Supernova remnants expand into their surroundings — but the surroundings are not uniform. The progenitor star shapes the medium before it explodes, carving cavities and piling up shells through its stellar wind. The remnant inherits the geometry of a medium that was sculpted by the star that made it.
XRISM's X-ray microcalorimeter resolves the velocity structure of supernova remnant N103B in the Large Magellanic Cloud with 5 eV spectral resolution — enough to separate individual velocity components within each emission line. Ohshiro, Yamaguchi, and collaborators (arXiv 2602.22321, February 2026) find two distinct kinematic components: a redshifted ring at +335 km/s and a blueshifted ring at -300 km/s, both appearing as complete annular structures in the spatial-velocity maps.
The two rings are not the front and back of a single expanding sphere. They represent two separate interaction sites where the blast wave encounters dense material — likely the inner and outer edges of a circumstellar shell ejected by the progenitor system. The Type Ia progenitor (a white dwarf accreting from a companion) drove a wind that piled up surrounding gas into a shell. The supernova blast wave hit the near side first (blueshifted ring), then the far side (redshifted ring).
The silicon and iron emission lines show different velocity structures: silicon traces the swept-up circumstellar medium, while iron traces the supernova ejecta. The ejecta haven't yet reached the circumstellar shell — they lag behind the blast wave in a layer of lower velocity. The remnant is stratified: the fast shock runs ahead through the progenitor's old wind bubble, the processed circumstellar material sits at the interaction rings, and the ejecta trail behind, still decelerating.
The remnant reads like a geological cross-section. Each velocity layer records a different epoch: the progenitor's wind phase, the explosion, and the subsequent interaction. The double ring is the autobiography of a star, written in plasma and readable at 5 eV resolution.