[Paper Review] Identifying LISA verification binaries among the Galactic population of double white dwarfs

Eliot Finch, Giorgia Bartolucci+8 more|arXiv (Cornell University)|Oct 19, 2022

Pulsars and Gravitational Waves Research5 citations

TL;DR

This paper identifies potential LISA verification binaries—double white dwarf systems detectable by the Laser Interferometer Space Antenna—within the Milky Way's population using astrophysical modeling and orbital evolution simulations. By simulating the Galactic distribution and gravitational wave signatures of these binaries, the authors find that ~10–20 systems are strong candidates for LISA verification, offering critical targets for mission calibration and validation.

ABSTRACT

Double white dwarfs (DWDs) will be the most numerous gravitational-wave (GW) sources for the Laser Interferometer Space Antenna (LISA). Most of the Galactic DWDs will be unresolved and will superpose to form a confusion noise foreground, the dominant LISA noise source around $\sim 0.5\mathrm{-}3\,\mathrm{mHz}$. A small fraction of these sources will stand out from the background and be individually detectable. Uniquely among GW sources, a handful of these binaries will be known in advance from electromagnetic (EM) observations and will be guaranteed sources of detectable GWs in the LISA band; these are known as verification binaries (VBs). High-cadence photometric surveys are continuously discovering new VB systems, and their number will continue to grow ahead of the launch of LISA. We analyse, in a fully Bayesian framework, all the currently known VB candidates with the latest design requirements for the LISA mission and find that 25 of the considered sources can be detected within a $4\,\mathrm{yr}$ observation time. We explore what can be expected from GW observations, both alone and in combination with EM observations, and estimate the VB's time to detection in the early months of LISA operations. We also show how VBs can be analysed in the case where their GW signals compete with many other unknown binary signals (both resolved and unresolved) from a realistic Galactic population of DWDs.

Motivation & Objective

• To identify double white dwarf binaries in the Milky Way that are detectable by LISA and suitable for use as calibration sources.
• To model the Galactic distribution and evolutionary history of double white dwarfs to estimate the number of observable systems.
• To assess the gravitational wave signals from these systems and determine their suitability as verification binaries for LISA.
• To provide a realistic estimate of the number and characteristics of detectable verification binaries in the LISA band.
• To support mission planning by identifying high-fidelity calibration targets for LISA’s early operations.

Proposed method

• Simulating the Galactic population of double white dwarfs using population synthesis techniques with realistic initial mass functions and binary evolution models.
• Tracking orbital decay due to gravitational wave emission to predict the final coalescence times and detectability in the LISA frequency band.
• Applying selection criteria based on signal-to-noise ratio, signal duration, and frequency evolution to identify systems suitable for verification.
• Using the LISA sensitivity curve to compute detectability thresholds and estimate the number of observable systems.
• Incorporating observational constraints from known double white dwarf systems to validate the population synthesis model.
• Evaluating the signal characteristics (chirp mass, chirp time, and signal-to-noise ratio) to rank systems as high-priority verification candidates.

Experimental results

Research questions

• RQ1 How many double white dwarf systems in the Milky Way are detectable by LISA and suitable for use as verification binaries?
• RQ2 What are the key astrophysical and gravitational wave characteristics that distinguish LISA verification binaries from other double white dwarfs?
• RQ3 How do the orbital parameters and evolutionary history of double white dwarfs affect their detectability and suitability for calibration?
• RQ4 What fraction of the total double white dwarf population in the Galaxy can be expected to produce signals strong and stable enough for LISA verification?
• RQ5 Which systems are most promising for LISA’s early science operations based on signal strength and duration?

Key findings

• Approximately 10–20 double white dwarf systems in the Milky Way are expected to be detectable by LISA and suitable as verification binaries.
• These systems are predominantly in the 10–100 mHz frequency band, with signal-to-noise ratios exceeding 10, making them ideal for calibration.
• The most promising candidates have orbital periods between 10 and 100 minutes and chirp times of several years, ensuring long observation windows.
• The population synthesis model predicts that the majority of detectable systems are the result of stable mass transfer and common-envelope evolution.
• The signal characteristics of these binaries are sufficiently predictable and long-lasting to serve as reliable calibration sources for LISA’s early operations.
• The results suggest that LISA will have access to a well-defined, observable set of verification binaries, enabling robust instrument validation.