Eclipsing white dwarf from the Zwicky Transient Facility: II. Seven eclipsing double white dwarfs

TL;DR

This paper reports the discovery and detailed analysis of seven eclipsing double white dwarf binaries from ZTF data, providing insights into their orbital parameters, evolution, and potential for future gravitational wave detection.

Contribution

It presents the first measurement of non-zero eccentricity in compact double white dwarf binaries and predicts observable relativistic effects within a decade.

Findings

Six binaries are low-mass, double helium-core white dwarfs.

One binary is a carbon-oxygen -- helium core white dwarf.

The systems will merge within 36Myr to 1.2Gyr, with measurable eclipse timing delays.

Abstract

In a systematic search for eclipsing white dwarfs using Zwicky transient facility (ZTF) data, we found seven eclipsing double white dwarfs with orbital periods ranging from 45 minutes to 3 hours. We collected high-speed light curves, archival multi-wavelength data, and optical spectra for all systems and determined the binary parameters for each of them. We show that six of the systems are low-mass, double helium-core white dwarf binaries, with the last one a carbon-oxygen -- helium core white dwarf binary. These binaries slowly spiral inwards due to gravitational wave energy losses and are expected to merge within 36Myr--1.2Gyr, and we predict that the shortest orbital period binary will show a measurable eclipse arrival time delay within a decade. The two longest systems show a delay in the arrival time of the secondary eclipse, which we attribute to a small eccentricity of $\approx 2\times10^{-3}$. This is the first time that a non-zero eccentricity is measured in a compact double white dwarf binary. We suggest that these systems emerged from the common envelope with this small eccentricity, and because of the relatively long orbital period, gravitational wave emission has not yet circularised the binaries. Finally, we predict that relativistic apsidal precession will result in a change in the delay of the secondary eclipse that is measurable within a decade.