An 8.8 minute orbital period eclipsing detached double white dwarf binary

Abstract

We report the discovery of ZTF J2243+5242, an eclipsing double white dwarf binary with an orbital period of just \(8.8\) minutes, the second known eclipsing binary with an orbital period less than ten minutes. The system likely consists of two low-mass white dwarfs, and will merge in approximately 400,000 years to form either an isolated hot subdwarf or an R Coronae Borealis star. Like its \(6.91\, \rm min\) counterpart, ZTF J1539+5027, ZTF J2243+5242 will be among the strongest gravitational wave sources detectable by the space-based gravitational-wave detector The Laser Space Interferometer Antenna (LISA) because its gravitational-wave frequency falls near the peak of LISA's sensitivity. Based on its estimated distance of \(d=2120^{+131}_{-115}\,\rm kpc\), LISA should detect the source within its first few months of operation, and should achieve a signal-to-noise ratio of \(87\pm5\) after four years. We find component masses of \(M_A= 0.349^{+0.093}_{-0.074}\,M_\odot\) and \(M_B=0.384^{+0.114}_{-0.074}\,M_\odot\), radii of \(R_A=0.0308^{+0.0026}_{-0.0025}\,R_\odot\) and \(R_B = 0.0291^{+0.0032}_{-0.0024}\,R_\odot\), and effective temperatures of \(T_A=22200^{+1800}_{-1600}\,\rm K\) and \(T_B=16200^{+1200}_{-1000}\,\rm K\). We determined all of these properties, and the distance to this system, using only photometric measurements, demonstrating a feasible way to estimate parameters for the large population of optically faint (\(r>21 \, m_{\rm AB}\)) gravitational-wave sources which the Vera Rubin Observatory (VRO) and LISA should identify.