  The geometric structure of supernova remnants (SNR) provides a clue to
unveiling the pre-explosion evolution of their progenitors. Here we present an
X-ray study of N103B (0509-68.7), a Type Ia SNR in the Large Magellanic Cloud,
that is known to be interacting with dense circumstellar matter (CSM). Applying
our novel method for feature extraction to deep Chandra observations, we have
successfully resolved the CSM, Fe-rich ejecta, and intermediate-mass element
(IME) ejecta components, and revealed each of their spatial distribution.
Remarkably, the IME ejecta component exhibits a double-ring structure, implying
that the SNR expands into an hourglass-shape cavity and thus forms bipolar
bubbles of the ejecta. This interpretation is supported by more quantitative
spectroscopy that reveals a clear bimodality in the distribution of the
ionization state of the IME ejecta. These observational results can be
naturally explained if the progenitor binary system had formed a dense CSM
torus on the orbital plane prior to the explosion, providing further evidence
that the SNR N103B originates from a single-degenerate progenitor.
