[Paper Review] Galactic binaries with eLISA

G. Nelemans|arXiv (Cornell University)|Feb 1, 2013

Pulsars and Gravitational Waves Research1 references3 citations

TL;DR

This paper reviews eLISA's expected detection of thousands of Galactic binary systems, primarily ultra-compact double white dwarfs, with 8 known verification binaries already identified. It highlights that precise gravitational wave parameter estimation—especially amplitude—can be significantly improved using electromagnetic data on inclination, particularly for non-eclipsing systems.

ABSTRACT

I review what eLISA will see from Galactic binaries -- double stars with orbital periods less than a few hours and white dwarf (or neutron star/black hole) components. I discuss the currently known binaries that are guaranteed (or verification) sources and explain why the expected total number of eLISA Galactic binaries is several thousand, even though there are large uncertainties in our knowledge of this population, in particular that of the interacting AM CVn systems. I very briefly sketch the astrophysical questions that can be addressed once these thousands of systems are detected. I close with a short outline of the electro-magnetic facilities that will come on line before eLISA will fly and the importance of developing analysis plans using both electro-magnetic and gravitational wave data.

Motivation & Objective

• To assess the expected number and detectability of Galactic binaries by eLISA, focusing on ultra-compact systems with periods under an hour.
• To identify and validate known verification binaries—systems bright enough for eLISA to detect with high signal-to-noise ratio.
• To evaluate uncertainties in population models, especially for interacting AM CVn systems, which remain poorly constrained.
• To explore the astrophysical insights eLISA can provide, including insights into common envelope evolution and the progenitors of Type Ia supernovae.
• To advocate for integrated analysis of electromagnetic and gravitational wave data to improve source parameter estimation.

Proposed method

• Population synthesis modeling is used to estimate the total number of detectable Galactic binaries, particularly double white dwarfs, with predictions ranging from several thousand to over ten thousand systems.
• Verification binaries are identified based on their expected signal-to-noise ratio (S/N > 7) in eLISA’s sensitivity curve, using known orbital parameters and gravitational wave strain amplitudes.
• Electromagnetic data—such as optical spectroscopy showing Doppler shifts and emission line profiles—are used to infer source geometry, including orbital inclination and mass ratios.
• Joint analysis techniques are explored to improve gravitational wave parameter estimation by incorporating prior electromagnetic constraints, especially on inclination.
• The impact of GAIA and transient surveys (e.g., LSST, PTF) on pre-eLISA identification of binaries is evaluated, particularly for detecting eclipsing systems.
• Statistical modeling of signal-to-noise and confusion limits is used to separate individual sources from the foreground gravitational wave background.

Experimental results

Research questions

• RQ1 How many Galactic binary systems with orbital periods under one hour are expected to be individually detectable by eLISA?
• RQ2 Which known binaries qualify as verification sources for eLISA, and what electromagnetic evidence supports their identification?
• RQ3 Why is the predicted number of AM CVn systems significantly higher than observed, and how does this affect population modeling?
• RQ4 To what extent can electromagnetic data improve the accuracy of gravitational wave parameter estimation, especially for amplitude and inclination?
• RQ5 How will upcoming electromagnetic surveys (e.g., LSST, GAIA) enhance pre-eLISA identification and characterization of Galactic binaries?

Key findings

• eLISA is expected to detect several thousand Galactic binaries, primarily double white dwarfs with orbital periods between 5 and 10 minutes.
• Eight known binaries—HM Cnc, V407 Vul, ES Cet, SDSS J0651+2844, AM CVn, HP Lib, V803 Cen, and CR Boo—are confirmed as verification binaries with S/N > 7.
• SDSS J0651+2844 is the only known detached double white dwarf system showing orbital decay consistent with gravitational wave energy loss, confirming its nature.
• Electromagnetic observations of HM Cnc revealed high-velocity spectral lines at the photometric period, confirming its 5.4-minute orbital period as the true orbital frequency.
• Incorporating electromagnetic data on source inclination can dramatically improve the accuracy of gravitational wave amplitude estimation, especially for face-on systems.
• GAIA is projected to detect around 200 eclipsing double white dwarfs, primarily at short periods, due to its high-cadence sky scanning.