A TESS search for donor-star pulsations in High-Mass X-ray Binaries

TL;DR

This study systematically analyzes TESS data for 23 High-Mass X-ray Binaries, revealing common short-term quasi-periodic variability likely due to stellar pulsations, and identifies outbursts in two systems, enhancing understanding of their optical variability.

Contribution

It is the first comprehensive TESS-based investigation of optical pulsations and outbursts in HMXBs, linking stellar pulsations with X-ray activity in these systems.

Findings

All 23 HMXBs show short-term quasi-periodic variability.

Detected outbursts in V725 Tau and HD 249179, with V725 Tau's outburst coinciding with an X-ray flare.

Variability characteristics are consistent with pulsations seen in OB and Be stars.

Abstract

Ground-based optical photometry of the counterparts of High-Mass X-ray Binaries (HMXBs) has revealed the presence of periodic modulations on timescales of ~0.3-0.5 d. More recent space-based observations Corot and TESS of OB and Be stars have shown that pulsations caused by p and g modes are common in early type stars. We have therefore undertaken a systematic search for variability in the optical counterparts of 23 HMXBs (mostly neutron star systems, but including one black hole, Cyg X-1) using TESS data primarily in 2 min cadence mode. After removing the orbital period modulation in four systems, we find that all 23 sources show evidence for quasi-periodic variability on periods shorter than ~1 d. We compare their power spectra with those from observations of other OB and Be type stars. In two systems, V725 Tau and HD 249179 (which may not be a HMXB), we find evidence for an outburst, the former being simultaneous with an X-ray flare. We search for changes in the power spectra over the outburst duration, and compare them with outbursts seen in other Be systems.