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

Abstract

We report the discovery of a 1201 s orbital period binary, the third shortest-period detached binary known. SDSS J232230.20+050942.06 contains two He-core white dwarfs orbiting with a 27 • inclination. Located 0.76 kpc from the Sun, the binary has an estimated LISA 4-yr signal-to-noise ratio of 40. J2322+0509 is the first He+He white dwarf LISA verification binary, a source class that is predicted to account for one-third of resolved LISA ultra-compact binary detections.

Tidally excited oscillations in hot white dwarfs

“…As we have predicted that the dynamical tide could lead to significant perturbation to the observed flux, it would be nice to compare them with existing observations. We note that there are two 20-min He WD binaries discovered recently, J0533 (Burdge et al 2019) and J2322 (Brown et al 2020b), that are potentially interesting.…”

“…The primary in J2322 (the DA WD) is more similar to our T18 model. However, this system is viewed nearly pole-on and therefore does not show a prominent pulsation signature at 2Ω orb but an upper limit of 0.3% is reported in Brown et al (2020b). If we use the most-likely inclination angle of Θ0 = 27 • as reported in Brown et al (2020b), then our T18 model would predict a flux of 1% due to the dynamical tide.…”

“…The exact transition location from the weakly nonlinear to the strongly nonlinear regimes is also of great significance. The observations of J0538 (Burdge et al 2020) (and potentially J2322; Brown et al 2020b) seem to suggest that gravity waves break before Equation (24). We note that the wave-breaking condition has been studied extensively in the context of solar-type stars where the maximum shear happens at the core (e.g., Ogilvie & Lin 2007;Barker & Ogilvie 2011).…”

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

“…It is also worth noting that the characteristic strains plotted in Figure 9 are a quantity that does not account for inclination, and as discussed earlier, a near edge-on inclination significantly diminishes the gravitational-wave signal expected from a binary compared to a faceon system. Many of the ZTF sources consist of two He-core WDs, whereas the ELM survey is dominated by He WDs orbiting higher-mass CO-core counterparts, with the notable exception of the system reported in Brown et al (2020a), meaning that on average the chirp mass is lower in the ZTF sample of objects than in the ELM survey.…”

“…By targeting objects in this manner in an all-sky survey, we can search millions of candidates, in contrast to the narrower selection criteria used in surveys such as the highly successful Extremely Low Mass (ELM) survey. The ELM survey spectroscopically followed up all candidates in a narrow parameter space, resulting in the discovery of 98 detached double-white dwarfs (DWDs), over half of the known doubledegenerate population (Brown et al 2010(Brown et al , 2012(Brown et al , 2013(Brown et al , 2016(Brown et al , 2020bKilic et al 2011Kilic et al , 2012Gianninas et al 2015), including several systems that are strong candidate LISAdetectable gravitational-wave sources (Brown et al 2011(Brown et al , 2020aKilic et al 2014). Until recently, there were only seven known detached binaries with periods less than an hour, two of which were eclipsing, all discoveries made by the ELM survey (Brown et al 2011(Brown et al , 2017(Brown et al , 2020b.…”