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Observing Orbital Decay in the Ultracompact Hot Subdwarf Binary System ZTFJ213056.71+442046.5

Paul Teckenburg, Thomas Kupfer, Alex J. Brown, Martin M. Roth, Fatma Ben Daya, Jörg Knoche, Stella Vješnica, Paško Roje, Mike Kretlow, Stefan Cikota

Abstract

Ultracompact Galactic binary systems (UCBs) emit low-frequency gravitational waves (GWs). The emission of GWs is causing these systems to lose angular momentum, which is detectable by observing an orbital period decay. ZTFJ213056.71+442046.5 (ZTFJ2130) is an UCB with a period of 39.3401(1) minutes consisting of a Roche lobe-filling hot subdwarf and a white dwarf companion. We attempt to measure the orbital decay rate $\dot{P}$ caused by GW emission of ZTFJ2130 and predict the expected GW signal for LISA. High-speed photometry was conducted using the FLI Kepler KL4040FI CMOS camera, mounted to the 1.2-meter Oskar Lühning telescope at the Hamburg Observatory as well as the Hamamatsu ORCA-Quest 2 qCMOS camera at the 1.23-meter telescope at CAHA in Spain. ZTFJ2130 was observed on six nights between August 2024 and September 2025. The obtained lightcurves combined with previous high-cadence observations were used to conduct an O-C timing analysis. Additionally, we employed the LISA data analysis tool ldasoft to model the expected GW data. We measure a period change of $(-1.97\pm0.05)\times10^{-12}\,\mathrm{ss^{-1}}$. Assuming only GW emission, this result was used to calculate a chirp mass of $(0.408\pm0.006)\,\mathrm{M_{\odot}}$. From ldasoft we predict that LISA will be able to measure the chirp mass with an uncertainty of 5%. We measure $\dot{P}$ with an uncertainty of only 2% and show that modern (q)CMOS detectors are well suited to provide precise timing measurements, enabling the measurement of the orbital decay of UCBs with high precision with modest size telescopes. The derived orbital decay is fully consistent with predictions from spectral and lightcurve modeling. We show that future observations with LISA can potentially provide a deviation from only gravitational wave effects, e.g. due to accretion, if the effect is sufficiently large.

Observing Orbital Decay in the Ultracompact Hot Subdwarf Binary System ZTFJ213056.71+442046.5

Abstract

Ultracompact Galactic binary systems (UCBs) emit low-frequency gravitational waves (GWs). The emission of GWs is causing these systems to lose angular momentum, which is detectable by observing an orbital period decay. ZTFJ213056.71+442046.5 (ZTFJ2130) is an UCB with a period of 39.3401(1) minutes consisting of a Roche lobe-filling hot subdwarf and a white dwarf companion. We attempt to measure the orbital decay rate caused by GW emission of ZTFJ2130 and predict the expected GW signal for LISA. High-speed photometry was conducted using the FLI Kepler KL4040FI CMOS camera, mounted to the 1.2-meter Oskar Lühning telescope at the Hamburg Observatory as well as the Hamamatsu ORCA-Quest 2 qCMOS camera at the 1.23-meter telescope at CAHA in Spain. ZTFJ2130 was observed on six nights between August 2024 and September 2025. The obtained lightcurves combined with previous high-cadence observations were used to conduct an O-C timing analysis. Additionally, we employed the LISA data analysis tool ldasoft to model the expected GW data. We measure a period change of . Assuming only GW emission, this result was used to calculate a chirp mass of . From ldasoft we predict that LISA will be able to measure the chirp mass with an uncertainty of 5%. We measure with an uncertainty of only 2% and show that modern (q)CMOS detectors are well suited to provide precise timing measurements, enabling the measurement of the orbital decay of UCBs with high precision with modest size telescopes. The derived orbital decay is fully consistent with predictions from spectral and lightcurve modeling. We show that future observations with LISA can potentially provide a deviation from only gravitational wave effects, e.g. due to accretion, if the effect is sufficiently large.

Paper Structure

This paper contains 7 sections, 13 equations, 2 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. $O-C$ Timing Analysis
  4. Discussion
  5. Comparison to prediction and previous measurements
  6. Predictions for future space based GW detectors
  7. Conclusions and Summary

Figures (2)

  • Figure 1: (a) Lightcurve of ZTF J2130 taken with the CMOS camera at the OLT in Hamburg on Aug 12, 2024 and (b) lightcurve of ZTF J2130 taken with the 1.23-m telescope at CAHA on Sep 24, 2025. Overplotted are lightcurve models generated using lcurve.
  • Figure 2: $O-C$ diagram for ZTF J2130 with data points from the Nordic Optical Telescope (NOT) and the HiPERCAM instrument at the Gran Telescopio Canarias (GTC), taken from deshmukh. The new data points have been taken with the OLT in Hamburg and the 1.23-meter telescope at CAHA in Spain.