A New LISA-Detectable Type Ia Supernova Progenitor in the Southern Sky: SMSS J1138-5139

We present the discovery and analysis of a nearby eclipsing ultra-compact accreting binary at coordinates 11:38:10.91 - 51:39:49.15 (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=27.69 plus or minus0.03 min; twice the photometric period seen in 2-minute cadence data from TESS Sector 37. We model our optical spectroscopy together with new simultaneous multi-band 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=0.24 M pre-white dwarf donor with a massive M 1=0.99 M white dwarf accretor at orbital inclination i=88.7 deg. 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 plus or minus0.3 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 signal-to-noise 7-10 after a 48-month mission.