Orbital Decay in a 20 Minute Orbital Period Detached Binary with a Hydrogen-poor Low-mass White Dwarf

Abstract

We report the discovery of a detached double white dwarf binary with an orbital period of ≈ 20.6 minutes, PTF J053332.05+020911.6. The visible object in this binary, PTF J0533+0209B, is a ≈ 0.17 M mass white dwarf with a helium-dominated atmosphere containing traces of hydrogen (DBA). This object exhibits ellipsoidal variations due to tidal deformation, and is the visible component in a single-lined spectroscopic binary with a velocity semi-amplitude of K B = 618.7 ± 6.9 km s −1 . We have detected significant orbital decay due to the emission of gravitational radiation, and we expect that the Laser Interferometer Space Antenna (LISA) (Amaro-Seoane et al. 2017) will detect this system with a signal to noise of 8.4 +4.2 −3.0 after four years of operation. Because this system already has a well determined orbital period, radial velocity semi-amplitude, temperature, atmospheric composition, surface gravity, and orbital decay rate, a LISA signal will help fully constrain the properties of this system by providing a direct measurement of its inclination. Thus, this binary demonstrates the synergy between electromagnetic and gravitational radiation for constraining the physical properties of an astrophysical object.

An 8.8 Minute Orbital Period Eclipsing Detached Double White Dwarf Binary

“…One way to prepare for LISA is by developing ground-and space-based instrumentation optimized to best characterize the optically detectable portion of its source population in an efficient manner. We hope that in this work, Burdge et al (2019aBurdge et al ( , 2019bBurdge et al ( , 2020, and Coughlin et al (2020), we have demonstrated that high-speed photometers, which can obtain densely sampled high-S/N light curves with high temporal resolution, will be one of the most powerful tools for such characterization. Such instruments on 10 and 30 m class telescopes could be used to characterize binaries like ZTF J2243+5242 and ZTF J1539+5027 to 10-30 kpc, distances well matched to LISAʼs sensitivity threshold.…”

A 1201 s Orbital Period Detached Binary: The First Double Helium Core White Dwarf LISA Verification Binary

“…This is the third shortest-period detached binary after J0651+2844 (Brown et al 2011) and ZTF J1539+5027 (Burdge et al 2019a). While similar in period to J0935+4411 (Kilic et al 2014) and PTF J0533+0209 (Burdge et al 2019b), J2322+0509 has a face-on orientation i = 27 • . It has no detectable light curve or binary astrometric signal; only time-series spectroscopy is able to detect its period.…”

A Systematic Search of Zwicky Transient Facility Data for Ultracompact Binary LISA-detectable Gravitational-wave Sources

“…We originally discovered ZTF J2130+4420, which is the brightest object in the sample, using the analysis-of-variance (AOV) algorithm (Schwarzenberg-Czerny 1996) in a search of bright sources between the WD track and the main sequence. It was later recovered by both the conditional entropy algorithm and as part of a search of the hot subdwarf catalog (Geier et al 2017) as discussed in Kupfer et al (2020b; PTF J0533+0209 was also discovered with the AOV algorithm: see Burdge et al 2019b).…”

“…In 2018 June, using ZTF's first internal data release, we discovered the shortest-orbital-period eclipsing binary system known, ZTF J1539+5027, with an orbital period of just 6.91 minutes (Burdge et al 2019a). We originally tested the viability of a photometric selection strategy using archival Palomar Transient Factory (PTF) data (Law et al 2009), and we discovered PTF J0533+0209, a ≈20.6 minute orbital period detached helium-rich DBA WD binary (Burdge et al 2019b). Since these two discoveries, we have confirmed 13 additional short-period binary systems either spectroscopically via radial velocity shifts or photometrically if they exhibit eclipses (an unambiguous indicator of binarity).…”