The Nature and Geometry of the Light Echo from SN 2006X

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We report on the discovery of the geometry producing the light echo emanating from supernova 2006X, a nearby but underluminous Type Ia supernova (SN Ia) in M100 (=NGC 4321). This offers a rare chance to study the environment of a SN Ia. Contrary to previous reports, there is little evidence of a circumstellar component in the light echo morphology or in the light curve of the unresolved SN point source. Instead, the obvious and dominant echo contribution comes from what is probably a relatively thin sheet of material some 26 pc in front of SN 2006X. Of the four known SN Ia light echoes, three show no evidence of a circumstellar echo and the fourth needs to be confirmed. We consider other evidence for circumstellar material around SN Ia, which may be rare.