Double White Dwarf ZTF J0538\(\boldsymbol{+}\)1953 As the Brightest Verification Binary for Space Laser Interferometers

Abstract

A decrease in the orbital period of the ultracompact binary white dwarf ZTF J0538\(+\)1953, which is one of the Galactic verification binaries in the millihertz frequency range for planned space laser interferometers, has been measured. Based on photometric observations carried out at the 2.5-m telescope of the Caucasian Mountain Observatory of Sternberg Astronomical Institute of Moscow State University (CMO SAI MSU), an O-C diagram is constructed. It can be described by quadratic elements of the brightness variations, which is in agreement to a decrease rate of the orbital period of the system of \(dP/dt={-}(1.16\pm 0.22)\times 10^{-11}\) s/s. The decrease rate of the orbital period in the quadrupole approximation for the emission of gravitational waves by a binary system corresponds to its chirp mass \(\mathcal{M}=0.434\pm 0.05\;M_{\odot}\), which turned out to be \({\sim}30\%\) higher than the value obtained earlier from spectroscopic mass determination. The chirp mass of ZTF J0538\(+\)1953 inferred from the measured orbital decay rate makes this system the brightest Galactic verification binary for LISA and TianQin space interferometers with a signal-to-noise ratio of \({\approx}119\) and \({\approx}30\) over 5 and 2.5 years of observations, respectively.