergiant X-ray binaries

Abstract: 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 such variability with the Galactic population of other early-type stars. We analyzed a sample of 14 high-mass X-ray binaries with supergiant companions observed by the Transiting Exoplanet Survey Satellite (TESS) and 4 Be/X-ray binaries with persistent X-ray emission for comparison. All sources exhibit fast aperiodic light variations. The shape of the periodogram is well described by a red noise component at intermediate frequencies ($\sim 1-10$ 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 that 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 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. 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.