Orbital Decay in a 20 Minute Orbital Period Detached Binary with a Hydrogen Poor Low Mass White Dwarf

TL;DR

This paper reports the discovery of a unique, short-period detached double white dwarf binary system exhibiting orbital decay due to gravitational waves, with implications for multi-messenger astronomy and system characterization.

Contribution

It presents the first detailed observation of a 20-minute orbital period white dwarf binary with measurable orbital decay, demonstrating the synergy between electromagnetic and gravitational wave data.

Findings

Detected significant orbital decay consistent with gravitational wave emission.

System's parameters are well constrained, enabling future gravitational wave detection.

Predicted LISA detection with high signal-to-noise after four years.

Abstract

We report the discovery of a detached double white dwarf binary with an orbital period of $\approx20.6\,\rm minutes$, PTF J053332.05+020911.6. The visible object in this binary, PTF J0533+0209B, is a $\approx 0.17\,M_\odot$ 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 $K_B=618.7\pm6.9 \, \rm km \, s^{-1}$. We have detected significant orbital decay due to the emission of gravitational radiation, and we expect that the Laser Interferometer Space Antenna (LISA) 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.