Deep MUSE observations of SNR 0509–67.5 reveal a double-degenerate merger progenitor

Deep Multi Unit Spectroscopic Explorer (MUSE) observations of SNR (supernova remnant) 0509–67.5 reveal that the coronal [Fe xiv] λ5303 emission line appears with either one or two velocity components across the entire remnant, arising from reverse-shocked ejecta moving towards and away from the observer. A supervised dense neural network classifies each spaxel and fits Gaussian profiles plus a linear function to the observed line emission. We measure a bulk Doppler velocity of -1000± 60~~km~s⁻¹, interpreted as the line-of-sight component of the primary white dwarf’s orbital velocity in a double-degenerate merger. The red- and blue-shifted ejecta map shows a flattened edge along the north-eastern rim, consistent with the companion’s shadow, indicating a binary companion was present at the explosion. Modelling this feature as a cone anchored at the explosion centre and applying Bayesian inference, we recover the cone’s orientation and half-opening angle. We then use the Eggleton Roche-lobe relation to infer properties of the companion. The companion was likely a ~ 0.6~M_ white dwarf with radius ~ 9800 km and orbital velocity ~ 1700~km~s⁻¹ at the time of explosion. Together, these results provide a complete dynamical picture of a Type Ia supernova progenitor system whose maximum-light spectrum is independently constrained by light echo observations.