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Abstract We present the discovery and analysis of a nearby eclipsing ultracompact accreting binary SMSS J1138−5139, the first well-constrained LISA-detectable Type Ia supernova progenitor. Our time-series optical spectroscopy identifies its orbital period through radial velocity monitoring at P orb,RV = 27.682 minutes , twice the photometric period seen in 2 minute cadence data from TESS Sector 37. We model our optical spectroscopy together with new simultaneous multiband time-series photometry from Gemini to place constraints on the binary parameters. Our light-curve modeling finds that SMSS J1138−5139 contains an M 2 = 0.25 ± 0.01 M ⊙ pre-white-dwarf donor with a massive M 1 = 1.02 ± 0.05 M ⊙ white dwarf accretor at orbital inclination i = 88 . ∘ 6 ± 0 . ∘ 1 . Based on our photometrically derived system parameters, we expect that gravitational-wave radiation will drive SMSS J1138−5139 to a merger within τ = 5.7 ± 0.3 Myr and result in a Type Ia supernova. Even without a direct merger event, the component masses of SMSS J1138−5139 and active hydrogen accretion suggest that eventual helium accretion will likely also trigger a Type Ia supernova explosion through the dynamically driven double-degenerate double-detonation (D6) channel. We expect LISA to detect the gravitational-wave emission from SMSS J1138−5139 with a signal-to-noise ratio of 7–10 after a 48 month mission.
Kosakowski et al. (Wed,) studied this question.