Long-term eclipse time variations in white dwarf binaries

TL;DR

This study analyzes eclipse timing variations in 43 white dwarf binary systems over many years, finding evidence that stellar magnetic activity mechanisms likely drive most observed variations, though some cases suggest possible planetary influences.

Contribution

It provides a comprehensive analysis of long-term eclipse timing variations in white dwarf binaries, supporting magnetic activity as the primary cause over planetary hypotheses.

Findings

Systems with fully convective companions show higher ETV levels.

Magnetic activity mechanisms are the most probable cause of observed ETVs.

Large rapid variations remain unexplained by current models.

Abstract

The overwhelming majority of eclipsing white dwarf (WD) binary systems show quasi-periodic variations in eclipse timings on many year timescales. Currently, the mechanism behind these eclipse time variations (ETVs) is not known, with the main competing theories being the planetary hypothesis and the Applegate/Lanza mechanisms. Here, we present a comprehensive study of 43 WD binary systems, the vast majority of which have more than a decade of eclipse timing measurements, analysing their global properties to determine which driving force is the likely origin of the observed ETVs. Long-term, high-speed photometry data obtained with ULTRACAM, ULTRASPEC and HiPERCAM have allowed us to track the evolution of the ETVs in these systems, and analyse any previously unseen trends. From this analysis, we find a clear difference in the level of observed ETVs past the fully convective boundary, where systems with partially radiative companion stars consistently showing high levels of variation. While some systems may be affected by the presence of an unknown planet, the results from this study strongly indicates that an Applegate- or Lanza-like mechanism is the most likely driving force for the timing variations seen in the majority of systems in this sample. However, as found in previous studies, the Applegate/Lanza mechanisms are still not able to reproduce the large and rapid timing variations seen in the vast majority of systems, with the companion star to the WD unable to provide sufficient energy on these short timescales.