Double white dwarf ZTF J0538+1953 as the brightest verification binary for space laser interferometers

TL;DR

This work analyzes eight nights of Rc-band photometry of the ultrashort-period binary ZTF J0538+1953 to measure its orbital-period decay via an O-C diagram. The quadratic term in the timing residuals yields $dP/dt=-(1.16\pm0.22)\times10^{-11}$ s s$^{-1}$, from which the GW-driven chirp mass is inferred as $\mathcal{M}=0.434\pm0.05\,M_\odot$, about 30% higher than the spectroscopic value. This higher chirp mass predicts strong GW signals, with SNRs of $\rho\approx119$ for LISA (5 years) and $\rho\approx30$ for TianQin (2.5 years), rendering ZTF J0538+1953 the brightest Galactic verification binary for these detectors. The results underscore the importance of long-term photometric monitoring for calibrating space-based GW interferometers and for constraining binary evolution, with future data expected to tighten $\dot P$ and GW-parameter estimates.

Abstract

A decrease in the orbital period of the ultrashort-period binary white dwarf \ZTF, 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 on the 2.5-m telescope of the Caucasian Mountain Observatory of the Sternberg Astronomical Institute of Moscow State University (CMO SAI MSU), a diagram \textit{O-C} is constructed. It can be described by quadratic elements of the brightness variation, which correspond 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 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 years and 2.5 years of observations, respectively.

Figures (4)

• Figure 1: The ZTF J0538+1953 field. Obj - the object, R - standard stars, C - comparison stars. North is up, east to the left.
• Figure 2: Phase light curves if ZTF J0538+1953 in individual nights calculated using ephemerid (1).
• Figure 3: Phase light curve of ZTF J0538+1953 calculated from six observational nights (JD 2460584-60952).
• Figure 4: The O–C diagram of ZTF J0538+1953 relative to the linear ephemerid (1). The solid curve corresponds to quadratic elements (2).